Portrait of Dr James Urquhart

Dr James Urquhart

Lecturer in Physics and Astrophysics
Head of Centre of Astronomy and Planetary Sciences


Dr James Urquhart graduated from the University of Sussex in 2001 with an MPhys in Physics and Astrophysics. His PhD research was completed at the University of Kent under the supervision of Professor Glenn J. White, and he graduated in 2005 with a thesis on 'Radio surveys of triggered star formation within bright-rimmed clouds'. 

James went on to hold postdoctoral positions at the University of Leeds, UK, the Australia Telescope National Facility in Sydney, Australia and the Max-Planck Institute for Radio Astronomy in Bonn, Germany. In 2015, he moved to the University of Kent to take up a lectureship in Physics and Astrophysics.

Research interests

Dr Urquhart's primary research interest is in the area of massive star formation – an area that underpins many fields in astrophysics; it provides an opportunity to link star formation with the large-scale structure of the Milky Way and obtain a better understanding of star formation in the nearby universe. 

Other areas of interest include: 

  • investigating the modes and efficiency of triggered star formation mechanisms
  • the evolution of the earliest stages of the most massive stars and their galactic distribution
  • galactic structure and the influence of the spiral arms in the star formation process
  • using rotational transitions of simple molecules to probe the structure of the interstellar medium
  • star formation in extreme environments (high pressures and densities, strong UV radiation and cosmic ray fields) such as found in the galactic centre and starburst galaxies.


James teaches a range of physics and astrophysics topics including space science and cosmology, thermodynamics and matter, and data analysis techniques.


Showing 50 of 149 total publications in the Kent Academic Repository. View all publications.


  • Mottram, J., Beuther, H., Ahmadi, A., Klaassen, P., Beltrán, M., Csengeri, T., Feng, S., Gieser, C., Henning, T., Johnston, K., Kuiper, R., Leurini, S., Linz, H., Longmore, S., Lumsden, S., Maud, L., Moscadelli, L., Palau, A., Peters, T., Pudritz, R., Ragan, S., Sánchez-Monge, Á, Semenov, D., Urquhart, J., Winters, J. and Zinnecker, H. (2020). From clump to disc scales in W3 IRS4 : A case study of the IRAM NOEMA large programme CORE. Astronomy and Astrophysics [Online] 636. Available at: https://doi.org/10.1051/0004-6361%2F201834152.
    Context. High-mass star formation typically takes place in a crowded environment, with a higher likelihood of young forming stars affecting and being affected by their surroundings and neighbours, as well as links between different physical scales affecting the outcome. However, observational studies are often focused on either clump or disc scales exclusively.
    Aims. We explore the physical and chemical links between clump and disc scales in the high-mass star formation region W3 IRS4, a region that contains a number of different evolutionary phases in the high-mass star formation process, as a case-study for what can be achieved as part of the IRAM NOrthern Extended Millimeter Array (NOEMA) large programme named CORE: “Fragmentation and disc formation in high-mass star formation”.
    Methods. We present 1.4 mm continuum and molecular line observations with the IRAM NOEMA interferometer and 30 m telescope, which together probe spatial scales from ~0.3−20′′ (600−40 000 AU or 0.003−0.2 pc at 2 kpc, the distance to W3). As part of our analysis, we used XCLASS to constrain the temperature, column density, velocity, and line-width of the molecular emission lines.
    Results. The W3 IRS4 region includes a cold filament and cold cores, a massive young stellar object (MYSO) embedded in a hot core, and a more evolved ultra-compact (UC)H II region, with some degree of interaction between all components of the region that affects their evolution. A large velocity gradient is seen in the filament, suggesting infall of material towards the hot core at a rate of 10−3−10−4 M⊙ yr−1, while the swept up gas ring in the photodissociation region around the UCH II region may be squeezing the hot core from the other side. There are no clear indications of a disc around the MYSO down to the resolution of the observations (600 AU). A total of 21 molecules are detected, with the abundances and abundance ratios indicating that many molecules were formed in the ice mantles of dust grains at cooler temperatures, below the freeze-out temperature of CO (≲35 K). This contrasts with the current bulk temperature of ~50 K, which was obtained from H2CO.
    Conclusions. CORE observations allow us to comprehensively link the different structures in the W3 IRS4 region for the first time. Our results argue that the dynamics and environment around the MYSO W3 IRS4 have a significant impact on its evolution. This context would be missing if only high resolution or continuum observations were available.
  • Wang, Y., Bihr, S., Beuther, H., Rugel, M., Soler, J., Ott, J., Kainulainen, J., Schneider, N., Klessen, R., Glover, S., McClure-Griffiths, N., Goldsmith, P., Johnston, K., Menten, K., Ragan, S., Anderson, L., Urquhart, J., Linz, H., Roy, N., Smith, R., Bigiel, F., Henning, T. and Longmore, S. (2020). Cloud formation in the atomic and molecular phase: H I self absorption (HISA) towards a giant molecular filament. Astronomy and Astrophysics [Online]. Available at: https://doi.org/10.1051/0004-6361/201935866.
    Molecular clouds form from the atomic phase of the interstellar medium. However, characterizing the transition between the atomic and the molecular interstellar medium (ISM) is a difficult observational task. Here we address cloud formation processes by combining H I self absorption (HISA) with molecular line data. Column density probability density functions (N-PDFs) are a common tool to examine molecular clouds. One scenario proposed by numerical simulations is that the N-PDF evolves from a log-normal shape at early times to a power-law-like shape at later times. To date, investigations of N-PDFs are mostly limited to the molecular component of the cloud. In this paper, we study the cold atomic component of the giant molecular filament GMF38.1-32.4a (GMF38a, distance=3.4 kpc, length∼ 230 pc), calculate its N-PDFs and study its kinematics. We identify an extended HISA feature, which is partly correlated with the \(^{13}\)CO emission. The peak velocities of the HISA and \(^{13}\)CO observations agree well on the eastern side of the filament, whereas a velocity offset of approximately 4 km s\(^{−1}\) is found on the western side. The sonic Mach number we derive from the linewidth measurements shows that a large fraction of the HISA, which is ascribed to the cold neutral medium (CNM), is at subsonic and transonic velocities. The column density of the CNM part is on the order of 10\(^{20}\) to 10\(^{21}\) cm\(^{−2}\). The column density of molecular hydrogen, traced by \(^{13}\)CO, is an order of magnitude higher. The N-PDFs from HISA (CNM), H I emission (the warm and cold neutral medium), and \(^{13}\)CO (molecular component) are well described by log-normal functions, which is in agreement with turbulent motions being the main driver of cloud dynamics. The N-PDF of the molecular component also shows a power law in the high column-density region, indicating self-gravity. We suggest that we are witnessing two different evolutionary stages within the filament. The eastern subregion seems to be forming a molecular cloud out of the atomic gas, whereas the western subregion already shows high column density peaks, active star formation and evidence of related feedback processes.
  • Wang, Y., Beuther, H., Rugel, M., Soler, J., Stil, J., Ott, J., Bihr, S., McClure-Griths, N., Anderson, L., Klessen, R., Goldsmith, P., Roy, N., Glover, S., Urquhart, J., Heyer, M., Linz, H., Smith, R., Bigiel, F., Dempsey, J. and Henning, T. (2020). The HI/OH/Recombination line survey of the inner Milky Way (THOR): data release 2 and Hi overview. Astronomy & Astrophysics [Online] 634:A83. Available at: http://dx.doi.org/10.1051/0004-6361/201937095.
    Context. The Galactic plane has been observed extensively by a large number of Galactic plane surveys from infrared to radio wavelengths at an angular resolution below 40". However, a 21 cm line and continuum survey with comparable spatial resolution is still missing. Aims. The first half of THOR data (l = 14.0 37.9, and l = 47.1 51.2, |b| < 1.25) has been published in our data release 1 paper (Beuther et al. 2016). With this data release 2 paper, we publish all the remaining spectral line data and Stokes I continuum data with high angular resolution (1000–4000) including a new H i dataset for the whole THOR survey region (l = 14.0 67.4 and |b| < 1.25). As we have published the results of OH lines and continuum emission elsewhere, we concentrate on the H i analysis in this paper. Methods. With the Karl G. Jansky Very Large Array (VLA) in C-configuration, we observed a large portion of the first Galactic quadrant achieving an angular resolution of < 40. At L Band, the WIDAR correlator at the VLA was set to cover the 21 cm H i line, four OH transitions, a series of Hn↵ radio recombination lines (RRLs; n = 151 to 186), and eight 128 MHz wide continuum spectral windows (SPWs) simultaneously. Results. We publish all OH and RRL data from the C-configuration observations, and a new H i dataset combining VLA C+D+GBT (VLA D-configuration and GBT data are from the VLA Galactic Plane Survey, Stil et al. 2006) for the whole survey. The H i emission shows clear filamentary substructures at negative velocities with low velocity crowding. The emission at positive velocities is more smeared-out likely due to higher spatial and velocity crowding of structures at the positive velocities. Comparing to the spiral arm model of the Milky Way, the atomic gas follows the Sagittarius and Perseus Arm well but with significant material in the inter-arm regions. With the C-configuration-only H i+continuum data, we produced a H i optical depth map of the THOR areal coverage from 228 absorption spectra with the nearest-neighbor method. With this ⌧ map, we corrected the H i emission for optical depth and the derived column density is 38% higher than the column density with optically thin assumption. The total H i mass with optical depth correction in the survey region is 4.7⇥108 M, 31% more than the mass derived assuming the emission is optically thin. If we apply this 31% correction to the whole Milky Way, the total atomic gas mass would be 9.4–10.5⇥109 M. Comparing the H i with existing CO data, we find a significant increase in the atomic-to-molecular gas ration from the spiral arms to the inter-arm regions. Conclusions. The high sensitivity and resolution THOR H i dataset provides an important new window on the physical and kinematic properties of gas in the inner Galaxy. Although the optical depth we derive is a lower limit, our study shows that the optical depth correction is significant for H i column density and mass estimation. Together with the OH, RRL and continuum emission from the THOR survey, these new H i data provide the basis for high angular-resolution studies of the interstellar medium (ISM) in different phases.
  • Chakraborty, A., Roy, N., Wang, Y., Datta, A., Beuther, H., Medina, S., Menten, K., Urquhart, J., Brunthaler, A. and Dzib, S. (2020). Characterization of unresolved and unclassified sources detected in radio continuum surveys of the Galactic plane. Monthly Notices of the Royal Astronomical Society [Online] 492:2236-2240. Available at: http://dx.doi.org/10.1093/mnras/stz3621.
    The continuum emission from 1 to 2 GHz of The HI/OH/Recombination line survey of the inner Milky Way (THOR) at < 18 resolution covers ∼ 132 square degrees of the Galactic plane and detects 10387 sources. Similarly, the first data release of the Global View of Star Formation in the Milky Way (GLOSTAR) surveys covers ∼ 16 square degrees of the Galactic plane from 4-8 GHz at 18 resolution and detects 1575 sources. However, a large fraction of the unresolved discrete sources detected in these radio continuum surveys of the Galactic plane remain unclassified. Here, we study the Euclidean-normalized di˙erential source counts of unclassified and unresolved sources detected in these surveys and compare them with simulated extragalactic radio source populations as well as previously established source counts. We find that the di˙erential source counts for THOR and GLOSTAR surveys are in excellent agreement with both simulation and previous observations. We also estimate the angular two-point correlation function of unclassified and unresolved sources detected in THOR survey. We find a higher clustering amplitude in comparison with the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey up to the angular separation of 5◦. The decrease in angular correlation with increasing flux cut and the excellent agreement of clustering pattern of sources above 1 mJy with high z samples (z > 0.5) of the FIRST survey indicates that these sources might be high z extragalactic compact objects. The similar pattern of one-point and two-point statistics of unclassified and compact sources with extragalactic surveys and simulations confirms the extragalactic origin of these sources.
  • Rigby, A., Moore, T., Eden, D., Urquhart, J., Ragan, S., Peretto, N., Plume, R., Thompson, M., Currie, M. and Park, G. (2019). CHIMPS: Physical properties of molecular clumps across the inner Galaxy. Astronomy & Astrophysics [Online] 632. Available at: https://doi.org/10.1051/0004-6361/201935236.
    The latest generation of high-angular-resolution unbiased Galactic plane surveys in molecular-gas tracers are enabling the interiors of molecular clouds to be studied across a range of environments. The CO Heterodyne Inner Milky Way Plane Survey (CHIMPS) simultaneously mapped a sector of the inner Galactic plane, within 27.8\(^\text{o}\) \(\lesssim\) \(\ell\) \(\lesssim\) 46.2\(^\text{o}\) and |\(\textit{b}\)| ≤ 0.5\(^{\rm o}\), in \({^1}{^3}\)CO (3–2) and C\({^1}{^8}\)O (3–2) at an angular resolution of 15 arcsec. The combination of the CHIMPS data with \({^1}{^2}\)CO (3–2) data from the CO High Resolution Survey (COHRS) has enabled us to perform a voxel-by-voxel local-thermodynamic-equilibrium (LTE) analysis, determining the excitation temperature, optical depth, and column density of \({^1}{^3}\)CO at each \(\ell\), \(\textit{b}\), \(\textit{v}\) position. Distances to discrete sources identified by FellWalker in the \({^1}{^3}\)CO (3–2) emission maps were determined, allowing the calculation of numerous physical properties of the sources, and we present the first source catalogues in this paper. We find that, in terms of size and density, the CHIMPS sources represent an intermediate population between large-scale molecular clouds identified by CO and dense clumps seen in thermal dust continuum emission, and therefore represent the bulk transition from the diffuse to the dense phase of molecular gas. We do not find any significant systematic variations in the masses, column densities, virial parameters, mean excitation temperature, or the turbulent pressure over the range of Galactocentric distance probed, but we do find a shallow increase in the mean volume density with increasing Galactocentric distance. We find that inter-arm clumps have significantly narrower linewidths, and lower virial parameters and excitation temperatures than clumps located in spiral arms. When considering the most reliable distance-limited subsamples, the largest variations occur on the clump-to-clump scale, echoing similar recent studies that suggest that the star-forming process is largely insensitive to the Galactic-scale environment, at least within the inner disc.
  • Gieser, C., Semenov, D., Beuther, H., Ahmadi, A., Mottram, J., Henning, T., Beltran, M., Maud, L., Bosco, F., Leurini, S., Peters, T., Klaassen, P., Kuiper, R., Feng, S., Urquhart, J., Moscadelli, L., Csengeri, T., Lumsden, S., Winters, J., Suri, S., Zhang, Q., Pudritz, R., Palau, A., Menten, K., Galvan-Madrid, R., Wyrowski, F., Schilke, P., Sanchez-Monge, A., Linz, H., Johnston, K., Jiménez-Serra, I., Longmore, S. and Möller, T. (2019). Chemical complexity in high-mass star formation: An observational and modeling case-study of the AFGL2591 VLA3 hot core. Astronomy and Astrophysics [Online] 631:A142. Available at: http://dx.doi.org/10.1051/0004-6361/201935865.
    Aims: In order to understand the observed molecular diversity in high-mass star-forming regions, one has to discern the underlying physical and chemical structure of those regions at high angular resolution and over a range of evolutionary stages. Methods: We present a detailed observational and modeling study of the hot core VLA3 in the high-mass star-forming region AFGL 2591, which is a target region of the NOrthern Extended Millimeter Array (NOEMA) large program CORE. Using NOEMA observations at 1.37mm with an angular resolution of ~0.0042 (1 400 au at 3.33 kpc), we derived the physical and chemical structure of the source. We modeled the observed molecular abundances with the chemical evolution code MUSCLE (MUlti Stage ChemicaL codE. Results: With the kinetic temperature tracers CH3CN and H2CO we observe a temperature distribution with a power-law index of q = 0:41+- 0:08. Using the visibilities of the continuum emission we derive a density structure with a power-law index of p = 1:7 +- 0:1. The hot core spectra reveal high molecular abundances and a rich diversity in complex molecules. The majority of the molecules have an asymmetric spatial distribution around the forming protostar(s), which indicates a complex physical structure on scales < 1 400 au. Using MUSCLE, we are able to explain the observed molecular abundance of 10 out of 14 modeled species at an estimated hot core chemical age of ~21 100 years. In contrast to the observational analysis, our chemical modeling predicts a lower density power-law index of p < 1:4. Reasons for this discrepancy are discussed. Conclusions: Combining high spatial resolution observations with detailed chemical modeling allows us to derive a concise picture of the physical and chemical structure of the famous AFGL 2591 hot core. The next steps are to conduct similar analysis for the whole CORE sample and by that constrain the chemical diversity in high-mass star formation to much greater depth.
  • Lin, Y., Csengeri, T., Wyrowski, F., Urquhart, J., Schuller, F., Weiss, A. and Menten, K. (2019). Fragmentation and filaments at the onset of star and cluster formation: SABOCA 350 μm view of ATLASGAL selected massive clumps. Astronomy & Astrophysics [Online] 631:A72. Available at: https://doi.org/10.1051/0004-6361/201935410.
    Context. The structure formation of the dense interstellar material and the fragmentation of clumps into cores is a fundamental step to understand how stars and stellar clusters form. Aims. We aim to establish a statistical view of clump fragmentation at sub-parsec scales based on a large sample of massive clumps selected from the ATLASGAL survey. Methods. We used the APEX/SABOCA camera at 350 µm to image clumps at a resolution of 8.005, corresponding to physical scales of <0.2 pc at a distance <5 kpc. The majority of the sample consists of massive clumps that are weak or in absorption at 24 µm. We resolve spherical and filamentary structures and identify the population of compact sources. Complemented with archival Herschel data, we derive the physical properties, such as dust temperature, mass and bolometric luminosity of clumps and cores. We use association with mid-infrared 22-24 µm and 70 µm point sources to pin down the star formation activity of the cores. We then statistically assess their physical properties, and the fragmentation characteristics of massive clumps. Results. We detect emission at 350 µm towards all targets and find that it typically exhibits a filamentary(-like) morphology and hosts a population of compact sources. Using Gaussclumps we identify 1120 compact sources and derive the physical parameters and star formation activity for 971 of these, 874 of which are associated with 444 clumps. We find a moderate correlation between the clump fragmentation levels with the clump gas density and the predicted number of fragments with pure Jeans fragmentation scenario. We find a strong correlation between the mass of the most massive fragment and the total clump mass, suggesting that the self-gravity may play an important role in the clumps’ small scale structure formation. Finally, due to the improved angular resolution compared to ATLASGAL, we are able to identify 27 massive quiescent cores with Mcore > 100 M within 5 kpc; these are massive enough to be self-gravitating but do not yet show any sign of star-formation. This sample comprises, therefore, promising candidates of massive pre-stellar cores, or deeply embedded high-mass protostars. Conclusions. The submillimeter observations of the massive clumps that are weak or completely dark at 24 µm reveal rich filamentary structures and an embedded population of compact cores. The maximum core mass is likely determined by the self-gravity of the clump. The rarity of massive pre-stellar core candidates implies short collapse time-scales for dense structures.
  • Billington, S., Urquhart, J., König, C., Moore, T., Eden, D., Breen, S., Kim, W., Thompson, M., Ellingsen, S., Menten, K., Wyrowski, F. and Leurini, S. (2019). ATLASGAL – physical parameters of dust clumps associated with 6.7 GHz methanol masers. Monthly Notices of the Royal Astronomical Society [Online] 490:2779-2798. Available at: https://doi.org/10.1093/mnras/stz2691.
    We have constructed the largest sample of dust-associated class II 6.7 GHz methanol masers yet obtained. New measurements from the the Methanol MultiBeam (MMB) Survey were combined with the 870 µm APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) and the 850 µm JCMT Plane Survey (JPS). Together with two previous studies we have now identified the host clumps for 958 methanol masers across the Galactic Plane, covering approximately 99 per cent of the MMB catalogue and increasing the known sample of dust-associated masers by over 30 per cent. We investigate correlations between the physical properties of the clumps and masers using distances and luminosities drawn from the literature. Clumps hosting methanol masers are significantly more compact and have higher volume densities than the general population of clumps. We determine a minimum volume density threshold of \(n(H^2)\) ≥ \(10^4\)\(cm^{−3}\) for the efficient formation of intermediate- and high-mass stars. We find 6.7 GHz methanol masers are associated with a distinct part of the evolutionary process \((L_{bol}/M_{fwhm}\) ratios of between \(10^{0.6}\) and \(10^{2.2}\)) and have well defined turning on and termination points. We estimate the lower limit for the mass of embedded objects to be ≥ \(6 M_\bigodot\) and the statistical lifetime of the methanol maser stage to be ∼ \(3.3×10^4\) yrs. This suggests that methanol masers are indeed reliable tracers of high mass star formation, and indicates that the evolutionary period traced by this marker is relatively rapid.
  • Keown, J., Francesco, J., Rosolowsky, E., Singh, A., Figura, C., Kirk, H., Anderson, L., Chen, M., Elia, D., Friesen, R., Ginsburg, A., Marston, A., Pezzuto, S., Schisano, E., Bontemps, S., Caselli, P., Liu, H., Longmore, S., Motte, F., Myers, P., Offner, S., Sanhueza, P., Schneider, N., Stephens, I. and Urquhart, J. (2019). KFPA Examinations of Young STellar Object Natal Environments (KEYSTONE): Hierarchical Ammonia Structures in Galactic Giant Molecular Clouds. The Astrophysical Journal [Online] 884:4. Available at: https://doi.org/10.3847/1538-4357%2Fab3e76.
    We present initial results from the K-band Focal Plane Array Examinations of Young STellar Object Natal Environments survey, a large project on the 100 m Green Bank Telescope mapping ammonia emission across 11 giant molecular clouds at distances of 0.9–3.0 kpc (Cygnus X North, Cygnus X South, M16, M17, Mon R1, Mon R2, NGC 2264, NGC 7538, Rosette, W3, and W48). This data release includes the \(NH^3\) (1,1) and (2,2) maps for each cloud, which are modeled to produce maps of kinetic temperature, centroid velocity, velocity dispersion, and ammonia column density. Median cloud kinetic temperatures range from 11.4 ± 2.2 K in the coldest cloud (Mon R1) to 23.0 ± 6.5 K in the warmest cloud (M17). Using dendrograms on the \(NH^3\) (1,1) integrated intensity maps, we identify 856 dense gas clumps across the 11 clouds. Depending on the cloud observed, 40%–100% of the clumps are aligned spatially with filaments identified in \(H^2\) column density maps derived from spectral energy distribution fitting of dust continuum emission. A virial analysis reveals that 523 of the 835 clumps (~63%) with mass estimates are bound by gravity alone. We find no significant difference between the virial parameter distributions for clumps aligned with the dust-continuum filaments and those unaligned with filaments. In some clouds, however, hubs or ridges of dense gas with unusually high mass and low virial parameters are located within a single filament or at the intersection of multiple filaments. These hubs and ridges tend to host water maser emission, multiple 70 μm detected protostars, and have masses and radii above an empirical threshold for forming massive stars.
  • Bosco, F., Beuther, H., Ahmadi, A., Mottram, J., Kuiper, R., Linz, H., Maud, L., Winters, J., Henning, T., Feng, S., Peters, T., Semenov, D., Klaassen, P., Schilke, P., Urquhart, J., Beltran, M., Lumsden, S., Leurini, S., Moscadelli, L., Cesaroni, R., Sanchez-Monge, A., Palau, A., Pudritz, R., Wyrowski, F., Longmore, S. and CORE team, (2019). Fragmentation, rotation and outflows in the high-mass star-forming region IRAS 23033+5951: A case study of the IRAM NOEMA large program CORE. Astronomy & Astrophysics [Online] 629. Available at: https://doi.org/10.1051/0004-6361/201935318.
    Context. The formation process of high-mass stars (> 8 M�) is poorly constrained, particularly, the effects of clump fragmentation creating multiple
    systems and the mechanism of mass accretion onto the cores.
    Aims. We study the fragmentation of dense gas clumps, and trace the circumstellar rotation and outflows by analyzing observations of the highmass (∼ 500 M�) star-forming region IRAS 23033+5951.
    Methods. Using the Northern Extended Millimeter Array (NOEMA) in three configurations and the IRAM 30-m single-dish telescope at 220 GHz,
    we probe the gas and dust emission at an angular resolution of ∼0.4500, corresponding to 1900 au.
    Results. In the mm continuum emission, we identify a protostellar cluster with at least four mm-sources, where three of them show a significantly
    higher peak intensity well above a signal-to-noise ratio of 100. Hierarchical fragmentation from large to small spatial scales is discussed. Two
    fragments are embedded in rotating structures and drive molecular outflows, traced by 13CO (2–1) emission. The velocity profiles across two of the
    cores are similar to Keplerian but are missing the highest velocity components close to the center of rotation, which is a common phenomena from
    observations like these, and other rotation scenarios are not excluded entirely. Position-velocity diagrams suggest protostellar masses of ∼ 6 and
    19 M�. Rotational temperatures from fitting CH3CN (12K − 11K) spectra are used for estimating the gas temperature and by that the disk stability
    against gravitational fragmentation, utilizing Toomre’s Q parameter. Assuming that the candidate disk is in Keplerian rotation about the central
    stellar object and considering different disk inclination angles, we identify only one candidate disk to be unstable against gravitational instability
    caused by axisymmetric perturbations.
    Conclusions. The dominant sources cover different evolutionary stages within the same maternal gas clump. The appearance of rotation and
    outflows of the cores are similar to those found in low-mass star-forming regions.
  • Beuther, H., Walsh, A., Wang, Y., Rugel, M., Soler, J., Linz, H., Klessen, R., Anderson, L., Urquhart, J., Glover, S., Billington, S., Kainulainen, J., Menten, K., Roy, N., Longmore, S. and Bigiel, F. (2019). OH Maser Emission in the THOR Survey of the Northern Milky Way. Astronomy & Astrophysics [Online] 628. Available at: https://doi.org/10.1051/0004-6361/201935936.
    OH masers trace diverse physical processes, from the expanding envelopes around evolved stars to star-forming regions or supernovae remnants. Providing a survey of the ground-state OH maser transitions in the northern hemisphere inner Milky Way facilitates the study of a broad range of scientific topics.
  • Cesaroni, R., Beuther, H., Ahmadi, A., Beltran, M., Csengeri, T., Galvan-Madrid, R., Gieser, C., Henning, T., Johnston, K., Klaassen, P., Kuiper, R., Leurini, S., Linz, H., Longmore, S., Lumsden, S., Maud, L., Moscadelli, L., Mottram, J., Palau, A., Peters, T., Pudritz, R., Sanchez-Monge, A., Schilke, P., Semenov, D., Suri, S., Urquhart, J., Winters, J., Zhang, Q. and Zinnecker, H. (2019). IRAS 23385+6053: An embedded massive cluster in the making. Astronomy and Astrophysics [Online] 627. Available at: https://doi.org/10.1051/0004-6361/201935506.
    Context. This study is part of the project “CORE”, an IRAM/NOEMA large program consisting of observations of the millimeter continuum and molecular line emission towards 20 selected high-mass star forming regions. The goal of the program is to search for circumstellar accretion disks, study the fragmentation process of molecular clumps, and investigate the chemical composition of the gas in these regions.

    Aims. We focus on IRAS 23385+6053, which is believed to be the least evolved source of the CORE sample. This object is characterized by a compact molecular clump that is IR dark shortward of 24 µm and is surrounded by a stellar cluster detected in the near-IR. Our aim is to study the structure and velocity field of the clump.

    Methods. The observations were performed at ∼1.4 mm and employed three configurations of NOEMA and additional single-dish maps, merged with the interferometric data to recover the extended emission. Our correlator setup covered a number of lines from well-known hot core tracers and a few outflow tracers. The angular (∼0.′′45–0.′′9) and spectral (0.5 km s−1) resolutions were sufficient to resolve the clump in IRAS 23385+6053 and investigate the existence of large-scale motions due to rotation, infall, or expansion.

    Results. We find that the clump splits into six distinct cores when observed at sub-arcsecond resolution. These are identified through their 1.4 mm continuum and molecular line emission. We produce maps of the velocity, line width, and rotational temperature from the methanol and methyl cyanide lines, which allow us to investigate the cores and reveal a velocity and temperature gradient in the most massive core. We also find evidence of a bipolar outflow, possibly powered by a low-mass star. Conclusions. We present the tentative detection of a circumstellar self-gravitating disk lying in the most massive core and powering a largescale outflow previously known in the literature. In our scenario, the star powering the flow is responsible for most of the luminosity of IRAS 23385+6053 (∼3000 L⊙). The other cores, albeit with masses below the corresponding virial masses, appear to be accreting material
    from their molecular surroundings and are possibly collapsing or on the verge of collapse. We conclude that we are observing a sample of star-forming cores that is bound to turn into a cluster of massive stars.
  • Medina, S., Urquhart, J., Dzib, S., Brunthaler, A., Cotton, W., Menten, K., Wyrowski, F., Beuther, H., Billington, S., Carrasco-Gonzalez, C., Csengeri, T., Gong, Y., Hofner, P., Nguyen, H., Ortiz-León, G., Pandian, J., Roy, N. and Sarkar, E. (2019). GLOSTAR — Radio Source Catalog I: 28◦ < ℓ < 36◦ and |b| < 1◦⋆. Astronomy and Astrophysics [Online] 627:A175. Available at: https://doi.org/10.1051/0004-6361/201935249.
    Context. Radio continuum surveys of the Galactic plane are an excellent way to identify different source populations such as planetary nebulae, H ii regions, and radio stars and characterize their statistical properties. The GLOSTAR survey will study the star formation in the Galactic plane between −2◦ < ℓ < 85◦ and |b| < 1◦ with unprecedented sensitivity in both, flux density (∼40 µJy beam−1) and range of angular scales (∼1.′′5 to the largest radio structures in the Galaxy). Aims. In this paper we present the first results obtained from a radio continuum map of a 16 square degree sized region of the Galactic plane centered on ℓ = 32◦ and b = 0◦ (28◦ < ℓ < 36◦ and |b| < 1◦). This map has a resolution of 18′′ and a sensitivity of ∼60-150 µJy beam−1. Methods. We present data acquired in 40 hours of observations with the VLA in D-configuration. Two 1 GHz wide sub-bands were observed simultaneously and they were centred at 4.7 and 6.9 GHz. These data were calibrated and imaged using the Obit software package. The source extraction has been performed using the BLOBCAT software package and verified through a combination of visual inspection and cross-matching with other radio and mid-infrared surveys. Results. The final catalog consists of 1575 discrete radio sources and 27 large scale structures (including W43 and W44). By crossmatching with other catalogs and calculating the spectral indices (S (ν) ∝ να), we have classified 231 continuum sources as H ii regions, 37 as ionization fronts, and 46 as planetary nebulae. The longitude and latitude distribution and negative spectral indices are all consistent with the vast majority of the unclassified sources being extragalactic background sources. Conclusions. We present a catalog of 1575 radio continuum sources and discuss their physical properties, emission nature and relation with previously reported. These first GLOSTAR results have increased the number of reliable H ii regions in this part of the Galaxy by a factor of four.
  • Djordjevic, J., Thompson, M., Urquhart, J. and Forbrich, J. (2019). Beyond the Solar Circle − Trends in Massive Star Formation Between the Inner and Outer Galaxy. Monthly Notices of the Royal Astronomical Society [Online]. Available at: https://doi.org/10.1093/mnras/stz1262.
    We have compiled the most complete compact and ultracompact H II region catalogue to date via
    multi-wavelength inspection of survey data. We utilise data from the recently available SASSy
    850 µm survey to identify massive star forming clumps in the outer Galaxy (RGC > 8.5 kpc)
    and cross-match with infrared and radio data of known UC H II regions from the RMS database.
    For the inner Galaxy sample (RGC < 8.5 kpc), we adopt the compact H II regions from previous
    works that used similar methods to cross match ATLASGAL with either CORNISH or RMS,
    depending on the location within the Galactic plane. We present a new UC H II region catalogue
    that more than doubles the original sample size of previous work, totalling 536 embedded H II
    regions and 445 host clumps. We examine the distance independent values of NLy/M and
    Lbol/M as proxies for massive star formation efficiency and overall star formation efficiency,
    respectively. We find a significant trend showing that Lbol/M decreases with increasing RGC,
    suggesting that the overall star formation per unit mass is less in the outer Galaxy.
  • Navarete, F., Leurini, S., Giannetti, A., Wyrowski, F., Urquhart, J., König, C., Csengeri, T., Güsten, R., Damineli, A. and Menten, K. (2019). ATLASGAL-selected high-mass clumps in the inner Galaxy. VII. Characterisation of mid-J CO emission. Astronomy & Astrophysics [Online] 622. Available at: https://doi.org/10.1051/0004-6361/201629777.
    Context: High-mass stars are formed within massive molecular clumps, where a large number of stars form close together. The evolution of the clumps with different masses and luminosities is mainly regulated by its high-mass stellar content and the formation of such objects is still not well understood. Aims. In this work, we characterise the mid-J CO emission in a statistical sample of 99 clumps (TOP100) selected from the ATLASGAL survey that are representative of the Galactic proto-cluster population.

    Methods: High-spatial resolution APEX-CHAMP+ maps of the CO (6–5) and CO (7–6) transitions were obtained and combined with additional single-pointing APEX-FLASH+ spectra of the CO (4–3) line. The data were convolved to a common angular resolution of 13??.4. We analysed the line profiles by fitting the spectra with up to three Gaussian components, classified as narrow or broad, and computed CO line luminosities for each transition. Additionally, we defined a distance-limited sample of 72 sources within 5 kpc to check the robustness of our analysis against beam dilution effects. We study the correlations of the line luminosities and profiles for the three CO transitions with the clump properties and investigate if and how they change as a function of the evolution.

    Results: All sources were detected above 3-? in all three CO transitions and most of the sources exhibit broad CO emission likely associated with molecular outflows. We found that the extension of the mid-J CO emission is correlated with the size of the dust emission traced by the Herschel-PACS 70 µm maps. The CO line luminosity (LCO) is correlated with the luminosity and mass of the clumps. However, it does not correlate with the luminosity-to-mass ratio.

    Conclusions: The dependency of the CO luminosity with the properties of the clumps is steeper for higher-J transitions. Our data seem to exclude that this trend is biased by self-absorption features in the CO emission, but rather suggest that different J transitions arise from different regions of the inner envelope. Moreover, high-mass clumps show similar trends in CO luminosity as lower mass clumps, but are systematically offset towards larger values, suggesting that higher column density and/or temperature (of unresolved) CO emitters are found inside high-mass clumps.
  • Soler, J., Beuther, H., Rugel, M., Wang, Y., Anderson, L., Clark, P., Glover, S., Goldsmith, P., Goodman, A., Hennebelle, P., Henning, T., Heyer, M., Kainulainen, J., Klessen, R., Longmore, S., McClure-Griffiths, N., Menten, K., Mottram, J., Ott, J., Ragan, S., Smith, R., Urquhart, J., Bigiel, F., Roy, N. and Schilke, P. (2019). Histogram of oriented gradients: a technique for the study of molecular cloud formation. Astronomy & Astrophysics [Online] 622:A166. Available at: http://dx/doi.org/10.1051/0004-6361/201834300.
    We introduce the histogram of oriented gradients (HOG), a tool developed for machine vision that we propose as a new metric for the systematic characterization of spectral line observations of atomic and molecular gas and the study of molecular cloud formation models. In essence, the HOG technique takes as input extended spectral-line observations from two tracers and provides an estimate of their spatial correlation across velocity channels. We characterized HOG using synthetic observations of HI and 13CO (J = 1 → 0) emission from numerical simulations of magnetohydrodynamic (MHD) turbulence leading to the formation of molecular gas after the collision of two atomic clouds. We found a significant spatial correlation between the two tracers in velocity channels where vHI ≈ v13CO, almost independent of the orientation of the collision with respect to the line of sight. Subsequently, we used HOG to investigate the spatial correlation of the HI, from The HI/OH/recombination line survey of the inner Milky Way (THOR), and the 13CO (J = 1 → 0) emission from the Galactic Ring Survey (GRS), toward the portion of the Galactic plane 33°.75 ≤l ≤ 35°.25 and |b| ≤ 1°.25. We found a significant spatial correlation between the two tracers in extended portions of the studied region. Although some of the regions with high spatial correlation are associated with HI self-absorption (HISA) features, suggesting that it is produced by the cold atomic gas, the correlation is not exclusive to this kind of region. The HOG results derived for the observational data indicate significant differences between individual regions: some show spatial correlation in channels around vHI ≈ v13CO while others present spatial correlations in velocity channels separated by a few kilometers per second. We associate these velocity offsets to the effect of feedback and to the presence of physical conditions that are not included in the atomic-cloud-collision simulations, such as more general magnetic field configurations, shear, and global gas infall.
  • Rugel, M., Rahner, D., Beuther, H., Pellegrini, E., Wang, Y., Soler, J., Ott, J., Brunthaler, A., Anderson, L., Mottram, J., Henning, T., Goldsmith, P., Heyer, M., Klessen, R., Bihr, S., Menten, K., Smith, R., Urquhart, J., Ragan, S., Glover, S., McClure-Griths, N., Bigiel, F. and Roy, N. (2019). Feedback in W94A diagnosed with Radio Recombination Lines and Models. Astronomy & Astrophysics [Online] 622:A48. Available at: https://doi.org/10.1051/0004-6361/201834068.
    We present images of radio recombination lines (RRLs) at wavelengths around 18 cm from the star-forming region W49A to determine the kinematics of ionized gas in the THOR survey (The Hi/OH/Recombination line survey of the inner Milky Way) at an angular resolution of 16:08 X 13:08. The distribution of ionized gas appears to be affected by feedback processes from the star clusters inW49A. The velocity structure of the RRLs shows a complex behaviour with respect to the molecular gas. We find a shell-like distribution of ionized gas as traced by RRL emission surrounding the central cluster of OB stars in W49A. We describe the evolution of the shell with the recent feedback model code warpfield that includes the important physical processes and has previously been applied to the 30 Doradus region in the Large Magellanic Cloud. The cloud structure and dynamics of W49A are in agreement with a feedbackdriven shell that is re-collapsing. The shell may have triggered star formation in other parts of W49A. We suggest that W49A is a potential candidate for star formation regulated by feedback-driven and re-collapsing shells.
  • Leurini, S., Schisano, E., Pillai, T., Giannetti, A., Urquhart, J., Csengeri, T., Casu, S., Cunningham, M., Elia, D., Jones, P., König, C., Molinari, S., Stanke, T., Testi, L., Wyrowski, F. and Menten, K. (2019). Characterising the high-mass star forming filament G351.776–0.527 with Herschel* and APEX** dust continuum and gas observations. Astronomy & Astrophysics [Online] 621. Available at: https://doi.org/10.1051/0004-6361/201833612.
    G351.776-0.527 is among the most massive, closest, and youngest filaments in the inner Galactic plane and therefore it is an ideal laboratory to study the kinematics of dense gas and mass replenishment on a large scale. In this paper, we present far-infrared (FIR) and submillimetre wavelength continuum observations combined with spectroscopic C18O(2–1) data of the entire region to study its temperature, mass distribution, and kinematics. The structure is composed of a main elongated region with an aspect ratio of ~ 23, which is associated with a network of filamentary structures. The main filament has a remarkably constant width of 0.2 pc. The total
    mass of the network (including the main filament) is > 2600M, while we estimate a mass of ~ 2000M for the main structure. Therefore, the network harbours a large reservoir of gas and dust that could still be accreted onto the main structure. From the analysis of the gas kinematics, we detect two velocity components in the northern part of the main filament. The data also reveal velocity oscillations in C18O along the spine in the main filament and in at least one of the branches. Considering the region as a single structure, we find that it is globally close to virial equilibrium indicating that the entire structure is approximately in a stable state.
  • Urquhart, J., Figura, C., Wyrowski, F., Giannetti, A., Kim, W., Wienen, M., Leurini, S., Pillai, T., Csengeri, T., Gibson, S., Menten, K., Moore, T. and Thompson, M. (2019). ATLASGAL – molecular fingerprints of a sample of massive star-forming clumps. Monthly Notices of the Royal Astronomical Society [Online] 484:4444-4470. Available at: https://doi.org/10.1093/mnras/stz154.
    We have conducted a 3-mm molecular-line survey towards 570 high-mass star-forming
    clumps, using the Mopra telescope. The sample is selected from the 10,000 clumps identified
    by the ATLASGAL survey and includes all of the most important embedded evolutionary
    stages associated with massive star formation, classified into five distinct categories (quiescent,
    protostellar, young stellar objects, HII regions and photo-dominated regions). The observations
    were performed in broadband mode with frequency coverage of 85.2 to 93.4 GHz
    and a velocity resolution of ?0.9 km s
  • Mattern, M., Kauffmann, J., Csengeri, T., Urquhart, J., Leurini, S., Wyrowski, F., Giannetti, A., Barnes, P., Beuther, H., Bronfman, L., Duarte-Cabral, A., Henning, T., Kainulainen, J., Menten, K., Schisano, E. and Schuller, F. (2018). SEDIGISM: The kinematics of ATLASGAL filaments. Astronomy & Astrophysics [Online] 619. Available at: https://doi.org/10.1051/0004-6361/201833406.
    Analysing the kinematics of filamentary molecular clouds is a crucial step towards understanding their role in the star formation process. Therefore, we study the kinematics of 283 filament candidates in the inner Galaxy, that were previously identified in the ATLASGAL dust continuum data. The 13CO(2 – 1) and C18O(2 – 1) data of the SEDIGISM survey (Structure, Excitation, and Dynamics of the Inner Galactic Inter Stellar Medium) allows us to analyse the kinematics of these targets and to determine their physical properties at a resolution of 30'' and 0.25 km s?1 . To do so, we developed an automated algorithm to identify all velocity components along the line-of-sight correlated with the ATLASGAL dust emission, and derive size, mass, and kinematic properties for all velocity components. We
    find two-third of the filament candidates are coherent structures in position-position-velocity space. The remaining candidates appear to be the result of a superposition of two or three filamentary structures along the line-of-sight. At the resolution of the data, on average the filaments are in agreement with Plummer-like radial density profiles with a power-law exponent of p ? 1.5 ± 0.5, indicating that they are typically embedded in a molecular cloud and do not have a well-defined outer radius. Also, we find a correlation between the observed mass per unit length and the velocity dispersion of the filament of m ? ?V^2. We show that this relation can be explained by a virial balance between self-gravity and pressure. Another possible explanation could be radial collapse of the filament, where we can exclude infall motions close to the free-fall velocity.
  • Wang, Y., Bihr, S., Rugel, M., Beuther, H., Johnston, K., Ott, J., Soler, J., Brunthaler, A., Anderson, L., Urquhart, J., Klessen, R., Linz, H., McClure-Griffiths, N., Glover, S., Menten, K., Bigiel, F., Hoare, M. and Longmore, S. (2018). Radio continuum emission in the northern Galactic plane: Sources and spectral indices from the THOR survey. Astronomy and Astrophysics [Online] 619. Available at: https://doi.org/10.1051/0004-6361/201833642.
    Context: Radio continuum surveys of the Galactic plane can find and characterize H II regions, supernova remnants (SNRs), planetary nebulae (PNe), and extragalactic sources. A number of surveys at high angular resolution (≤25″) at different wavelengths exist to study the interstellar medium (ISM), but no comparable high-resolution and high-sensitivity survey exists at long radio wavelengths around 21 cm.

    Aims: Our goal is to investigate the 21 cm radio continuum emission in the northern Galactic plane at < 25″ resolution.

    Methods: We observed a large percentage of the Galactic plane in the first quadrant of the Milky Way (l = 14.0−67.4° and |b| ≤ 1.25°) with the Karl G. Jansky Very Large Array (VLA) in the C-configuration covering six continuum spectral windows (SPW). These data provide a detailed view on the compact as well as extended radio emission of our Galaxy and thousands of extragalactic background sources.

    Results: We used the BLOBCAT software and extracted 10 916 sources. After removing spurious source detections caused by the side lobes of the synthesized beam, we classified 10 387 sources as reliable detections. We smoothed the images to a common resolution of 25″ and extracted the peak flux density of each source in each SPW to determine the spectral indices α (assuming I(ν) ∝ να). By cross-matching with catalogs of H II regions, SNRs, PNe, and pulsars, we found radio counterparts for 840 H II regions, 52 SNRs, 164 PNe, and 38 pulsars. We found 79 continuum sources that are associated with X-ray sources. We identified 699 ultra-steep spectral sources (α < −1.3) that could be high-redshift galaxies. About 9000 of the sources we extracted are not classified specifically, but based on their spatial and spectral distribution, a large percentage of these are likely to be extragalactic background sources. More than 7750 sources do not have counterparts in the SIMBAD database and more than 3760 sources do not have counterparts in the NED database.

    Conclusions: Studying the long wavelengths centimeter continuum emission and the associated spectral indices allows us to characaterize a large percentage of Galactic and extragalactic radio sources in the area of the northern inner Milky Way. This database will be extremely useful for future studies of a diverse set of astrophysical objects.
  • Billington, S., Urquhart, J., Figura, C., Eden, D. and Moore, T. (2018). The RMS Survey: Ammonia mapping of the environment of young massive stellar objects II. Monthly Notices of the Royal Astronomical Society [Online] 483:3146-3167. Available at: https://doi.org/10.1093/mnras/sty3053.
    We present the results from NH3 mapping observations towards 34 regions identified by the Red MSX Source (RMS) survey. We have used the Australia Telescope Compact Array to map ammonia (1, 1) and (2, 2) inversion emission spectra at a resolution of 10 arcsec with velocity channel resolution of 0.4 km s−1 towards the positions of embedded massive star formation. Complementary data have been used from the ATLASGAL and GLIMPSE Legacy Surveys in order to improve the understanding of the regions and to estimate physical parameters for the environments. The fields have typical masses of ∼1000 M⊙, radii of ∼0.15 pc, and distances of ∼3.5 kpc. Luminosities range between ∼103 and ∼106 L⊙ and kinetic temperatures between 10 and 40 K. We classify each field into one of two subsets in order to construct an evolutionary system for massive star formation in these regions based on the morphology and relative positions of the NH3 emission, RMS sources, and ATLASGAL thermal dust emission. Differences in morphology between NH3 emission and ATLASGAL clumps are shown to correspond to evolutionary stages of ongoing massive star formation in these regions. The study has been further refined by including the positions of known methanol and water masers in the regions to gain insight into possible protostellar regions and triggered star formation.
  • Rugel, M., Beuther, H., Bihr, S., Wang, Y., Ott, J., Brunthaler, A., Walsh, A., Glover, S., Goldsmith, P., Anderson, L., Schneider, N., Menten, K., Ragan, S., Urquhart, J., Klessen, R., Soler, J., Roy, N., Kainulainen, J., Henning, T., Bigiel, F., Smith, R., Wyrowski, F. and Longmore, S. (2018). OH absorption in the first quadrant of the Milky Way as seen by THOR. Astronomy & Astrophysics [Online] 618. Available at: https://dx.doi.org/10.1051/0004-6361/201731872.
    Context: The hydroxyl radical (OH) is present in the diffuse molecular and partially atomic phases of the interstellar medium (ISM), but its abundance relative to hydrogen is not clear.

    Aims: We aim to evaluate the abundance of OH with respect to molecular hydrogen using OH absorption against cm-continuum sources over the first Galactic quadrant.

    Methods: This OH study is part of the Hi/OH/Recombination line survey of the inner Milky Way (THOR). THOR is a Karl G. Jansky Very Large Array (VLA) large program of atomic, molecular and ionized gas in the range 15? ? l ? 67? and |b| ? 1?. It is the highest-resolution unbiased OH absorption survey to date towards this region. We combine the optical depths derived from these observations with literature 13CO(1-0) and Hi observations to determine the OH abundance. Results. We detect absorption in the 1665 and 1667MHz transitions, that is, the “main” hyperfine structure lines, for continuum sources stronger than Fcont ? 0.1 Jy beam?1. OH absorption is found against approximately 15% of these continuum sources with increasing fractions for stronger sources. Most of the absorption occurs in molecular clouds that are associated with Galactic Hii regions. We find OH and 13CO gas to have similar kinematic properties. The data indicate that the OH abundance decreases with increasing hydrogen column density. The derived OH abundance with respect to the total hydrogen nuclei column density (atomic and molecular phase) is in agreement with a constant abundance for AV < 10 ? 20. Towards the lowest column densities, we find sources that exhibit OH absorption but no 13CO emission, indicating that OH is a well suited tracer of the low column density molecular gas. We also present spatially resolved OH absorption towards the prominent extended Hii-region W43.

    Conclusions: The unbiased nature of the THOR survey opens a new window onto the gas properties of the interstellar medium. The characterization of the OH abundance over a large range of hydrogen gas column densities contributes to the understanding of OH as a molecular gas tracer and provides a starting point for future investigations.
  • Ahmadi, A., Beuther, H., Mottram, J., Bosco, F., Linz, H., Henning, T., Winters, J., Kuiper, R., Pudritz, R., Sanchez-Monge, A., Keto, E., Beltran, M., Bontemps, S., Cesaroni, R., Csengeri, T., Feng, S., Galvan-Madrid, R., Johnston, K., Klaassen, P., Leurini, S., Longmore, S., Lumsden, S., Maud, L., Menten, K., Moscadelli, l, Motte, F., Palau, A., Peters, T., Ragan, S., Schilke, P., Urquhart, J., Wyrowski, F. and Zinnecker, H. (2018). Core fragmentation and Toomre stability analysis of W3(H2O) A case study of the IRAM NOEMA large program CORE. Astronomy and Astrophysics [Online] 618. Available at: https://doi.org/10.1051/0004-6361/201732548.
    The fragmentation mode of high-mass molecular clumps and the properties of the central rotating structures surrounding the most luminous objects have yet to be comprehensively characterised. Using the IRAM NOrthern Extended Millimeter Array (NOEMA) and the IRAM 30-m telescope, the CORE survey has obtained high-resolution observations of 20 well-known highly luminous star-forming regions in the 1.37 mm wavelength regime in both line and dust continuum emission. We present the spectral line setup of the CORE survey and a case study for W3(H2O). At ~0.35" (700 AU at 2 kpc) resolution, the W3(H2O) clump fragments into two cores (West and East), separated by ~2300 AU. Velocity shifts of a few km/s are observed in the dense-gas tracer, CH3CN, across both cores, consistent with rotation and perpendicular to the directions of two bipolar outflows, one emanating from each core. The kinematics of the rotating structure about W3(H2O) W shows signs of differential rotation of material, possibly in a disk-like object. The observed rotational signature around W3(H2O) E may be due to a disk-like object, an unresolved binary (or multiple) system, or a combination of both. We fit the emission of CH3CN (12-11) K = 4-6 and derive a gas temperature map with a median temperature of ~165 K across W3(H2O). We create a Toomre Q map to study the stability of the rotating structures against gravitational instability. The rotating structures appear to be Toomre unstable close to their outer boundaries, with a possibility of further fragmentation in the differentially-rotating core W3(H2O) W. Rapid cooling in the Toomre-unstable regions supports the fragmentation scenario. Combining millimeter dust continuum and spectral line data toward the famous high-mass star-forming region W3(H2O), we identify core fragmentation on large scales, and indications for possible disk fragmentation on smaller spatial scales.
  • Beuther, H., Mottram, J., Ahmadi, A., Bosco, F., Linz, H., Hennings, T., Klaassen, P., Winters, J., Maud, L., Kuiper, R., Semenov, D., Gieser, C., Peters, T. and Urquhart, J. (2018). Fragmentation and disk formation during high-mass star formation. IRAM NOEMA (Northern Extended Millimeter Array) large program CORE. Astronomy & Astrophysics [Online] 617. Available at: https://doi.org/10.1051/0004-6361/201833021.
    Context. High-mass stars form in clusters, but neither the early fragmentation processes nor the detailed physical processes leading to the most massive stars are well understood. Aims. We aim to understand the fragmentation as well as the disk formation, outflow generation and chemical processes during highmass star formation on spatial scales of individual cores. Methods. Using the IRAM Northern Extended Millimeter Array (NOEMA) in combination with the 30 m telescope, we have observed in the IRAM large program CORE the 1.37 mm continuum and spectral line emission at high angular resolution (?0.4??) for a sample of 20 well-known high-mass star-forming regions with distances below 5.5 kpc and luminosities larger than 104 L?. Results. We present the overall survey scope, the selected sample, the observational setup and the main goals of CORE. Scientifically, we concentrate on the mm continuum emission on scales on the order of 1000 AU. We detect strong mm continuum emission from all regions, mostly due to the emission from cold dust. The fragmentation properties of the sample are diverse. We see extremes where some regions are dominated by a single high-mass core whereas others fragment into as many as 20 cores. A minimum-spanning-tree analysis finds fragmentation at scales on the order of the thermal Jeans length or smaller suggesting that turbulent fragmentation is less important than thermal gravitational fragmentation. The diversity of highly fragmented versus singular regions can be explained by varying initial density structures and/or different initial magnetic field strengths.
    Conclusions. A large sample of high-mass star-forming regions at high spatial resolution allows us to study the fragmentation properties of young cluster-forming regions. The smallest observed separations between cores are found around the angular resolution limit which indicates that further fragmentation likely takes place on even smaller spatial scales. The CORE project with its numerous spectral line detections will address a diverse set of important physical and chemical questions in the field of high-mass star formation.
  • Csengeri, T., Bontemps, S., Wyrowski, F., Belloche, A., Menten, K., Leurini, S., Beuther, H., Bronfman, L., Commerçon, B., Chapillon, E., Longmore, S., Palau, A., Tan, J. and Urquhart, J. (2018). The search for high-mass protostars with ALMA revealed up to kilo-parsec scales (SPARKS). Astronomy and Astrophysics [Online] 617:A89. Available at: https://dx.doi.org/10.1051/0004-6361/201832753.
    The conditions leading to the formation of the most massive O-type stars, are still an enigma in modern astrophysics. To assess the physical conditions of high-mass protostars in their main accretion phase, here we present a case study of a young massive clump selected from the ATLASGAL survey, G328.2551-0.5321. The source exhibits a bolometric luminosity of 1.3 × 104 L?, which allows us to estimate its current protostellar mass to be between ?11 and 16 M?. We show high angular-resolution observations with ALMA reaching a physical scale of ?400 au. To reveal the structure of this high-mass protostellar envelope in detail at a ?0.17?? resolution, we use the thermal dust continuum emission and spectroscopic information, amongst others from the CO (J=3–2) line, which is sensitive to the high velocity molecular outflow, the SiO (J=8–7), and SO2 (J=82,6 ? 71,7) lines tracing shocks along the outflow, as well as several CH3OH and HC3N lines that probe the gas of the inner envelope in the closest vicinity of the protostar. The dust continuum emission reveals a single high-mass protostellar envelope, down to our resolution limit. We find evidence for a compact, marginally resolved continuum source, which is surrounded by azimuthal elongations that could be consistent
    with a spiral pattern. We also report on the detection of a rotational line of CH3OH within its 3t = 1 torsionally excited state. This shows two bright peaks of emission spatially offset from the dust continuum peak, and exhibiting a distinct velocity component ±4.5 km s?1 offset compared to the source vlsr. Rotational diagram analysis and models based on local thermodynamic equilibrium (LTE) assumption require high CH3OH column densities reaching N(CH3OH)=1.2 ? 2 × 1019 cm?2
    , and kinetic temperatures of the order of 160-200 K at the position of these peaks. A comparison of their morphology and kinematics with those of the outflow component of the CO line, and the SO2 line suggests that the high excitation CH3OH spots are associated with the innermost regions of the envelope. While the HC3N 37 = 0 (J=37–36) line is also detected in the outflow, the HC3N 37 = 1e (J=38–37) rotational transition within the molecule’s vibrationally excited state shows a compact morphology. We find that the velocity shifts at the position of the observed high excitation CH3OH spots correspond well to the expected Keplerian velocity around a central object with 15 M? consistent with the mass estimate based on the source’s bolometric luminosity. We propose a picture where the CH3OH emission peaks trace the accretion shocks around the centrifugal barrier, pinpointing the interaction region between the collapsing envelope and an accretion disk. The physical properties of the accretion disk inferred from these observations suggest a specific angular momentum several times larger than typically observed towards low-mass protostars. This is consistent with a scenario of global collapse setting on at larger scales that could carry a more significant amount of kinetic energy compared to the core collapse models of low-mass star formation. Furthermore, our results suggest that vibrationally exited HC3N emission could be a new tracer for compact accretion disks around high-mass protostars.
  • Irabor, T., Hoare, M., Oudmaijer, R., Urquhart, J., Kurtz, S., Lumsden, S., Purcell, C., Zijlstra, A. and Umana, G. (2018). The coordinated radio and infrared survey for high-mass star formation – IV. A new radio-selected sample of compact galactic planetary nebulae. Monthly Notices of the Royal Astronomical Society [Online] 124:939-955. Available at: https://doi.org/10.1093/mnras/sty1929.
    We present a new radio-selected sample of PNe from the CORNISH survey. This is a radio continuum survey of the inner Galactic plane covering Galactic longitude, 10
    < l < 65 and latitude, |b| < 1 with a resolution of 1.5" and sensitivity better than 0.4 mJy/beam. The radio regime, being unbiased by dust extinction, allows for a more complete sample selection, especially towards the Galactic mid-plane. Visual inspection of the CORNISH data, in combination with data from multi-wavelength surveys of the Galactic plane, allowed the CORNISH team to identify 169 candidate PNe. Here, we explore the use of multi-wavelength diagnostic plots and analysis to verify and classify the candidate PNe. We present the multiwavelength properties of this new PNe sample. We find 90 new PNe, of which 12 are newly discovered and 78 are newly classified as PN. A further 47 previously suspected PNe are confirmed as such from the analysis presented here and 24 known PNe are detected. Eight sources are classified as possible PNe or other source types. Our sample includes a young sub-sample, with physical diameters < 0.12 pc, brightness temperatures (> 1000 K) and located closer than 7 kpc. Within this sample is a water-maser PN with a spectral index of - 0.55 ± 0.08, which indicates non-thermal radio emission. Such a radio-selected sample, unaffected by extinction, will be particularly useful to compare with population synthesis models and should contribute to the understanding of the formation and evolution of PNe.
  • Eden, D., Moore, T., Urquhart, J., Elia, D., Plume, R., König, C., Baldeschi, A., Schisano, E., Rigby, A., Morgan, L. and Thompson, M. (2018). Extreme star formation in the Milky Way: Luminosity distributions of young stellar objects in W49A and W51. Monthly Notices of the Royal Astronomical Society [Online] 477:3369-3382. Available at: https://doi.org/10.1093/mnras/sty606.
    We have compared the star-formation properties of the W49A and W51 regions by using far-infrared data from the Herschel infrared Galactic Plane Survey (Hi-GAL) and 850-µm observations from the James Clerk Maxwell Telescope (JCMT) to obtain luminosities and masses, respectively, of associated compact sources. The former are infrared luminosities from the catalogue of Elia et al. (2017), while the latter are from the JCMT Plane survey source catalogue as well as measurements from new data. The clump-mass distributions of the two regions are found to be consistent with each other, as are the clump-formation efficiency and star-formation efficiency analogues. However, the frequency distributions of the luminosities of the young stellar objects are significantly different. While the luminosity distribution in W51 is consistent with Galaxy-wide samples, that of W49A is top-heavy. The differences are not dramatic, and are concentrated in the central regions of W49A. However, they suggest that physical conditions there, which are comparable in part to those in extragalactic starbursts, are significantly affecting the star-formation properties or evolution of the dense clumps in the region.
  • Ragan, S., Moore, T., Eden, D., Hoare, M., Urquhart, J., Elia, D. and Molinari, S. (2018). The role of spiral arms in Milky Way star formation. Monthly Notices of the Royal Astronomical Society [Online] 479:2361-2373. Available at: https://doi.org/10.1093/mnras/sty1672.
    What role does Galactic structure play in star formation? We have used the Herschel Hi-GAL compact-clump catalogue to examine trends in evolutionary stage over large spatial scales in the inner Galaxy. We examine the relationship between the fraction of clumps with embedded star formation (the star-forming fraction, or SFF) and other measures of star-formation activity. Based on a positive correlation between SFF and evolutionary indicators such as the luminosity-to-mass ratio, we assert that the SFF principally traces the average evolutionary state of a sample and must depend on the local fraction of rapidly-evolving, high-mass young stellar objects. The spiralarm tangent point longitudes show small excesses in the SFF, though these can be accounted for by a small number of the most massive clusters, just 7.6% of the total number of clumps in the catalogue. This suggests that while the arms tend to be home to the Galaxy’s massive clusters, the remaining 92.4% of Hi-GAL clumps in our catalogue do not show an enhancement of star formation within arms. Globally, the SFF is highest at the Galactic midplane and inner longitudes. We find no significant trend in evolutionary stage as a function of position across spiral arms at the tangentpoint longitudes. This indicates that the angular offset observed between gas and stars, if coordinated by a density wave, is not evident at the clump phase; alternatively, the onset of star formation is not triggered by the spiral density wave.
  • Kim, J., Urquhart, J., Wyrowski, F., Menten, K. and Csengeri, T. (2018). New detections of (sub)millimeter hydrogen radio recombination lines towards high-mass star-forming clumps. Astronomy & Astrophysics [Online]. Available at: https://doi.org/10.1051/0004-6361/201732330.
  • Dalgleish, H., Longmore, S., Peters, T., Henshaw, J., Veitch-Michaelis, J. and Urquhart, J. (2018). Ionised gas kinematics in bipolar H II regions. Monthly Notices of the Royal Astronomical Society [Online] 478:3530-3543. Available at: https://doi.org/10.1093/mnras/sty1109.
    Stellar feedback plays a fundamental role in shaping the evolution of galaxies. Here we explore the use of ionised gas kinematics in young, bipolar H II regions as a probe of early feedback in these star-forming environments. We have undertaken a multi-wavelength study of a young, bipolar H II region in the Galactic disc, G316.81–0.06, which lies at the centre of a massive (?103 M?) infrared-dark cloud filament. It is still accreting molecular gas as well as driving a ?0.2 pc ionised gas outflow perpendicular to the filament. Intriguingly, we observe a large velocity gradient (47.81 ± 3.21 km s?1 pc?1) across the ionised gas in a direction perpendicular to the outflow. This kinematic signature of the ionised gas shows a reasonable correspondence with the simulations of young H II regions. Based on a qualitative comparison between our observations and these simulations, we put forward a possible explanation for the velocity gradients observed in G316.81–0.06. If the velocity gradient perpendicular to the outflow is caused by rotation of the ionised gas, then we infer that this rotation is a direct result of the initial net angular momentum in the natal molecular cloud. If this explanation is correct, this kinematic signature should be common in other young (bipolar) H II regions. We suggest that further quantitative analysis of the ionised gas kinematics of young H II regions, combined with additional simulations, should improve our understanding of feedback at these early stages.
  • Kalcheva, E., Hoare, M., Urquhart, J., Kurtz, S., Lumsden, S., Purcell, C. and Zijlstra, A. (2018). The Coordinated Radio and Infrared Survey for High-Mass Star Formation III. A catalogue of northern ultra-compact H II regions. Astronomy & Astrophysics [Online] 615. Available at: https://doi.org/10.1051/0004-6361/201832734.
    A catalogue of 239 ultra-compact H II regions (UCHIIs) found in the CORNISH survey at 5 GHz and 1.5″ resolution in the region 10° < l < 65°, |b| < 1° is presented. This is the largest complete and well-selected sample of UCHIIs to date and provides the opportunity to explore the global and individual properties of this key state in massive star formation at multiple wavelengths. The nature of the candidates was validated, based on observational properties and calculated spectral indices, and the analysis is presented in this work. The physical sizes, luminosities and other physical properties were computed by utilising literature distances or calculating the distances whenever a value was not available. The near- and mid-infrared extended source fluxes were measured and the extinctions towards the UCHIIs were computed. The new results were combined with available data at longer wavelengths and the spectral energy distributions (SEDs) were reconstructed for 177 UCHIIs. The bolometric luminosities obtained from SED fitting are presented. By comparing the radio flux densities to previous observational epochs, we find about 5% of the sources appear to be time variable. This first high-resolution area survey of the Galactic plane shows that the total number of UCHIIs in the Galaxy is ~750 – a factor of 3–4 fewer than found in previous large area radio surveys. It will form the basis for future tests of models of massive star formation.
  • Tang, X., Henkel, C., Wyrowski, F., Giannetti, A., Menten, K., Csengeri, T., Leurini, S., Urquhart, J., Koenig, C., Guesten, R., Lin, Y., Zheng, X., Esimbek, J. and Zhou, J. (2018). ATLASGAL-selected massive clumps in the inner Galaxy. VI. Kinetic temperature and spatial density measured with formaldehyde. Astronomy & Astrophysics [Online] 611. Available at: https://doi.org/10.1051/0004-6361/201732168.
    Context: Formaldehyde (H2CO) is a reliable tracer to accurately measure the physical parameters of dense gas in star-forming regions.

    Aim: We aim to determine directly the kinetic temperature and spatial density with formaldehyde for the ~100 brightest ATLASGAL-selected clumps (the TOP100 sample) at 870 ?m representing various evolutionary stages of high-mass star formation.

    Methods: Ten transitions (J = 3–2 and 4–3) of ortho- and para-H2CO near 211, 218, 225, and 291 GHz were observed with the Atacama Pathfinder EXperiment (APEX) 12 m telescope.

    Results: Using non-LTE models with RADEX, we derived the gas kinetic temperature and spatial density with the measured para-H2CO 321–220/303–202, 422–321/404–303, and 404–303/303–202 ratios. The gas kinetic temperatures derived from the para-H2CO 321–220/303–202 and 422–321/404–303 line ratios are high, ranging from 43 to >300 K with an unweighted average of 91 ± 4 K. Deduced Tkin values from the J = 3–2 and 4–3 transitions are similar. Spatial densities of the gas derived from the para-H2CO 404–303/303–202 line ratios yield 0.6–8.3 × 106 cm?3 with an unweighted average of 1.5 (±0.1) × 106 cm?3. A comparison of kinetic temperatures derived from para-H2CO, NH3, and dust emission indicates that para-H2CO traces a distinctly higher temperature than the NH3 (2, 2)/(1, 1) transitions and the dust, tracing heated gas more directly associated with the star formation process. The H2CO line widths are found to be correlated with bolometric luminosity and increase with the evolutionary stage of the clumps, which suggests that higher luminosities tend to be associated with a more turbulent molecular medium. It seems that the spatial densities measured with H2CO do not vary significantly with the evolutionary stage of the clumps. However, averaged gas kinetic temperatures derived from H2CO increase with time through the evolution of the clumps. The high temperature of the gas traced by H2CO may be mainly caused by radiation from embedded young massive stars and the interaction of outflows with the ambient medium. For Lbol/Mclump ? 10 L?/M?, we find a rough correlation between gas kinetic temperature and this ratio, which is indicative of the evolutionary stage of the individual clumps. The strong relationship between H2CO line luminosities and clump masses is apparently linear during the late evolutionary stages of the clumps, indicating that LH_2CO does reliably trace the mass of warm dense molecular gas. In our massive clumps H2CO line luminosities are approximately linearly correlated with bolometric luminosities over about four orders of magnitude in Lbol, which suggests that the mass of dense molecular gas traced by the H2CO line luminosity is well correlated with star formation.
  • Klaassen, P., Johnston, K., Urquhart, J., Mottram, J., Peters, T., Kuiper, R., Beuther, H., van der Tak, F. and Goddi, C. (2018). The evolution of young Hii regions?: I. Continuum emission and internal dynamics. Astronomy and Astrophysics [Online] 611. Available at: https://doi.org/10.1051/0004-6361/201731727.
    Context: High-mass stars form in much richer environments than those associated with isolated low-mass stars, and once they reach a certain mass, produce ionised (HII) regions. The formation of these pockets of ionised gas are unique to the formation of high-mass stars (M > 8 M?), and present an excellent opportunity to study the final stages of accretion, which could include accretion through the HII region itself.

    Aim: This study of the dynamics of the gas on both sides of these ionisation boundaries in very young HII regions aims to quantify the relationship between the HII regions and their immediate environments.

    Methods: We present high-resolution (~0.5?) ALMA observations of nine HII regions selected from the red MSX source survey with compact radio emission and bolometric luminosities greater than 104 L?. We focus on the initial presentation of the data, including initial results from the radio recombination line H29?, some complementary molecules, and the 256 GHz continuum emission.

    Results: Of the six (out of nine) regions with H29? detections, two appear to have cometary morphologies with velocity gradients across them, and two appear more spherical with velocity gradients suggestive of infalling ionised gas. The remaining two were either observed at low resolution or had signals that were too weak to draw robust conclusions. We also present a description of the interactions between the ionised and molecular gas (as traced by CS (J = 5 ? 4)), often (but not always) finding the HII region had cleared its immediate vicinity of molecules.

    Conclusions: Of our sample of nine, the observations of the two clusters expected to have the youngest HII regions (from previous radio observations) are suggestive of having infalling motions in the H29? emission, which could be indicative of late stage accretion onto the stars despite the presence of an HII region.
  • Yang, A., Thompson, M., Urquhart, J. and Tian, W. (2018). Massive Outflows Associated with Atlasgal Clumps. Astrophysical Journal Supplement [Online] 235. Available at: https://doi.org/10.3847/1538-4365/aaa297.
    We have undertaken the largest survey for outflows within the Galactic plane using simultaneously observed 13CO and C18O data. Out of a total of 919 ATLASGAL clumps, 325 have data suitable to identify outflows, and 225 (69% ± 3%) show high-velocity outflows. The clumps with detected outflows show significantly higher clump masses (Mclump), bolometric luminosities (Lbol), luminosity-to-mass ratios (Lbol/Mclump), and peak H2 column densities (NH2) compared to those without outflows. Outflow activity has been detected within the youngest quiescent clump (i.e., 70μm weak) in this sample, and we find that the outflow detection rate increases with Mclump, Lbol, Lbol/Mclump and NH2, approaching 90% in some cases (UC H ii regions = 93% ± 3%; masers = 86% ± 4%; HC H ii regions = 100%). This high detection rate suggests that outflows are ubiquitous phenomena of massive star formation (MSF). The mean outflow mass entrainment rate implies a mean accretion rate of ~10-4Myr-1) in full agreement with the accretion rate predicted by theoretical models of MSF. Outflow properties are tightly correlated with Mclump, Lbol, and Lbol/Mclump and show the strongest relation with the bolometric clump luminosity. This suggests that outflows might be driven by the most massive and luminous source within the clump. The correlations are similar for both low-mass and high-mass outflows over 7 orders of magnitude, indicating that they may share a similar outflow mechanism. Outflow energy is comparable to the turbulent energy within the clump; however, we find no evidence that outflows increase the level of clump turbulence as the clumps evolve. This implies that the origin of turbulence within clumps is fixed before the onset of star formation.
  • Wienen, M., Wyrowski, F., Menten, K., Urquhart, J., Walmsley, C., Csengeri, T., Koribalski, B. and Schuller, F. (2018). ATLASGAL - Ammonia observations towards the southern Galactic Plane. Astronomy and Astrophysics [Online] 609. Available at: https://doi.org/10.1051/0004-6361/201526384.
    Context: The initial conditions of molecular clumps in which high-mass stars form are poorly understood. In particular, a more detailed study of the earliest evolutionary phases is needed. The APEX Telescope Large Area Survey of the whole inner Galactic disk at 870 μm, ATLASGAL, has therefore been conducted to discover high-mass star-forming regions at different evolutionary phases.

    Aims: We derive properties such as velocities, rotational temperatures, column densities, and abundances of a large sample of southern ATLASGAL clumps in the fourth quadrant.

    Methods: Using the Parkes telescope, we observed the NH3 (1, 1) to (3, 3) inversion transitions towards 354 dust clumps detected by ATLASGAL within a Galactic longitude range between 300° and 359° and a latitude within ± 1.5°. For a subsample of 289 sources, the N2H+ (1–0) line was measured with the Mopra telescope.

    Results: We measured a median NH3 (1, 1) line width of ~ 2 km s-1, rotational temperatures from 12 to 28 K with a mean of 18 K, and source-averaged NH3 abundances from 1.6 × 10-6 to 10-8. For a subsample with detected NH3 (2, 2) hyperfine components, we found that the commonly used method to compute the (2, 2) optical depth from the (1, 1) optical depth and the (2, 2) to (1, 1) main beam brightness temperature ratio leads to an underestimation of the rotational temperature and column density. A larger median virial parameter of ~ 1 is determined using the broader N2H+ line width than is estimated from the NH3 line width of ~ 0.5 with a general trend of a decreasing virial parameter with increasing gas mass. We obtain a rising NH3 (1, 1)/N2H+ line-width ratio with increasing rotational temperature.

    Conclusions: A comparison of NH3 line parameters of ATLASGAL clumps to cores in nearby molecular clouds reveals smaller velocity dispersions in low-mass than high-mass star-forming regions and a warmer surrounding of ATLASGAL clumps than the surrounding of low-mass cores. The NH3 (1, 1) inversion transition of 49% of the sources shows hyperfine structure anomalies. The intensity ratio of the outer hyperfine structure lines with a median of 1.27 ± 0.03 and a standard deviation of 0.45 is significantly higher than 1, while the intensity ratios of the inner satellites with a median of 0.9 ± 0.02 and standard deviation of 0.3 and the sum of the inner and outer hyperfine components with a median of 1.06 ± 0.02 and standard deviation of 0.37 are closer to 1.
  • Giannetti, A., Leurini, S., König, C. and Urquhart, J. (2017). Galactocentric variation of the gas-to-dust ratio and its relation with metallicity. Astronomy and Astrophysics [Online] 606. Available at: https://doi.org/10.1051/0004-6361/201731728.
    Context: The assumption of a gas-to-dust mass ratio γ is a common approach to estimate the basic properties of molecular clouds, such as total mass and column density of molecular hydrogen, from (sub)mm continuum observations of the dust. In the Milky Way a single value is used at all galactocentric radii, independently of the observed metallicity gradients. Both models and extragalactic observations suggest that this quantity increases for decreasing metallicity Z, typical of the outer regions in disks, where fewer heavy elements are available to form dust grains.

    Aims: We aim to investigate the variation of the gas-to-dust ratio as a function of galactocentric radius and metallicity, to allow a more accurate characterisation of the quantity of molecular gas across the galactic disk, as derived from observations of the dust.

    Methods: Observations of the optically thin C18O (2–1) transition were obtained with the APEX telescope for a sample of 23 massive and dense star-forming regions in the far outer Galaxy (galactocentric distance greater than 14 kpc). From the modelling of this line and of the spectral energy distribution of the selected clumps we computed the gas-to-dust ratio and compared it to that of well-studied sources from the ATLASGAL TOP100 sample in the inner galactic disk.

    Results. The gradient in γ is found to be 0.087+0.047-0.025 dex kpc-1 (or equivalently γ ∝ Z-1.4+0.3-1.0). The dust-to-metal ratio, decreases with galactocentric radius, which is the most common situation also for external late-type galaxies. This suggests that grain growth dominates over destruction. The predicted γ is in excellent agreement with the estimates in Magellanic clouds, for the appropriate value of Z.
  • Anderson, L., Wang, Y., Bihr, S., Beuther, H., Bigiel, F., Churchwell, E., Glover, S., Goodman, A., Henning, T., Heyer, M., Klessen, R., Linz, H., Longmore, S., Menten, K., Ott, J., Roy, N., Rugel, M., Soler, J., Stil, J. and Urquhart, J. (2017). Galactic Supernova Remnant Candidates Discovered by THOR. Astronomy and Astrophysics [Online] 605. Available at: https://doi.org/10.1051/0004-6361/201731019.
    Context: There is a considerable deficiency in the number of known supernova remnants (SNRs) in the Galaxy compared to that expected. This deficiency is thought to be caused by a lack of sensitive radio continuum data. Searches for extended low-surface brightness radio sources may find new Galactic SNRs, but confusion with the much larger population of H ii regions makes identifying such features challenging. SNRs can, however, be separated from H ii regions using their significantly lower mid-infrared (MIR) to radio continuum intensity ratios.

    Aims: Our goal is to find missing SNR candidates in the Galactic disk by locating extended radio continuum sources that lack MIR counterparts.

    Methods: We use the combination of high-resolution 1–2  GHz continuum data from The HI, OH, Recombination line survey of the Milky Way (THOR) and lower-resolution VLA 1.4  GHz Galactic Plane Survey (VGPS) continuum data, together with MIR data from the Spitzer GLIMPSE, Spitzer MIPSGAL, and WISE surveys to identify SNR candidates. To ensure that the candidates are not being confused with H ii regions, we exclude radio continuum sources from the WISE Catalog of Galactic H ii Regions, which contains all known and candidate H ii regions in the Galaxy.

    Results: We locate 76 new Galactic SNR candidates in the THOR and VGPS combined survey area of 67.4° > ℓ > 17.5°, | b | ≤ 1.25° and measure the radio flux density for 52 previously-known SNRs. The candidate SNRs have a similar spatial distribution to the known SNRs, although we note a large number of new candidates near ℓ ≃ 30°, the tangent point of the Scutum spiral arm. The candidates are on average smaller in angle compared to the known regions, 6.4′ ± 4.7′ versus 11.0′ ± 7.8′, and have lower integrated flux densities.

    Conclusions: The THOR survey shows that sensitive radio continuum data can discover a large number of SNR candidates, and that these candidates can be efficiently identified using the combination of radio and MIR data. If the 76 candidates are confirmed as true SNRs, for example using radio polarization measurements or by deriving radio spectral indices, this would more than double the number of known Galactic SNRs in the survey area. This large increase would still, however, leave a discrepancy between the known and expected SNR populations of about a factor of two.
  • Urquhart, J., König, C., Giannetti, A., Leurini, S., Moore, T., Eden, D., Pillai, T., Thompson, M., Braiding, C., Burton, M., Csengeri, T., Dempsey, J., Figura, C., Froebrich, D., Menten, K., Schuller, F., Smith, M. and Wyrowski, F. (2017). ATLASGAL —- properties of a complete sample of Galactic clumps. Monthly Notices of the Royal Astronomical Society [Online] 473:1059-1102. Available at: https://doi.org/10.1093/mnras/stx2258.
    ATLASGAL is an unbiased 870 micron submillimetre survey of the inner Galactic plane. It provides a large and systematic inventory of all massive, dense clumps in the Galaxy (>1000 Msun) and includes representative samples of all embedded stages of high-mass star formation. Here we present the first detailed census of the properties (velocities, distances, luminosities and masses) and spatial distribution of a complete sample of ~8000 dense clumps located in the Galactic disk. We derive highly reliable velocities and distances to ~97% of the sample and use mid- and far-infrared survey data to develop an evolutionary classification scheme that we apply to the whole sample. Comparing the evolutionary subsamples reveals trends for increasing dust temperatures, luminosities and line-widths as a function of evolution indicating that the feedback from the embedded proto-clusters is having a significant impact on the structure and dynamics of their natal clumps. We find 88\,per\,cent are already associated with star formation at some level. We also find the clump mass to be independent of evolution suggesting that the clumps form with the majority of their mass in-situ. We estimate the statistical lifetime of the quiescent stage to be ~5 x 10^4 yr for clump masses ~1000 Msun decreasing to ~1 x 10^4 yr for clump masses >10000 Msun. We find a strong correlation between the fraction of clumps associated with massive stars and peak column density. The fraction is initially small at low column densities but reaching 100\,per\,cent for column densities above 10^{23} cm^{-2}; there are no clumps with column density clumps above this value that are not already associated with massive star formation. All of the evidence is consistent with a dynamic view of star formation wherein the clumps form rapidly and are initially very unstable so that star formation quickly ensues.
  • Giannetti, A., Leurini, S., Wyrowski, F., Urquhart, J., Csengeri, T., Menten, K., Koenig, C. and Guesten, R. (2017). ATLASGAL-selected massive clumps in the inner Galaxy. V. Temperature structure and evolution. Astronomy & Astrophysics [Online] 603. Available at: https://doi.org/10.1051/0004-6361/201630048.
    Context: Observational identification of a solid evolutionary sequence for high-mass star-forming regions is still missing. Spectroscopic observations give the opportunity to test possible schemes and connect the phases identified to physical processes.

    Aims: We aim to use the progressive heating of the gas caused by the feedback of high-mass young stellar objects to prove the statistical validity of the most common schemes used to observationally define an evolutionary sequence for high-mass clumps, and characterise the sensitivity of different tracers to this process.

    Methods: From the spectroscopic follow-ups carried out towards submillimeter continuum (dust) emission-selected massive clumps (the ATLASGAL TOP100 sample) with the IRAM 30?m, Mopra, and APEX telescopes between 84 GHz and 365 GHz, we selected several multiplets of CH3CN, CH3CCH, and CH3OH emission lines to derive and compare the physical properties of the gas in the clumps along the evolutionary sequence, fitting simultaneously the large number of lines that these molecules have in the observed band. Our findings are compared with results obtained from optically thin CO isotopologues, dust, and ammonia from previous studies on the same sample.

    Results: The chemical properties of each species have a major role on the measured physical properties. Low temperatures are traced by ammonia, methanol, and CO (in the early phases), the warm and dense envelope can be probed with CH3CN, CH3CCH, and, in evolved sources where CO is abundant in the gas phase, via its optically thin isotopologues. CH3OH and CH3CN are also abundant in the hot cores, and we suggest that their high-excitation transitions are good tools to study the kinematics in the hot gas associated with the inner envelope surrounding the young stellar objects that these clumps are hosting. All tracers show, to different degrees according to their properties, progressive warming with evolution. The relation between gas temperature and the luminosity-to-mass (L/M) ratio is reproduced by a simple toy model of a spherical, internally heated clump.

    Conclusions: The evolutionary sequence defined for the clumps is statistically valid and we could identify the physical processes dominating in different intervals of L/M. For L/M ? 2 L?M?-1 a large quantity of the gas is still accumulated and compressed at the bottom of the potential well. Between 2 L?M?-1 ? L/M ? 40 L?M?-1 the young stellar objects gain mass and increase in luminosity; the first hot cores hosting intermediate- or high-mass ZAMS stars appear around L/M ~ 10 L?M?-1. Finally, for L/M ? 40 L?M?-1 Hii regions become common, showing that dissipation of the parental clump dominates.
  • Kim, W., Wyrowski, F., Urquhart, J., Menten, K. and Csengeri, T. (2017). ATLASGAL-selected massive clumps in the inner Galaxy IV. Millimeter hydrogen recombination lines from associated HII regions. Astronomy and Astrophysics [Online] 602:A37. Available at: http://dx.doi.org/10.1051/0004-6361/201629764.
    Aims: Observations of millimeter wavelength radio recombination lines (mm-RRLs) are used to search for H ii regions in an unbiased way that is complementary to many of the more traditional methods previously used (e.g., radio continuum, far-infrared colors, maser emission). The mm-RRLs can be used to derive physical properties of H ii regions and to provide velocity information of ionized gas.

    Methods: We carried out targeted mm-RRL observations (39 ≤ principal quantum number (n) ≤ 65 and Δn = 1, 2, 3, and 4, named Hnα, Hnβ, Hnγ, and Hnδ) using the IRAM 30 m and Mopra 22 m telescopes. In total, we observed 976 compact dust clumps selected from a catalog of ~10 000 sources identified by the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). The sample was selected to ensure a representative mix of star-forming and quiescent clumps such that a variety of different evolutionary stages is represented. Approximately half of the clumps are mid-infrared quiet while the other half are mid-infrared bright.

    Results: We detected Hnα mm-RRL emission toward 178 clumps; Hnβ, Hnγ, and Hnδ were also detected toward 65, 23, and 22 clumps, respectively. This is the largest sample of mm-RRLs detections published to date. Comparing the positions of these clumps with radio continuum surveys we identified compact radio counterparts for 134 clumps, confirming their association with known H ii regions. The nature of the other 44 detections is unclear, but 8 detections are thought to be potentially new H ii regions while the mm-RRL emission from the others may be due to contamination from nearby evolved H ii regions. Broad linewidths are seen toward nine clumps (linewidth > 40 km s-1) revealing significant turbulent motions within the ionized gas; in the past, such wide linewidths were found toward very compact and dense H ii regions. We find that the systemic velocity of the associated dense molecular gas, traced by H13CO+(1−0), is consistent with the mm-RRL velocities and confirms them as embedded H ii regions. We also find that the linewidth of the H13CO+(1−0) emission is significantly wider than those without mm-RRL detection, indicating a physical connection between the embedded H ii region and their natal environments. We also find a correlation between the integrated fluxes of the mm-RRLs and the 6 cm continuum flux densities of their radio counterparts (the correlation coefficient, ρ, is 0.70). By calculating the electron densities we find that the mm-RRL emission is associated with H ii regions with ne < 105 cm-3 and H ii region diameter >0.03 pc.

    Conclusions: We detected mm-RRLs toward 178 clumps and identified eight new H ii region candidates. The broad mm-RRL from nine clumps may indicate that they arise in very young hyper-compact H ii regions. The mm-RRLs trace the radio continuum sources detected by high-resolution observations and their line parameters show associations with the embedded radio sources and their parental molecular clumps.
  • Longmore, S., Walsh, A., Purcell, C., Burke, D., Henshaw, J., Walker, D., Urquhart, J., Barnes, A., Whiting, M., Burton, M., Breen, S., Britton, T., Brooks, K., Cunningham, M., Green, J., Harvey-Smith, L., Hindson, L., Hoare, M., Indermuehle, B., Jones, P., Lo, N., Lowe, V., Moore, T., Thompson, M. and Voronkov, M. (2017). H2O Southern Galactic Plane Survey (HOPS): Paper III – Properties of Dense Molecular Gas across the Inner Milky Way. Monthly Notices of the Royal Astronomical Society [Online] 470:1462-1490. Available at: https://doi.org/10.1093/mnras/stx1226.
    The H2O Southern Galactic Plane Survey (HOPS) has mapped 100 deg2 of the Galactic plane for water masers and thermal molecular line emission using the 22 m Mopra telescope. We describe the automated spectral-line fitting pipelines used to determine the properties of emission detected in HOPS data cubes, and use these to derive the physical and kinematic properties of gas in the survey. A combination of the angular resolution, sensitivity, velocity resolution and high critical density of lines targeted make the HOPS data cubes ideally suited to finding precursor clouds to the most massive and dense stellar clusters in the Galaxy. We compile a list of the most massive HOPS ammonia regions and investigate whether any may be young massive cluster progenitor gas clouds. HOPS is also ideally suited to trace the flows of dense gas in the Galactic Centre. We find the kinematic structure of gas within the inner 500 pc of the Galaxy is consistent with recent predictions for the dynamical evolution of gas flows in the centre of the Milky Way. We confirm a recent finding that the dense gas in the inner 100 pc has an oscillatory kinematic structure with characteristic length-scale of 20 pc, and also identify similar oscillatory kinematic structure in the gas at radii larger than 100 pc. Finally, we make all of the above fits and the remaining HOPS data cubes across the 100 deg2 of the survey available to the community.
  • Schuller, F., Csengeri, T., Urquhart, J., Duarte-Cabral, A., Barnes, P., Giannetti, A., Hernandez, A., Leurini, S., Mattern, M., Medina, S., Agurto, C., Azagra, F., Anderson, L., Beltran, M. and Beuther, H. (2017). SEDIGISM: Structure, excitation, and dynamics of the inner Galactic interstellar medium. Astronomy & Astrophysics [Online] 601. Available at: https://doi.org/10.1051/0004-6361/201628933.
    Context: The origin and life-cycle of molecular clouds are still poorly constrained, despite their importance for understanding the evolution of the interstellar medium. Many large-scale surveys of the Galactic plane have been conducted recently, allowing for rapid progress in this field. Nevertheless, a sub-arcminute resolution global view of the large-scale distribution of molecular gas, from the diffuse medium to dense clouds and clumps, and of their relationshipto the spiral structure, is still missing.

    Aims: We have carried out a systematic, homogeneous, spectroscopic survey of the inner Galactic plane, in order to complement the many continuum Galactic surveys available with crucial distance and gas-kinematic information. Our aim is to combine this data set with recent infrared to sub-millimetre surveys at similar angular resolutions.

    Methods: The SEDIGISM survey covers 78?deg2 of the inner Galaxy (?60°??? 18°, |b|? 0.5°) in the J = 2–1 rotational transition of 13CO. This isotopologue of CO is less abundant than 12CO by factors up to 100. Therefore, its emission has low to moderate optical depths, and higher critical density, making it an ideal tracer of the cold, dense interstellar medium. The data have been observed with the SHFI single-pixel instrument at APEX. The observational setup covers the 13CO(2?1) and C18O(2?1) lines, plus several transitions from other molecules.

    Results: The observations have been completed. Data reduction is in progress, and the final data products will be made available in the near future. Here we give a detailed description of the survey and the dedicated data reduction pipeline. To illustrate the scientific potential of this survey, preliminary results based on a science demonstration field covering ?20°?? ? ?18.5° are presented. Analysis of the 13CO(2?1) data in this field reveals compact clumps, diffuse clouds, and filamentary structures at a range of heliocentric distances. By combining our data with data in the (1–0) transition of CO isotopologues from the ThrUMMS survey, we are able to compute a 3D realization of the excitation temperature and optical depth in the interstellar medium. Ultimately, this survey will provide a detailed, global view of the inner Galactic interstellar medium at an unprecedented angular resolution of ~30??.
  • Eden, D., Moore, T., Plume, R., Urquhart, J., Thompson, M., Parsons, H., Dempsey, J., Rigby, A., Morgan, L., Thomas, H., Berry, D., Buckle, J., Brunt, C., Butner, H., Carretero, D., Chrysostomou, A., Currie, M., deVilliers, H., Fich, M., Gibb, A., Graves, S., Hoare, M., Jenness, T., Manser, G., Mottram, J., Natario, C., Olguin, F., Peretto, N., Pestalozzi, M., Polychroni, D., Redman, R., Salji, C., Summers, L., Tahani, K., Traficante, A., diFrancesco, J., Evans, A., Fuller, G., Johnstone, D., Joncas, G., Longmore, S., Martin, P., Richer, J., Weferling, B., White, G. and Zhu, M. (2017). The JCMT Plane Survey: First complete data release — emission maps and compact source catalogue. Monthly Notices of the Royal Astronomical Society [Online] 469:2163-2183. Available at: https://doi.org/10.1093/mnras/stx874.
    We present the first data release of the James Clerk Maxwell Telescope Plane Survey (JPS), the JPS Public Release 1. JPS is an 850-?m continuum survey of six fields in the northern inner Galactic plane in a longitude range of ? = 7°–63°, made with the Submillimetre Common-User Bolometer Array 2. This first data release consists of emission maps of the six JPS regions with an average pixel-to-pixel noise of 7.19 mJy beam?1, when smoothed over the beam, and a compact source catalogue containing 7813 sources. The 95 per cent completeness limits of the catalogue are estimated at 0.04 Jy beam?1 and 0.3 Jy for the peak and integrated flux densities, respectively. The emission contained in the compact source catalogue is 42 ± 5 per cent of the total and, apart from the large-scale (greater than 8 arcmin) emission, there is excellent correspondence with features in the 500-?m Herschel maps. We find that, with two-dimensional matching, 98 ± 2 per cent of sources within the fields centred at ? = 20°, 30°, 40° and 50° are associated with molecular clouds, with 91 ± 3 per cent of the ? = 30° and 40° sources associated with dense molecular clumps. Matching the JPS catalogue to Herschel 70-?m sources, we find that 38 ± 1 per cent of sources show evidence of ongoing star formation. The JPS Public Release 1 images and catalogue will be a valuable resource for studies of star formation in the Galaxy and the role of environment and spiral arms in the star formation process.
  • Csengeri, T., Bontemps, S., Wyrowski, F., Motte, F., Menten, K., Beuther, H., Bronfman, L., Commerçon, B., Chapillon, E., Duarte-Cabral, A., Fuller, G., Henning, T., Leurini, S., Longmore, S., Palau, A., Peretto, N., Schuller, F., Tan, J., Testi, L., Traficante, A. and Urquhart, J. (2017). ALMA survey of massive cluster progenitors from ATLASGAL. Astronomy & Astrophysics [Online] 600. Available at: https://doi.org/10.1051/0004-6361/201629754.
    The early evolution of massive cluster progenitors is poorly understood. We investigate the fragmentation properties from 0.3 pc to 0.06 pc scales of a homogenous sample of infrared-quiet massive clumps within 4.5 kpc selected from the ATLASGAL survey. Using the ALMA 7 m array we detect compact dust continuum emission towards all targets and find that fragmentation, at these scales, is limited. The mass distribution of the fragments uncovers a large fraction of cores above 40 M⊙, corresponding to massive dense cores (MDCs) with masses up to ~400 M⊙. Seventy-seven percent of the clumps contain at most 3 MDCs per clump, and we also reveal single clumps/MDCs. The most massive cores are formed within the more massive clumps and a high concentration of mass on small scales reveals a high core formation efficiency. The mass of MDCs highly exceeds the local thermal Jeans mass, and we lack the observational evidence of a sufficiently high level of turbulence or strong enough magnetic fields to keep the most massive MDCs in equilibrium. If already collapsing, the observed fragmentation properties with a high core formation efficiency are consistent with the collapse setting in at parsec scales.
  • König, C., Urquhart, J., Csengeri, T., Leurini, S., Wyrowski, F., Giannetti, A., Wienen, M., Pillai, T., Kauffmann, J., Menten, K. and Schuller, F. (2017). ATLASGAL-selected massive clumps in the inner Galaxy III. Dust Continuum Characterization of an Evolutionary Sample. Astronomy & Astrophysics [Online] 599. Available at: https://dx.doi.org/10.1051/0004-6361/201526841.
    Context: Massive-star formation and the processes involved are still poorly understood. The ATLASGAL survey provides an ideal basis for detailed studies of large numbers of massive-star forming clumps covering the whole range of evolutionary stages. The ATLASGAL Top100 is a sample of clumps selected by their infrared and radio properties to be representative for the whole range of evolutionary stages.

    Aims: The ATLASGAL Top100 sources are the focus of a number of detailed follow-up studies that will be presented in a series of papers. In the present work we use the dust continuum emission to constrain the physical properties of this sample and identify trends as a function of source evolution.

    Methods: We determine flux densities from mid-infrared to submillimeter wavelength (8–870 μm) images and use these values to fit their spectral energy distributions and determine their dust temperature and flux. Combining these with recent distances from the literature including maser parallax measurements we determine clump masses, luminosities and column densities.

    Results: We define four distinct source classes from the available continuum data and arrange these into an evolutionary sequence. This begins with sources found to be dark at 70 μm, followed by 24 μm weak sources with an embedded 70 μm source, continues through mid-infrared bright sources and ends with infrared bright sources associated with radio emission (i.e., H ii regions). We find trends for increasing temperature, luminosity, and column density with the proposed evolution sequence, confirming that this sample is representative of different evolutionary stages of massive star formation. Our sources span temperatures from approximately 11 to 41 K, with bolometric luminosities in the range 57 L⊙−3.8 × 106L⊙. The highest masses reach 4.3 × 104M⊙ and peak column densities up to 1.1 × 1024 cm-1, and therefore have the potential to form the most massive O-type stars. We show that at least 93 sources (85%) of this sample have the ability to form massive stars and that most are gravitationally unstable and hence likely to be collapsing.

    Conclusions: The highest column density ATLASGAL sources cover the whole range of evolutionary stages from the youngest to the most evolved high-mass-star forming clumps. Study of these clumps provides a unique starting point for more in-depth research on massive-star formation in four distinct evolutionary stages whose well defined physical parameters afford more detailed studies. As most of the sample is closer than 5 kpc, these sources are also ideal for follow-up observations with high spatial resolution.
  • Nettke, W., Scott, D., Gibb, A., Thompson, M., Chrysostomou, A., Evans, A., Hill, T., Jenness, T., Joncas, G., Moore, T., Serjeant, S., Urquhart, J., Vaccari, M., Weferling, B., White, G. and Zhu, M. (2017). The SCUBA-2 Ambitious Sky Survey: a catalogue of beam-sized sources in the Galactic longitude range 120°–140°. Monthly Notices of the Royal Astronomical Society [Online] 468:250-260. Available at: https://doi.org/10.1093/mnras/stx339.
    The SCUBA-2 Ambitious Sky Survey (SASSy) is composed of shallow 850-μm imaging using the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) on the James Clerk Maxwell Telescope. Here we describe the extraction of a catalogue of beam-sized sources from a roughly 120 deg2 region of the Galactic plane mapped uniformly (to an rms level of about 40 mJy), covering longitude 120° < l < 140° and latitude |b| < 2∘.9. We used a matched-filtering approach to increase the signal-to-noise ratio (S/N) in these noisy maps and tested the efficiency of our extraction procedure through estimates of the false discovery rate, as well as by adding artificial sources to the real images. The primary catalogue contains a total of 189 sources at 850 μm, down to an S/N threshold of approximately 4.6. Additionally, we list 136 sources detected down to S/N = 4.3, but recognize that as we go lower in S/N, the reliability of the catalogue rapidly diminishes. We perform follow-up observations of some of our lower significance sources through small targeted SCUBA-2 images and list 265 sources detected in these maps down to S/N = 5. This illustrates the real power of SASSy: inspecting the shallow maps for regions of 850-μm emission and then using deeper targeted images to efficiently find fainter sources. We also perform a comparison of the SASSy sources with the Planck Catalogue of Compact Sources and the IRAS Point Source Catalogue, to determine which sources discovered in this field might be new, and hence potentially cold regions at an early stage of star formation.

Conference or workshop item

  • Thompson, M., Goedhart, S., Benaglia, P., Beuther, H., Blomme, R., Chrysostomou, A., Clark, J., Dickinson, C., Ellingsen, S., Fenech, D., Hindson, L., Longmore, S., van Langevelde, H., MacLeod, G., Molinari, S., Prinja, R., Purcell, C., Stevens, I., Umana, G., Urquhart, J., Vlemmings, W., van der Walt, J., Walsh, A., Yang, A. and Zijlstra, A. (2018). MeerGAL: the MeerKAT Galactic Plane Survey. In: MeerKAT Science: On the Pathway to the SKA (MeerKAT2016). POS. Available at: https://doi.org/10.22323/1.277.0015.
    Radio surveys of the Milky Way galaxy have transformed our understanding of star formation
    and stellar evolution. However, due to strong dependence of “survey cost” on frequency most
    large area surveys have so far been carried out at low frequencies (? a few GHz). These surveys
    select against dense plasma as the free-free turnover frequency scales directly with electron
    density which means that there are significant biases against the detection of the youngest and
    densest HII regions, Young Stellar Objects, jets, winds and Planetary Nebulae. Here we describe
    the MeerKAT Large Project MeerGAL, which aims to address this issue by making the first sensitive
    high frequency, high resolution multi-epoch survey of the Galactic Plane. Together with
    its Northern Hemisphere sister project KuGARS (the Ku-band Galactic Reconnaissance Survey),
    MeerGAL will revolutionise the study of massive star formation and stellar evolution, Galactic
    structure, and variability.


  • Schuller, F., Urquhart, J., Csengeri, T., Colombo, D., Duarte-Cabral, A., Mattern, M., Ginsburg, A., Pettitt, A., Wyrowski, F., Anderson, L., Azagra, F., Barnes, P., Beltran, M., Beuther, H., Billington, S., Bronfman, L., Cesaroni, R., Dobbs, C., Eden, D., Lee, M., Medina, S., Menten, K., Moore, T., Montenegro-Montes, F., Ragan, S., Rigby, A., Riener, M., Schisano, E., Sanchez-Monge, A., Traficante, A., Zavagno, A., Agurto, C., Bontemps, S., Finger, R., Giannetti, A., Gonzalez, E., Hernandez, A., Henning, T., Kainulainen, J., Kauffmann, J., Leurini, S., Lopez, S., Mac-Auliffe, F., Mazumdar, P., Molinari, S., Motte, F., Muller, E., Nguyen-Luong, Q., Parra, R., Perez-Beaupuits, J., Schilke, P., Schneider, N., Suri, S., Testi, L., Torstensson, K., Veena, V., Venegas, P., Wang, K., Wienen, M. and Russeil, D. (2020). The SEDIGISM survey: first data release and overview of the Galactic structure?. Monthly Notices of the Royal Astronomical Society [Online]. Available at: https://academic.oup.com/mnras.
    The SEDIGISM (Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium) survey used the APEX telescope to map 84 deg2 of the Galactic plane between ` = −60◦and ` = +31◦ in several molecular transitions, including 13CO (2 – 1) and C18O (2 –1), thus probing the moderately dense (∼103 cm−3 ) component of the interstellar medium. With an angular resolution of 3000 and a typical 1σ sensitivity of 0.8–1.0 K at 0.25 km s−1 velocity resolution, it gives access to a wide range of structures, from individual star-forming clumps to giant molecular clouds and complexes. The coverage includes a good fraction of the first and fourth Galactic quadrants, allowing us to constrain the large scale distribution of cold molecular gas in the inner Galaxy. In this paper we provide an updated overview of the full survey and the data reduction procedures used. We also assess the quality of these data and describe the data products that are being made publicly available as part of this first data release (DR1). We present integrated maps and position-velocity maps of the molecular gas and use these to investigate the correlation between the molecular gas and the large scale structural features of the Milky Way such as the spiral arms, Galactic bar and Galactic centre. We find that approximately 60 per cent of the molecular gas is associated with the spiral arms and these appear as strong intensity peaks in the derived Galactocentric distribution. We also find strong peaks in intensity at specific longitudes that correspond to the Galactic centre and well known star forming complexes, revealing that the 13CO emission is concentrated in a small number of complexes rather than evenly distributed along spiral arms.
  • Duarte-Cabral, A., Colombo, D., Urquhart, J., Ginsburg, A., Russeil, D., Schuller, F., Anderson, L., Barnes, P., Beltrán, M., Beuther, H., Bontemps, S., Bronfman, L., Csengeri, T., Dobbs, C., Eden, D., Giannetti, A., Kauffmann, J., Mattern, M., Medina, S., Menten, K., Lee, M., Pettitt, A., Riener, M., Rigby, A., Traficante, A., Veena, V., Wienen, M., Wyrowski, F., Agurto, C., Azagra, F., Cesaroni, R., Finger, R., Gonzalez, E., Henning, T., Hernandez, A., Kainulainen, J., Leurini, S., Lopez, S., Mac-Auliffe, F., Mazumdar, P., Molinari, S., Motte, F., Muller, E., Nguyen-Luong, Q., Parra, R., Perez-Beaupuits, J., Montenegro-Montes, F., Moore, T., Ragan, S., Sánchez-Monge, A., Sanna, A., Schilke, P., Schisano, E., Schneider, N., Suri, S., Testi, L., Torstensson, K., Venegas, P., Wang, K. and Zavagno, A. (2020). The SEDIGISM survey: Molecular clouds in the inner Galaxy. Monthly Notices of the Royal Astronomical Society [Online]. Available at: https://academic.oup.com/mnras.
    We use the 13CO (2-1) emission from the SEDIGISM (Structure, Excitation, and Dynamics of the Inner Galactic InterStellar Medium) high-resolution spectral-line survey of the inner Galaxy, to extract the molecular cloud population with a large dynamic range in spatial scales, using the Spectral Clustering for Interstellar Molecular Emission Segmentation (scimes) algorithm. This work compiles a cloud catalogue with a total of 10663 molecular clouds, 10300 of which we were able to assign distances and compute physical properties. We study some of the global properties of clouds using a science sample, consisting of 6664 well resolved sources and for which the distance estimates are reliable. In particular, we compare the scaling relations retrieved from SEDIGISM to those of other surveys, and we explore the properties of clouds with and without high-mass star formation. Our results suggest that there is no single global property of a cloud that determines its ability to form massive stars, although we find combined trends of increasing mass, size, surface density and velocity dispersion for the sub-sample of clouds with ongoing high-mass star formation. We then isolate the most extreme clouds in the SEDIGISM sample (i.e. clouds in the tails of the distributions) to look at their overall Galactic distribution, in search for hints of environmental effects. We find that, for most properties, the Galactic distribution of the most extreme clouds is only marginally different to that of the global cloud population. The Galactic distribution of the largest clouds, the turbulent clouds and the high-mass star-forming clouds are those that deviate most significantly from the global cloud population. We also find that the least dynamically active clouds (with low velocity dispersion or low virial parameter) are situated further afield, mostly in the least populated areas. However, we suspect that part of these trends may be affected by some observational biases (such as completeness and survey limitations), and thus require further follow up work in order to be confirmed
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