Dr Dirk Froebrich
After completing his degree in Physics at the Universität Leipzig in Germany, Dr Dirk Froebrich went on to study for his PhD at the Universität Jena. From Germany, he moved to a researcher position at the Dublin Institute for Advanced Studies and was appointed Lecturer in Astronomy/Astrophysics at the University of Kent in 2009.
Dr Dirk Froebrich’s main research areas are young protostars and their outflows, structure and properties of molecular clouds and the formation and evolution of star clusters.
More detailed information on Dirk’s research can be found at the research website link above.
Dirk teaches on a range of astrophysics and astronomy modules including atomic physics, cosmology and the interstellar medium, and the multiwavelength universe and exoplanets.
Showing 5 of 88 total publications in the Kent Academic Repository. View all publications.
Guo, Z., Lucas, P., Contreras Peña, C., Kurtev, R., Smith, L., Borissova, J., Alonso-García, J., Minniti, D., Caratti o Garatti, A. and Froebrich, D. (2019). Short and long term near-infrared spectroscopic variability of eruptive protostars from VVV. Monthly Notices of the Royal Astronomical Society [Online]. Available at: https://dx.doi.org/10.1093/mnras/stz3374.Numerous eruptive variable young stellar objects (YSOs), mostly Class I systems, were recently detected by the near-infrared Vista Variables in the Via Lactea (VVV) survey. We present an exploratory near-infrared spectroscopic variability study of 14 eruptive YSOs. The variations were sampled over 1-day and 1 to 2-year intervals and analysed in combination with VVV light curves. CO overtone absorption features are observed on 3 objects with FUor-like spectra: all show deeper absorption when they are brighter. This implies stronger emission from the circumstellar disc with a steeper vertical temperature gradient when the accretion rate is higher. This confirms the nature of fast VVV FUor-like events, in line with the accepted picture for classical FUors. The absence of Brγ emission in a FUor-like object declining to pre-outburst brightness suggests that reconstruction of the stellar magnetic field is a slow process. Within the 1-day timescale, 60% of H2-emitting YSOs show significant but modest variation, and 2/6 sources have large variations in Brγ. Over year-long timescales, H2 flux variations remain modest despite up to 1.8 mag variation in Ks. This indicates that emission from the molecular outflow usually arises further from the protostar and is unaffected by relatively large changes in accretion rate on year-long timescales.
Two objects show signs of on/off magnetospheric accretion traced by Brγ emission. In addition, a 60% inter-night brightening of the H2 outflow is detected in one YSO.
Monnier, J., Alicia, A., Absil, O., Anugu, N., Baines, E., Bayo, A., Berger, J., Cleeves, L., Dale, D., Danchi, W., de Wit, W., Defrère, D., Domagal-Goldman, S., Elvis, M., Froebrich, D., Gai, M., Gandhi, P., Garcia, P., Gardner, T., Gies, D., Gonzalez, J., Gunter, B., Hoenig, S., Ireland, M., Jorgensen, A., Kishimoto, M., Klarmann, L., Kloppenborg, B., Kluska, J., Knight, J., Kral, Q., Kraus, S., Labadie, L., Lawson, P., LeBouquin, J., Leisawitz, D., Lightsey, E., Linz, H., Lipscy, S., MacGregor, M., Matsuo, H., Mennesson, B., Meyer, M., Michael, E., Millour, F., Mozurkewich, D., Norris, R., Ollivier, M., Packham, C., Petrov, R., Pueyo, L., Pope, B., Quanz, S., Ragland, S., Rau, G., Regaly, Z., Riva, A., Roettenbacher, R., Savini, G., Setterholm, B., Sewilo, M., Smith, M., Spencer, L., ten Brummelaar, T., Turner, N., van Belle, G., Weigelt, G. and Wittkowski, M. (2019). A Realistic Roadmap to Formation Flying Space Interferometry. Bulletin of the American Astronomical Society [Online] 51:153. Available at: https://ui.adsabs.harvard.edu/abs/2019BAAS. 51g.153M/abstract.The ultimate astronomical observatory would be a formation flying space interferometer, combining sensitivity and stability with high angular resolution. The smallSat revolution offers a new and maturing prototyping platform for space interferometry and we put forward a realistic plan for achieving first stellar fringes in space by 2030.
Hillenbrand, L., Peña, C., Morrell, S., Naylor, T., Kuhn, M., Cutri, R., Rebull, L., Hodgkin, S., Froebrich, D. and Mainzer, A. (2018). Gaia 17bpi: An FU Ori–type Outburst. The Astrophysical Journal [Online] 869:146. Available at: https://doi.org/10.3847/1538-4357/aaf414.We report on the source Gaia 17bpi and identify it as a new, ongoing FU Ori–type outburst, associated with a young stellar object. The optical light curve from Gaia exhibited a 3.5 mag rise with the source appearing to plateau in mid-/late 2018. Mid-infrared observations from NEOWISE also show a >3 mag rise that occurred in two stages, with the second one coincident with the optical brightening, and the first one preceding the optical brightening by ~1.5 yr. We model the outburst as having started between October and December of 2014. This wavelength-dependent aspect of young star accretion-driven outbursts has never been documented before. Both the mid-infrared and the optical colors of the object become bluer as the outburst proceeds. Optical spectroscopic characteristics in the outburst phase include: a GK-type absorption spectrum, strong wind/outflow in, e.g., Mgb, NaD, H?, K i, O i, and Ca ii profiles, and detection of Li i 6707 Å. The infrared spectrum in the outburst phase is similar to that of an M-type spectrum, notably exhibiting prominent H2O and 12CO (2–0) bandhead absorption in the K band, and likely He i wind in the Y band. The new FU Ori source Gaia 17bpi is associated with a little-studied dark cloud in the galactic plane, located at a distance of 1.27 kpc.
Teixeira, G., Kumar, M., Smith, L., Lucas, P., Morris, C., Borissova, J., Monteiro, M., Carattie o Garatti, A., Contreras Peña, C., Froebrich, D. and Gameiro, J. (2018). Photometric variability of massive young stellar objects. Astronomy and Astrophysics [Online] 619. Available at: http://dx.doi.org/10.1051/0004-6361/201833667.The Vista Variables in the Via Lactea (VVV) survey has allowed for an unprecedented number of multi-epoch observations of the southern Galactic plane. In a recent paper, 13 massive young stellar objects (MYSOs) have already been identified within the highly variable (?Ks > 1 mag) YSO sample of another published work. This study aims to understand the general nature of variability in MYSOs. Here we present the first systematic study of variability in a large sample of candidate MYSOs. We examined the data for variability of the putative driving sources of all known Spitzer extended green objects (EGOs; 270) and bright 24 ?m sources coinciding with the peak of 870 ?m detected ATLASGAL clumps (448), a total of 718 targets. Of these, 190 point sources (139 EGOs and 51 non-EGOs) displayed variability (IQR > 0.05, ?Ks > 0.15 mag). 111 and 79 light-curves were classified as periodic and aperiodic respectively. Light-curves have been sub-classified into eruptive, dipper, fader, short-term-variable and long-period-variable-YSO categories. Lomb-Scargle periodogram analysis of periodic light-curves was carried out. 1–870 ?m spectral energy distributions of all the variable sources were fitted with YSO models to obtain the representative properties of the variable sources. 41% of the variable sources are represented by > 4 M? objects, and only 6% were modelled as > 8 M? objects. The highest-mass objects are mostly non-EGOs, and deeply embedded, as indicated by nearly twice the extinction when compared with EGO sources. By placing them on the HR diagram we show that most of the lower mass, EGO type objects are concentrated on the putative birth-line position, while the luminous non-EGO type objects group around the zero-age-main-sequence track. Some of the most luminous far infrared (FIR) sources in the massive clumps and infrared quiet driving sources of EGOs have been missed out by this study owing to an uniform sample selection method. A high rate of detectable variability in EGO targets (139 out of 153 searched) implies that near-infrared variability in MYSOs is closely linked to the accretion phenomenon and outflow activity.
Borissova, J., Ivanov, V., Lucas, P., Kurtev, R., Alonso-Garcia, J., Ramirez Alegria, S., Minniti, D., Froebrich, D., Hempel, M., Medina, N., Chene, A. and Kuhn, M. (2018). New Galactic Star Clusters Discovered in the Disk Area of the VVVX Survey. Monthly Notices of the Royal Astronomical Society [Online]. Available at: http://dx.doi.org/10.1093/mnras/sty2354.The “VISTA Variables in the V´?a L´actea eXtended (VVVX)” ESO Public Survey is a
near-infrared photometric sky survey that covers nearly 1700 deg2
towards the Galactic
disk and bulge. It is well-suited to search for new open clusters, hidden behind dust
and gas. The pipeline processed and calibrated KS-band tile images of 40% of the
disk area covered by VVVX was visually inspected for stellar over-densities. Then,
we identified cluster candidates by examination of the composite JHKS color images.
The color-magnitude diagrams of the cluster candidates are constructed. Whenever
possible the Gaia DR2 parameters are used to calculate the mean proper motions,
radial velocities, reddening and distances. We report the discovery of 120 new infrared
clusters and stellar groups. Approximately, half of them (47%) are faint, compact,
highly reddened, and they seem to be associated with other indicators of recent star
formation, such as nearby Young Stellar Objects, Masers, H ii regions or bubbles. The
preliminary distance determinations allow us to trace the clusters up to 4.5 kpc, but
most of the cluster candidates are centered at 2.2 kpc. The mean proper motions of
the clusters, show that in general, they follow the disk motion of the Galaxy.