A resolved, au-scale gas disk around the B[e] star HD 50138

Ellerbroek, L. E.; Benisty, M.; Kraus, S.; Perraut, K.; Kluska, J.; le Bouquin, J. B.; Borges Fernandes, M.; Domiciano de Souza, A.; Maaskant, K. M.; Kaper, L.; Tramper, F.; Mourard, D.; Tallon-Bosc, I.; ten Brummelaar, T.; Sitko, M. L.; Lynch, D. K.; Russell, R. W.

Astronomy & Astrophysics, Volume 573, A77 (2015)

ADS – Journal – arXiv


HD 50138 is a B[e] star surrounded by a large amount of circumstellar gas and dust. Its spectrum shows characteristics which may indicate either a pre- or a post-main-sequence system. Mapping the kinematics of the gas in the inner few au of the system contributes to a better understanding of its physical nature. We present the first high spatial and spectral resolution interferometric observations of the Brγ line of HD 50138, obtained with VLTI/AMBER. The line emission originates in a region more compact (up to 3 au) than the continuum-emitting region. Blue- and red-shifted emission originates from the two different hemispheres of an elongated structure perpendicular to the polarization angle. The velocity of the emitting medium decreases radially. An overall offset along the NW direction between the line- and continuum-emitting regions is observed. We compare the data with a geometric model of a thin Keplerian disk and a spherical halo on top of a Gaussian continuum. Most of the data are well reproduced by this model, except for the variability, the global offset and the visibility at the systemic velocity. The evolutionary state of the system is discussed; most diagnostics are ambiguous and may point either to a post-main-sequence or a pre-main-sequence nature.

Keywords: stars: formation; stars: emission-line; Be; stars: variables: T Tauri; Herbig Ae/Be; circumstellar matter; stars: individual: HD 50138; Astrophysics – Solar and Stellar Astrophysics

The properties of ten O-type stars in the low-metallicity galaxies IC 1613, WLM, and NGC 3109

Tramper, F.; Sana, H.; de Koter, A.; Kaper, L.; Ramírez-Agudelo, O. H.

Astronomy & Astrophysics, Volume 572, A36 (2014)

ADS – Journal – arXiv


Context. Massive stars likely played an important role in the reionization of the Universe, and the formation of the first black holes. They are potential progenitors of long-duration gamma-ray bursts, seen up to redshifts of about ten. Massive stars in low-metallicity environments in the local Universe are reminiscent of their high redshift counterparts, emphasizing the importance of the study of their properties and evolution. In a previous paper, we reported on indications that the stellar winds of low-metallicity O stars may be stronger than predicted, which would challenge the current paradigm of massive star evolution.
Aims: In this paper, we aim to extend our initial sample of six O stars in low-metallicity environments by four. The total sample of ten stars consists of the optically brightest sources in IC 1613, WLM, and NGC 3109. We aim to derive their stellar and wind parameters, and compare these to radiation-driven wind theory and stellar evolution models.
Methods: We have obtained intermediate-resolution VLT/X-shooter spectra of our sample of stars. We derive the stellar parameters by fitting synthetic fastwindline profiles to the VLT/X-shooter spectra using a genetic fitting algoritm. We compare our parameters to evolutionary tracks and obtain evolutionary masses and ages. We also investigate the effective temperature versus spectral type calibration for SMC and lower metallicities. Finally, we reassess the wind momentum versus luminosity diagram.
Results: The derived parameters of our target stars indicate stellar masses that reach values of up to 50 M. The wind strengths of our stars are, on average, stronger than predicted from radiation-driven wind theory and reminiscent of stars with an LMC metallicity. We discuss indications that the iron content of the host galaxies is higher than originally thought and is instead SMC-like. We find that the discrepancy with theory is reduced, but remains significant for this higher metallicity. This may imply that our current understanding of the wind properties of massive stars, both in the local universe as well as at cosmic distances, remains incomplete.

Keywords: stars: early-type; stars: massive; stars: winds; outflows; stars: mass-loss; stars: evolution; Astrophysics – Solar and Stellar Astrophysics

On the nature of WO stars: a quantitative analysis of the WO3 star DR1 in IC 1613

Tramper, F.; Gräfener, G.; Hartoog, O. E.; Sana, H.; de Koter, A.; Vink, J. S.; Ellerbroek, L. E.; Langer, N.; Garcia, M.; Kaper, L.; de Mink, S. E.

Astronomy & Astrophysics, Volume 559, A72 (2013)

ADS – Journal – arXiv


Context. Oxygen sequence Wolf-Rayet (WO) stars are thought to represent the final evolutionary stage of the most massive stars. The characteristic strong O vi emission possibly originates from an enhanced oxygen abundance in the stellar wind. Alternatively, the O vi emission can be caused by the high temperature of these stars, in which case the WO stars are the high-temperature extension of the more common carbon sequence Wolf-Rayet (WC) stars.
Aims: By constraining the physical properties and evolutionary status of DR1, a WO star in the low-metallicity Local Group dwarf galaxy IC 1613 and one of only two objects of its class known in a SMC-like metallicity environment, we aim to investigate the nature of WO stars and their evolutionary connection with WC stars.
Methods: We use the non-local thermodynamic equilibrium atmosphere code cmfgen to model the observed spectrum of DR1 and to derive its stellar and wind parameters. We compare our values with other studies of WC and WO stars, as well as with the predictions of evolutionary models. We also model the surrounding nebula using the photo-ionization code cloudy.
Results: The oxygen and carbon abundances that we obtain are comparable to values found for WC stars. The temperature and luminosity are, however, higher than those of WC stars. DR1 is embedded in the hottest known H ii region in the Local Group. The nebular properties can be consistently reproduced by cloudy models adopting DR1 as central ionizing source.
Conclusions: Comparison of the abundances and temperature of DR1 with core helium-burning models show that DR1 is currently well into the second half of helium burning. If the properties of DR1 are representative for the WO class, it would imply that WO stars are the high-temperature and high-luminosity extension of the WC stars, and do not necessarily represent a later evolutionary stage.

Keywords: stars: Wolf-Rayet; stars: massive; stars: individual: DR1; galaxies: individual: IC 1613; HII regions; Astrophysics – Solar and Stellar Astrophysics

RCW36: characterizing the outcome of massive star formation

Ellerbroek, L. E.; Bik, A.; Kaper, L.; Maaskant, K. M.; Paalvast, M.; Tramper, F.; Sana, H.; Waters, L. B. F. M.; Balog, Z.

Astronomy & Astrophysics, Volume 558, A102 (2013)

ADS – Journal – arXiv


Context. Massive stars play a dominant role in the process of clustered star formation, with their feedback into the molecular cloud through ionizing radiation, stellar winds, and outflows. The formation process of massive stars is poorly constrained because of their scarcity, the short formation timescale, and obscuration. By obtaining a census of the newly formed stellar population, the star formation history of the young cluster and the role of the massive stars within it can be unraveled.
Aims: We aim to reconstruct the formation history of the young stellar population of the massive star-forming region RCW 36. We study several dozen individual objects, both photometrically and spectroscopically, looking for signs of multiple generations of young stars and investigating the role of the massive stars in this process.
Methods: We obtain a census of the physical parameters and evolutionary status of the young stellar population. Using a combination of near-infrared photometry and spectroscopy we estimate the ages and masses of individual objects. We identify the population of embedded young stellar objects (YSOs) by their infrared colors and emission line spectra.
Results: RCW 36 harbors a stellar population of massive and intermediate-mass stars located around the center of the cluster. Class 0/I and II sources are found throughout the cluster. The central population has a median age of 1.1 ± 0.6 Myr. Of the stars that could be classified, the most massive ones are situated in the center of the cluster. The central cluster is surrounded by filamentary cloud structures; within these, some embedded and accreting YSOs are found.
Conclusions: Our age determination is consistent with the filamentary structures having been shaped by the ionizing radiation and stellar winds of the central massive stars. The formation of a new generation of stars is ongoing, as demonstrated by the presence of embedded protostellar clumps and two exposed protostellar jets.

Keywords: stars: formation; stars: massive; stars: pre-main sequence; stars: variables: T Tauri; Herbig Ae/Be; Astrophysics – Solar and Stellar Astrophysics

R144 revealed as a double-lined spectroscopic binary

Sana, H.; van Boeckel, T.; Tramper, F.; Ellerbroek, L. E.; de Koter, A.; Kaper, L.; Moffat, A. F. J.; Schnurr, O.; Schneider, F. R. N.; Gies, D. R.

Monthly Notices of the Royal Astronomical Society, Volume 432, L26 (2013)

ADS – Journal – arXiv


R144 is a WN6h star in the 30 Doradus region. It is suspected to be a binary because of its high luminosity and its strong X-ray flux, but no periodicity could be established so far. Here, we present new X-shooter multi-epoch spectroscopy of R144 obtained at the ESO Very Large Telescope. We detect variability in position and/or shape of all the spectral lines. We measure radial velocity variations with an amplitude larger than 250 km s-1 in N IV and N V lines. Furthermore, the N III and N V line Doppler shifts are anticorrelated and the N IV lines show a double-peaked profile on six of our seven epochs. We thus conclude that R144 is a double-lined spectroscopic binary. Possible orbital periods range from two to six months, although a period up to one year is allowed if the orbit is highly eccentric. We estimate the spectral types of the components to be WN5-6h and WN6-7h, respectively. The high luminosity of the system (log Lbol/L ≈ 6.8) suggests a present-day total mass content in the range of about 200-300 M, depending on the evolutionary stage of the components. This makes R144 the most massive binary identified so far, with a total mass content at birth possibly as large as 400 M. We briefly discuss the presence of such a massive object, 60 pc away from the R136 cluster core in the context of star formation and stellar dynamics.

Keywords: binaries: spectroscopic; stars: early-type; stars: formation; stars: individual: RMC 144; stars: Wolf-Rayet; Astrophysics – Solar and Stellar Astrophysics