@article{RamiaramanantsoaRatnasingamShenaretal.2018,
  author    = {Ramiaramanantsoa, Tahina and Ratnasingam, Rathish and Shenar, Tomer and Moffat, Anthony F. J. and Rogers, Tamara M. and Popowicz, Adam and Kuschnig, Rainer and Pigulski, Andrzej and Handler, Gerald and Wade, Gregg A. and Zwintz, Konstanze and Weiss, Werner W.},
  title     = {A BRITE view on the massive O-type supergiant V973 Scorpii},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {480},
  journal   = {Monthly notices of the Royal Astronomical Society},
  number    = {1},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnras/sty1897},
  pages     = {972 -- 986},
  year      = {2018},
  abstract  = {Stochastically triggered photospheric light variations reaching similar to 40 mmag peak-to-valley amplitudes have been detected in the O8 Iaf supergiant V973 Scorpii as the outcome of 2 months of high-precision time-resolved photometric observations with the BRIght Target Explorer (BRITE) nanosatellites. The amplitude spectrum of the time series photometry exhibits a pronounced broad bump in the low-frequency regime (less than or similar to 0.9 d(-1)) where several prominent frequencies are detected. A time-frequency analysis of the observations reveals typical mode lifetimes of the order of 5-10 d. The overall features of the observed brightness amplitude spectrum of V973 Sco match well with those extrapolated from two-dimensional hydrodynamical simulations of convectively driven internal gravity waves randomly excited from deep in the convective cores of massive stars. An alternative or additional possible source of excitation from a sub-surface convection zone needs to be explored in future theoretical investigations.},
  language  = {en}
}
@article{RamiaramanantsoaMoffatHarmonetal.2018,
  author    = {Ramiaramanantsoa, Tahina and Moffat, Anthony F. J. and Harmon, Robert and Ignace, R. and St-Louis, Nicole and Vanbeveren, Dany and Shenar, Tomer and Pablo, Herbert and Richardson, Noel D. and Howarth, Ian D. and Stevens, Ian R. and Piaulet, Caroline and St-Jean, Lucas and Eversberg, Thomas and Pigulski, Andrzej and Popowicz, Adam and Kuschnig, Rainer and Zoclonska, Elzbieta and Buysschaert, Bram and Handler, Gerald and Weiss, Werner W. and Wade, Gregg A. and Rucinski, Slavek M. and Zwintz, Konstanze and Luckas, Paul and Heathcote, Bernard and Cacella, Paulo and Powles, Jonathan and Locke, Malcolm and Bohlsen, Terry and Chen{\´e}, Andr{\´e}-Nicolas and Miszalski, Brent and Waldron, Wayne L. and Kotze, Marissa M. and Kotze, Enrico J. and B{\"o}hm, Torsten},
  title     = {BRITE-Constellation high-precision time-dependent photometry of the early O-type supergiant zeta Puppis unveils the photospheric drivers of its small- and large-scale wind structures},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {473},
  journal   = {Monthly notices of the Royal Astronomical Society},
  number    = {4},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnras/stx2671},
  pages     = {5532 -- 5569},
  year      = {2018},
  abstract  = {From 5.5 months of dual-band optical photometric monitoring at the 1 mmag level, BRITE-Constellation has revealed two simultaneous types of variability in the O4I(n)fp star ζ Puppis: one single periodic non-sinusoidal component superimposed on a stochastic component. The monoperiodic component is the 1.78-d signal previously detected by Coriolis/Solar Mass Ejection Imager, but this time along with a prominent first harmonic. The shape of this signal changes over time, a behaviour that is incompatible with stellar oscillations but consistent with rotational modulation arising from evolving bright surface inhomogeneities. By means of a constrained non-linear light-curve inversion algorithm, we mapped the locations of the bright surface spots and traced their evolution. Our simultaneous ground-based multisite spectroscopic monitoring of the star unveiled cyclical modulation of its He ii λ4686 wind emission line with the 1.78-d rotation period, showing signatures of corotating interaction regions that turn out to be driven by the bright photospheric spots observed by BRITE. Traces of wind clumps are also observed in the He ii λ4686 line and are correlated with the amplitudes of the stochastic component of the light variations probed by BRITE at the photosphere, suggesting that the BRITE observations additionally unveiled the photospheric drivers of wind clumps in ζ Pup and that the clumping phenomenon starts at the very base of the wind. The origins of both the bright surface inhomogeneities and the stochastic light variations remain unknown, but a subsurface convective zone might play an important role in the generation of these two types of photospheric variability.},
  language  = {en}
}
@phdthesis{Shenar2017,
  author    = {Shenar, Tomer},
  title     = {Comprehensive analyses of massive binaries and implications on stellar evolution},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-104857},
  school      = {Universit{\"a}t Potsdam},
  pages     = {187},
  year      = {2017},
  abstract  = {Via their powerful radiation, stellar winds, and supernova explosions, massive stars (Mini \& 8 M☉) bear a tremendous impact on galactic evolution. It became clear in recent decades that the majority of massive stars reside in binary systems. This thesis sets as a goal to quantify the impact of binarity (i.e., the presence of a companion star) on massive stars. For this purpose, massive binary systems in the Local Group, including OB-type binaries, high mass X-ray binaries (HMXBs), and Wolf-Rayet (WR) binaries, were investigated by means of spectral, orbital, and evolutionary analyses. The spectral analyses were performed with the non-local thermodynamic equillibrium (non-LTE) Potsdam Wolf-Rayet (PoWR) model atmosphere code. Thanks to critical updates in the calculation of the hydrostatic layers, the code became a state-of-the-art tool applicable for all types of hot massive stars (Chapter 2). The eclipsing OB-type triple system δ Ori served as an intriguing test-case for the new version of the PoWR code, and provided key insights regarding the formation of X-rays in massive stars (Chapter 3). We further analyzed two prototypical HMXBs, Vela X-1 and IGR J17544-2619, and obtained fundamental conclusions regarding the dichotomy of two basic classes of HMXBs (Chapter 4). We performed an exhaustive analysis of the binary R 145 in the Large Magellanic Cloud (LMC), which was claimed to host the most massive stars known. We were able to disentangle the spectrum of the system, and performed an orbital, polarimetric, and spectral analysis, as well as an analysis of the wind-wind collision region. The true masses of the binary components turned out to be significantly lower than suggested, impacting our understanding of the initial mass function and stellar evolution at low metallicity (Chapter 5). Finally, all known WR binaries in the Small Magellanic Cloud (SMC) were analyzed. Although it was theoretical predicted that virtually all WR stars in the SMC should be formed via mass-transfer in binaries, we find that binarity was not important for the formation of the known WR stars in the SMC, implying a strong discrepancy between theory and observations (Chapter 6).},
  language  = {en}
}
@article{AlmeidaSanaTayloretal.2017,
  author    = {Almeida, Leonardo A. and Sana, H. and Taylor, W. and Barb{\´a}, Rodolfo and Bonanos, Alceste Z. and Crowther, Paul and Damineli, Augusto and de Koter, A. and de Mink, Selma E. and Evans, C. J. and Gieles, Mark and Grin, Nathan J. and H{\´e}nault-Brunet, V. and Langer, Norbert and Lennon, D. and Lockwood, Sean and Ma{\´i}z Apell{\´a}niz, Jes{\´u}s and Moffat, A. F. J. and Neijssel, C. and Norman, C. and Ram{\´i}rez-Agudelo, O. H. and Richardson, N. D. and Schootemeijer, Abel and Shenar, Tomer and Soszyński, Igor and Tramper, Frank and Vink, J. S.},
  title     = {The tarantula massive binary monitoring},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {598},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201629844},
  pages     = {36},
  year      = {2017},
  abstract  = {Context: Massive binaries play a crucial role in the Universe. Knowing the distributions of their orbital parameters is important for a wide range of topics from stellar feedback to binary evolution channels and from the distribution of supernova types to gravitational wave progenitors, yet no direct measurements exist outside the Milky Way. Aims: The Tarantula Massive Binary Monitoring project was designed to help fill this gap by obtaining multi-epoch radial velocity (RV) monitoring of 102 massive binaries in the 30 Doradus region. Methods: In this paper we analyze 32 FLAMES/GIRAFFE observations of 93 O- and 7 B-type binaries. We performed a Fourier analysis and obtained orbital solutions for 82 systems: 51 single-lined (SB1) and 31 double-lined (SB2) spectroscopic binaries. Results: Overall, the binary fraction and orbital properties across the 30 Doradus region are found to be similar to existing Galactic samples. This indicates that within these domains environmental effects are of second order in shaping the properties of massive binary systems. A small difference is found in the distribution of orbital periods, which is slightly flatter (in log space) in 30 Doradus than in the Galaxy, although this may be compatible within error estimates and differences in the fitting methodology. Also, orbital periods in 30 Doradus can be as short as 1.1 d, somewhat shorter than seen in Galactic samples. Equal mass binaries (q> 0.95) in 30 Doradus are all found outside NGC 2070, the central association that surrounds R136a, the very young and massive cluster at 30 Doradus's core. Most of the differences, albeit small, are compatible with expectations from binary evolution. One outstanding exception, however, is the fact that earlier spectral types (O2-O7) tend to have shorter orbital periods than later spectral types (O9.2-O9.7). Conclusions: Our results point to a relative universality of the incidence rate of massive binaries and their orbital properties in the metallicity range from solar (Z⊙) to about half solar. This provides the first direct constraints on massive binary properties in massive star-forming galaxies at the Universe's peak of star formation at redshifts z ~ 1 to 2 which are estimated to have Z ~ 0.5 Z⊙.},
  language  = {en}
}
@article{AldorettaStLouisRichardsonetal.2016,
  author    = {Aldoretta, E. J. and St-Louis, N. and Richardson, N. D. and Moffat, Anthony F. J. and Eversberg, T. and Hill, G. M. and Shenar, Tomer and Artigau, E. and Gauza, B. and Knapen, J. H. and Kubat, Jiř{\´i} and Kubatova, Brankica and Maltais-Tariant, R. and Munoz, M. and Pablo, H. and Ramiaramanantsoa, T. and Richard-Laferriere, A. and Sablowski, D. P. and Simon-Diaz, S. and St-Jean, L. and Bolduan, F. and Dias, F. M. and Dubreuil, P. and Fuchs, D. and Garrel, T. and Grutzeck, G. and Hunger, T. and Kuesters, D. and Langenbrink, M. and Leadbeater, R. and Li, D. and Lopez, A. and Mauclaire, B. and Moldenhawer, T. and Potter, M. and dos Santos, E. M. and Schanne, L. and Schmidt, J. and Sieske, H. and Strachan, J. and Stinner, E. and Stinner, P. and Stober, B. and Strandbaek, K. and Syder, T. and Verilhac, D. and Waldschlaeger, U. and Weiss, D. and Wendt, A.},
  title     = {An extensive spectroscopic time series of three Wolf-Rayet stars - I. The lifetime of large-scale structures in the wind of WR 134},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {460},
  journal   = {Monthly notices of the Royal Astronomical Society},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnras/stw1188},
  pages     = {3407 -- 3417},
  year      = {2016},
  abstract  = {During the summer of 2013, a 4-month spectroscopic campaign took place to observe the variabilities in three Wolf-Rayet stars. The spectroscopic data have been analysed for WR 134 (WN6b), to better understand its behaviour and long-term periodicity, which we interpret as arising from corotating interaction regions (CIRs) in the wind. By analysing the variability of the He ii lambda 5411 emission line, the previously identified period was refined to P = 2.255 +/- 0.008 (s.d.) d. The coherency time of the variability, which we associate with the lifetime of the CIRs in the wind, was deduced to be 40 +/- 6 d, or similar to 18 cycles, by cross-correlating the variability patterns as a function of time. When comparing the phased observational grey-scale difference images with theoretical grey-scales previously calculated from models including CIRs in an optically thin stellar wind, we find that two CIRs were likely present. A separation in longitude of Delta I center dot a parts per thousand integral 90A degrees was determined between the two CIRs and we suggest that the different maximum velocities that they reach indicate that they emerge from different latitudes. We have also been able to detect observational signatures of the CIRs in other spectral lines (C iv lambda lambda 5802,5812 and He i lambda 5876). Furthermore, a DAC was found to be present simultaneously with the CIR signatures detected in the He i lambda 5876 emission line which is consistent with the proposed geometry of the large-scale structures in the wind. Small-scale structures also show a presence in the wind, simultaneously with the larger scale structures, showing that they do in fact co-exist.},
  language  = {en}
}
@article{CorcoranNicholsPabloetal.2015,
  author    = {Corcoran, Michael F. and Nichols, Joy S. and Pablo, Herbert and Shenar, Tomer and Pollock, Andy M. T. and Waldron, Wayne L. and Moffat, Anthony F. J. and Richardson, Noel D. and Russell, Christopher M. P. and Hamaguchi, Kenji and Huenemoerder, David P. and Oskinova, Lida and Hamann, Wolf-Rainer and Naze, Yael and Ignace, Richard and Evans, Nancy Remage and Lomax, Jamie R. and Hoffman, Jennifer L. and Gayley, Kenneth and Owocki, Stanley P. and Leutenegger, Maurice and Gull, Theodore R. and Hole, Karen Tabetha and Lauer, Jennifer and Iping, Rosina C.},
  title     = {A coordinated X-Ray and optical campaign of the nearest massive eclipsing binary, delta ORIONIS Aa. I. Overview of thr X-Ray spectrum},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {809},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  number    = {2},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0004-637X},
  doi       = {10.1088/0004-637X/809/2/132},
  pages     = {15},
  year      = {2015},
  abstract  = {We present an overview of four deep phase-constrained Chandra HETGS X-ray observations of delta Ori A. Delta Ori A is actually a triple system that includes the nearest massive eclipsing spectroscopic binary, delta Ori Aa, the only such object that can be observed with little phase-smearing with the Chandra gratings. Since the fainter star, delta Ori Aa2, has a much lower X-ray luminosity than the brighter primary (delta Ori Aa1), delta Ori Aa provides a unique system with which to test the spatial distribution of the X-ray emitting gas around delta Ori Aa1 via occultation by the photosphere of, and wind cavity around, the X-ray dark secondary. Here we discuss the X-ray spectrum and X-ray line profiles for the combined observation, having an exposure time of nearly 500 ks and covering nearly the entire binary orbit. The companion papers discuss the X-ray variability seen in the Chandra spectra, present new space-based photometry and ground-based radial velocities obtained simultaneously with the X-ray data to better constrain the system parameters, and model the effects of X-rays on the optical and UV spectra. We find that the X-ray emission is dominated by embedded wind shock emission from star Aa1, with little contribution from the tertiary star Ab or the shocked gas produced by the collision of the wind of Aa1 against the surface of Aa2. We find a similar temperature distribution to previous X-ray spectrum analyses. We also show that the line half-widths are about 0.3-0.5 times the terminal velocity of the wind of star Aa1. We find a strong anti-correlation between line widths and the line excitation energy, which suggests that longer-wavelength, lower-temperature lines form farther out in the wind. Our analysis also indicates that the ratio of the intensities of the strong and weak lines of Fe XVII and Ne X are inconsistent with model predictions, which may be an effect of resonance scattering.},
  language  = {en}
}
@article{GvaramadzeCheneKniazevetal.2014,
  author    = {Gvaramadze, V. V. and Chene, A.-N. and Kniazev, A. Y. and Schnurr, O. and Shenar, Tomer and Sander, Andreas Alexander Christoph and Hainich, Rainer and Langer, N. and Hamann, Wolf-Rainer and Chu, Y.-H. and Gruendl, R. A.},
  title     = {Discovery of a new Wolf-Rayet star and a candidate star cluster in the Large Magellanic Cloud with Spitzer},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {442},
  journal   = {Monthly notices of the Royal Astronomical Society},
  number    = {2},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnras/stu909},
  pages     = {929 -- 945},
  year      = {2014},
  abstract  = {We report the first-ever discovery of a Wolf-Rayet (WR) star in the Large Magellanic Cloud via detection of a circular shell with the Spitzer Space Telescope. Follow-up observations with Gemini-South resolved the central star of the shell into two components separated from each other by a parts per thousand 2 arcsec (or a parts per thousand 0.5 pc in projection). One of these components turns out to be a WN3 star with H and He lines both in emission and absorption (we named it BAT99 3a using the numbering system based on extending the Breysacher et al. catalogue). Spectroscopy of the second component showed that it is a B0 V star. Subsequent spectroscopic observations of BAT99 3a with the du Pont 2.5-m telescope and the Southern African Large Telescope revealed that it is a close, eccentric binary system, and that the absorption lines are associated with an O companion star. We analysed the spectrum of the binary system using the non-LTE Potsdam WR (powr) code, confirming that the WR component is a very hot (a parts per thousand 90 kK) WN star. For this star, we derived a luminosity of log L/ L-aS (TM) = 5.45 and a mass-loss rate of 10(- 5.8) M-aS (TM) yr(- 1), and found that the stellar wind composition is dominated by helium with 20 per cent of hydrogen. Spectroscopy of the shell revealed an He iii region centred on BAT99 3a and having the same angular radius (a parts per thousand 15 arcsec) as the shell. We thereby add a new example to a rare class of high-excitation nebulae photoionized by WR stars. Analysis of the nebular spectrum showed that the shell is composed of unprocessed material, implying that the shell was swept-up from the local interstellar medium. We discuss the physical relationship between the newly identified massive stars and their possible membership of a previously unrecognized star cluster.},
  language  = {en}
}
@article{RichardsonShenarRoyLoubieretal.2016,
  author    = {Richardson, Noel D. and Shenar, Tomer and Roy-Loubier, Olivier and Schaefer, Gail and Moffat, Anthony F. J. and St-Louis, Nicole and Gies, Douglas R. and Farrington, Chris and Hill, Grant M. and Williams, Peredur M. and Gordon, Kathryn and Pablo, Herbert and Ramiaramanantsoa, Tahina},
  title     = {The CHARA Array resolves the long-period Wolf-Rayet binaries WR 137 and WR 138},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {461},
  journal   = {Monthly notices of the Royal Astronomical Society},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnras/stw1585},
  pages     = {4115 -- 4124},
  year      = {2016},
  abstract  = {We report on interferometric observations with the CHARAArray of two classical Wolf-Rayet (WR) stars in suspected binary systems, namely WR 137 and WR 138. In both cases, we resolve the component stars to be separated by a few milliarcseconds. The data were collected in the H band, and provide a measure of the fractional flux for both stars in each system. We find that the WR star is the dominant H-band light source in both systems (fWR, 137 = 0.59 +/- 0.04; fWR, 138 = 0.67 +/- 0.01), which is confirmed through both comparisons with estimated fundamental parameters for WR stars and O dwarfs, as well as through spectral modelling of each system. Our spectral modelling also provides fundamental parameters for the stars and winds in these systems. The results on WR 138 provide evidence that it is a binary system which may have gone through a previous mass-transfer episode to create the WR star. The separation and position of the stars in the WR 137 system together with previous results from the IOTA interferometer provides evidence that the binary is seen nearly edgeon. The possible edge-on orbit of WR 137 aligns well with the dust production site imaged by the Hubble Space Telescope during a previous periastron passage, showing that the dust production may be concentrated in the orbital plane.},
  language  = {en}
}
@article{JaervinenHubrigIlyinetal.2017,
  author    = {Jaervinen, S. P. and Hubrig, Swetlana and Ilyin, Ilya and Shenar, Tomer and Schoeller, M.},
  title     = {A search for spectral variability in the highly magnetized O9.7 V star HD 54879},
  series = {Astronomische Nachrichten = Astronomical notes},
  volume    = {338},
  journal   = {Astronomische Nachrichten = Astronomical notes},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {0004-6337},
  doi       = {10.1002/asna.201713402},
  pages     = {952 -- 958},
  year      = {2017},
  abstract  = {The O9.7 V star HD 54879 possesses the second strongest magnetic field among the single, magnetic, O-type stars. In contrast to other magnetic O-type stars, the chemical abundance analysis of HD 54879 indicated a rather normal optical spectrum without obvious element enhancements or depletions. Furthermore, spectral variability was detected only in lines partly formed in the magnetosphere. As this star shows such a deviate, almost nonvariable, spectral behavior, we performed a deeper analysis of its spectral variability on different timescales using all currently available HARPSpol and FORS 2 spectropolarimetric observations. The longitudinal magnetic field strengths measured at different epochs indicate the presence of variability possibly related to stellar rotation, but the current data do not allow us yet to identify the periodicity of the field variation. As spectropolarimetric observations obtained at different epochs consist of subexposures with different integration times, we investigated spectral variability on timescales of minutes. The detected level of variability in line profiles of different elements is rather low, between 0.2 and 1.7\%, depending on the integration time of the exposures and the considered element.},
  language  = {en}
}
@article{MunozMoffatHilletal.2017,
  author    = {Munoz, Melissa and Moffat, Anthony F. J. and Hill, Grant M. and Shenar, Tomer and Richardson, Noel D. and Pablo, Herbert and St-Louis, Nicole and Ramiaramanantsoa, Tahina},
  title     = {WR 148: identifying the companion of an extreme runaway massive binary},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {467},
  journal   = {Monthly notices of the Royal Astronomical Society},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnras/stw2283},
  pages     = {3105 -- 3121},
  year      = {2017},
  abstract  = {WR 148 (HD 197406) is an extreme runaway system considered to be a potential candidate for a short-period (4.3173 d) rare WR + compact object binary. Provided with new high-resolution, high signal-to-noise spectra from the Keck observatory, we determine the orbital parameters for both the primary WR and the secondary, yielding respective projected orbital velocity amplitudes of 88.1 ± 3.8\&\#8201;km\&\#8201;s\&\#8722;1 and 79.2 ± 3.1\&\#8201;km\&\#8201;s\&\#8722;1 and implying a mass ratio of 1.1 ± 0.1. We then apply the shift-and-add technique to disentangle the spectra and obtain spectra compatible with a WN7ha and an O4-6 star. Considering an orbital inclination of \&\#8764;67°, derived from previous polarimetry observations, the system's total mass would be a mere 2-3M\&\#8857;\&\#8288;, an unprecedented result for a putative massive binary system. However, a system comprising a 37M\&\#8857; secondary (typical mass of an O5V star) and a 33M\&\#8857; primary (given the mass ratio) would infer an inclination of \&\#8764;18°. We therefore reconsider the previous methods of deriving the orbital inclination based on time-dependent polarimetry and photometry. While the polarimetric results are inconclusive requiring better data, the photometric results favour low inclinations. Finally, we compute WR 148's space velocity and retrace the runaway's trajectory back to the Galactic plane (GP). With an ejection velocity of 198 ± 27\&\#8201;km\&\#8201;s\&\#8722;1 and a travel time of 4.7 ± 0.8 Myr to reach its current location, WR 148 was most likely ejected via dynamical interactions in a young cluster.},
  language  = {en}
}
@article{RichardsonRussellStJeanetal.2017,
  author    = {Richardson, Noel D. and Russell, Christopher M. P. and St-Jean, Lucas and Moffat, Anthony F. J. and St-Louis, Nicole and Shenar, Tomer and Pablo, Herbert and Hill, Grant M. and Ramiaramanantsoa, Tahina and Corcoran, Michael and Hamuguchi, Kenji and Eversberg, Thomas and Miszalski, Brent and Chene, Andre-Nicolas and Waldron, Wayne and Kotze, Enrico J. and Kotze, Marissa M. and Luckas, Paul and Cacella, Paulo and Heathcote, Bernard and Powles, Jonathan and Bohlsen, Terry and Locke, Malcolm and Handler, Gerald and Kuschnig, Rainer and Pigulski, Andrzej and Popowicz, Adam and Wade, Gregg A. and Weiss, Werner W.},
  title     = {The variability of the BRITE-est Wolf-Rayet binary, gamma(2) Velorum-I. Photometric and spectroscopic evidence for colliding winds},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {471},
  journal   = {Monthly notices of the Royal Astronomical Society},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnras/stx1731},
  pages     = {2715 -- 2729},
  year      = {2017},
  abstract  = {We report on the first multi-colour precision light curve of the bright Wolf-Rayet binary gamma(2) Velorum, obtained over six months with the nanosatellites in the BRITE-Constellation fleet. In parallel, we obtained 488 high-resolution optical spectra of the system. In this first report on the data sets, we revise the spectroscopic orbit and report on the bulk properties of the colliding winds. We find a dependence of both the light curve and excess emission properties that scales with the inverse of the binary separation. When analysing the spectroscopic properties in combination with the photometry, we find that the phase dependence is caused only by excess emission in the lines, and not from a changing continuum. We also detect a narrow, high-velocity absorption component from the He perpendicular to lambda 5876 transition, which appears twice in the orbit. We calculate smoothed-particle hydrodynamical simulations of the colliding winds and can accurately associate the absorption from He perpendicular to to the leading and trailing arms of the wind shock cone passing tangentially through our line of sight. The simulations also explain the general strength and kinematics of the emission excess observed in wind lines such as C III lambda 5696 of the system. These results represent the first in a series of investigations into the winds and properties of gamma(2) Velorum through multi-technique and multi-wavelength observational campaigns.},
  language  = {en}
}
@article{KubatovaSzecsiSanderetal.2019,
  author    = {Kubatova, Brankica and Szecsi, D. and Sander, Andreas Alexander Christoph and Kubat, Jiř{\´i} and Tramper, F. and Krticka, Jiri and Kehrig, C. and Hamann, Wolf-Rainer and Hainich, Rainer and Shenar, Tomer},
  title     = {Low-metallicity massive single stars with rotation},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {623},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201834360},
  pages     = {32},
  year      = {2019},
  abstract  = {Context. Metal-poor massive stars are assumed to be progenitors of certain supernovae, gamma-ray bursts, and compact object mergers that might contribute to the early epochs of the Universe with their strong ionizing radiation. However, this assumption remains mainly theoretical because individual spectroscopic observations of such objects have rarely been carried out below the metallicity of the Small Magellanic Cloud. Aims. Here we explore the predictions of the state-of-the-art theories of stellar evolution combined with those of stellar atmospheres about a certain type of metal-poor (0.02 Z(circle dot)) hot massive stars, the chemically homogeneously evolving stars that we call Transparent Wind Ultraviolet INtense (TWUIN) stars. Methods. We computed synthetic spectra corresponding to a broad range in masses (20 130 M-circle dot) and covering several evolutionary phases from the zero-age main-sequence up to the core helium-burning stage. We investigated the influence of mass loss and wind clumping on spectral appearance and classified the spectra according to the Morgan-Keenan (MK) system. Results. We find that TWUIN stars show almost no emission lines during most of their core hydrogen-burning lifetimes. Most metal lines are completely absent, including nitrogen. During their core helium-burning stage, lines switch to emission, and even some metal lines (oxygen and carbon, but still almost no nitrogen) are detected. Mass loss and clumping play a significant role in line formation in later evolutionary phases, particularly during core helium-burning. Most of our spectra are classified as an early-O type giant or supergiant, and we find Wolf-Rayet stars of type WO in the core helium-burning phase. Conclusions. An extremely hot, early-O type star observed in a low-metallicity galaxy could be the result of chemically homogeneous evolution and might therefore be the progenitor of a long-duration gamma-ray burst or a type Ic supernova. TWUIN stars may play an important role in reionizing the Universe because they are hot without showing prominent emission lines during most of their lifetime.},
  language  = {en}
}
@article{ShenarHamannTodt2015,
  author    = {Shenar, Tomer and Hamann, Wolf-Rainer and Todt, Helge Tobias},
  title     = {The impact of rotation on the line profiles of Wolf-Rayet stars},
  series = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015},
  journal   = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-88008},
  pages     = {193 -- 196},
  year      = {2015},
  abstract  = {The distribution of angular momentum in massive stars is a critical component of their evolution, yet not much is known on the rotation velocities of Wolf-Rayet stars. There are various indications that rapidly rotating Wolf-Rayet stars should exist. Unfortunately, due to their expanding atmospheres, rotational velocities of Wolf-Rayet stars are very difficult to measure. In this work, we model the effects of rotation on the atmospheres of Wolf-Rayet stars by implementing a 3D integration scheme in the PoWR code. We further investigate whether the peculiar spectra of five Wolf-Rayet stars may imply rapid rotation, infer the corresponding rotation parameters, and discuss the implications of our results. We find that rotation helps to reproduce the unique spectra analyzed here. However, if rotation is indeed involved, the inferred rotational velocities at the stellar surface are large (∼ 200 km/s), and the implied co-rotation radii (∼ 10R∗) suggest the existence of very strong photospheric magnetic fields (∼ 20 kG).},
  language  = {en}
}
@article{SanderHamannHainichetal.2015,
  author    = {Sander, A. and Hamann, Wolf-Rainer and Hainich, Rainer and Shenar, Tomer and Todt, Helge Tobias},
  title     = {Hydrodynamic modeling of massive star atmospheres},
  series = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015},
  journal   = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-87857},
  pages     = {139 -- 142},
  year      = {2015},
  abstract  = {In the last decades, stellar atmosphere codes have become a key tool in understanding massive stars, including precise calculations of stellar and wind parameters, such as temperature, massloss rate, and terminal wind velocity. Nevertheless, for these models the hydrodynamic equation is not solved in the wind. Motivated by the results of the CAK theory, the models typically use a beta velocity law, which however turns out not to be adequate for stars with very strong winds, and treat the mass-loss rate as a free parameter. In a new branch of the Potsdam Wolf-Rayet model atmosphere (PoWR) code, we solve the hydrodynamic equation consistently throughout the stellar atmosphere. The PoWR code performs the calculation of the radiative force without approximations (e.g. Sobolev). We show the impact of hydrodynamically consistent modelling on OB and WR stars in comparison to conventional models and discuss the obtained velocity fields and their impact on the observed spectral lines.},
  language  = {en}
}
@article{KubatovaHamannTodtetal.2015,
  author    = {Kub{\´a}tov{\´a}, Brankica and Hamann, Wolf-Rainer and Todt, Helge Tobias and Sander, A. and Steinke, M. and Hainich, Rainer and Shenar, Tomer},
  title     = {Macroclumping in WR 136},
  series = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015},
  journal   = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-87823},
  pages     = {125 -- 128},
  year      = {2015},
  abstract  = {Macroclumping proved to resolve the discordance between different mass-loss rate diagnostics for O-type stars, in particular between Hα and the P v resonance lines. In this paper, we report first results from a corresponding investigation for WR stars. We apply our detailed 3-D Monte Carlo (MC) line formation code to the P v resonance doublet and show, for the Galactic WNL star WR136, that macroclumping is require to bring this line in accordance with the mass-loss rate derived from the emission-line spectrum.},
  language  = {en}
}
@article{HainichPasemannTodtetal.2015,
  author    = {Hainich, Rainer and Pasemann, Diana and Todt, Helge Tobias and Shenar, Tomer and Sander, Andreas Alexander Christoph and Hamann, Wolf-Rainer},
  title     = {Wolf-Rayet stars in the Small Magellanic Cloud I. Analysis of the single WN stars},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {581},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201526241},
  pages     = {30},
  year      = {2015},
  abstract  = {Context. Wolf-Rayet (WR) stars have a severe impact on their environments owing to their strong ionizing radiation fields and powerful stellar winds. Since these winds are considered to be driven by radiation pressure, it is theoretically expected that the degree of the wind mass-loss depends on the initial metallicity of WR stars. Aims. Following our comprehensive studies of WR stars in the Milky Way, M31, and the LMC, we derive stellar parameters and mass-loss rates for all seven putatively single WN stars known in the SMC. Based on these data, we discuss the impact of a low-metallicity environment on the mass loss and evolution of WR stars. Methods. The quantitative analysis of the WN stars is performed with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The physical properties of our program stars are obtained from fitting synthetic spectra to multi-band observations. Results. In all SMC WN stars, a considerable surface hydrogen abundance is detectable. The majority of these objects have stellar temperatures exceeding 75 kK, while their luminosities range from 10(5.5) to 10(6.1) L-circle dot. The WN stars in the SMC exhibit on average lower mass-loss rates and weaker winds than their counterparts in the Milky Way, M31, and the LMC. Conclusions. By comparing the mass-loss rates derived for WN stars in different Local Group galaxies, we conclude that a clear dependence of the wind mass-loss on the initial metallicity is evident, supporting the current paradigm that WR winds are driven by radiation. A metallicity effect on the evolution of massive stars is obvious from the HRD positions of the SMC WN stars at high temperatures and high luminosities. Standard evolution tracks are not able to reproduce these parameters and the observed surface hydrogen abundances. Homogeneous evolution might provide a better explanation for their evolutionary past.},
  language  = {en}
}
@article{ShenarHamannTodt2014,
  author    = {Shenar, Tomer and Hamann, Wolf-Rainer and Todt, Helge Tobias},
  title     = {The impact of rotation on the line profiles of Wolf-Rayet stars},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {562},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/201322496},
  pages     = {13},
  year      = {2014},
  abstract  = {Context. Massive Wolf-Rayet stars are recognized today to be in a very common, but short, evolutionary phase of massive stars. While our understanding of Wolf-Rayet stars has increased dramatically over the past decades, it remains unclear whether rapid rotators are among them. There are various indications that rapidly rotating Wolf-Rayet stars should exist. Unfortunately, due to their expanding atmospheres, rotational velocities of Wolf-Rayet stars are very difficult to measure. However, recently observed spectra of several Wolf-Rayet stars reveal peculiarly broad and round emission lines. Could these spectra imply rapid rotation? Aims. In this work, we model the effects of rotation on the atmospheres of Wolf-Rayet stars. We further investigate whether the peculiar spectra of five Wolf-Rayet stars may be explained with the help of stellar rotation, infer appropriate rotation parameters, and discuss the implications of our results. Methods. We make use of the Potsdam Wolf-Rayet (PoWR) non-LTE model atmosphere code. Since the observed spectra of WolfRayet stars are mainly formed in their expanding atmospheres, rotation must be accounted for with a 3D integration scheme of the formal integral. For this purpose, we assume a rotational velocity field consisting of an inner co-rotating domain and an outer domain, where the angular momentum is conserved. Results. We find that rotation can reproduce the unique spectra analyzed here. However, the inferred rotational velocities at the stellar surface are large (similar to 200 km s(-1)), and the inferred co-rotation radii (similar to 10R.) suggest the existence of very strong photospheric magnetic fields (similar to 20 kG).},
  language  = {en}
}
@article{GrunerHainichSanderetal.2018,
  author    = {Gruner, David and Hainich, Rainer and Sander, Andreas Alexander Christoph and Shenar, Tomer and Todt, Helge Tobias and Oskinova, Lida and Ramachandran, Varsha and Ayres, T. and Hamann, Wolf-Rainer},
  title     = {The extreme O-type spectroscopic binary HD 93129A A quantitative, multiwavelength analysis},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {621},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201833178},
  pages     = {16},
  year      = {2018},
  abstract  = {Context. HD 93129A was classified as the earliest O-type star in the Galaxy (O2 If*) and is considered as the prototype of its spectral class. However, interferometry shows that this object is a binary system, while recent observations even suggest a triple configuration. None of the previous spectral analyses of this object accounted for its multiplicity. With new high-resolution UV and optical spectra, we have the possibility to reanalyze this key object, taking its binary nature into account for the first time. Aims. We aim to derive the fundamental parameters and the evolutionary status of HD 93129A, identifying the contributions of both components to the composite spectrum Results. Despite the similar spectral types of the two components, we are able to find signatures from each of the components in the combined spectrum, which allows us to estimate the parameters of both stars. We derive log(L/L-circle dot) = 6.15, T-eff = 52 kK, and log (M)over dot = -4.7[M-circle dot yr(-1)] for the primary Aa, and log(L/L-circle dot) = 5.58, T-eff = 45 kK, and log (M)over dot = -5.8 [M(circle dot)yr(-1)] for the secondary Ab. Conclusions. Even when accounting for the binary nature, the primary of HD 93129A is found to be one of the hottest and most luminous O stars in our Galaxy. Based on the theoretical decomposition of the spectra, we assign spectral types O2 If* and O3 III(f*) to components Aa and Ab, respectively. While we achieve a good fit for a wide spectral range, specific spectral features are not fully reproduced. The data are not sufficient to identify contributions from a hypothetical third component in the system.},
  language  = {en}
}
@article{RamachandranHamannHainichetal.2018,
  author    = {Ramachandran, Varsha and Hamann, Wolf-Rainer and Hainich, Rainer and Oskinova, Lida and Shenar, Tomer and Sander, Andreas Alexander Christoph and Todt, Helge Tobias and Gallagher, John S.},
  title     = {Stellar population of the superbubble N206 in the LMC II. Parameters of the OB and WR stars, and the total massive star feedback},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {615},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201832816},
  pages     = {72},
  year      = {2018},
  abstract  = {Context. Clusters or associations of early-type stars are often associated with a "superbubble" of hot gas. The formation of such superbubbles is caused by the feedback from massive stars. The complex N206 in the Large Magellanic Cloud (LMC) exhibits a superbubble and a rich massive star population. Aims. Our goal is to perform quantitative spectral analyses of all massive stars associated with the N206 superbubble in order to determine their stellar and wind parameters. We compare the superbubble energy budget to the stellar energy input and discuss the star formation history of the region. Results. We present the stellar and wind parameters of the OB stars and the two Wolf-Rayet (WR) binaries in the N206 complex. Twelve percent of the sample show Oe/Be type emission lines, although most of them appear to rotate far below critical. We found eight runaway stars based on their radial velocity. The wind-momentum luminosity relation of our OB sample is consistent with the expectations. The Hertzsprung-Russell diagram (HRD) of the OB stars reveals a large age spread (1-30 Myr), suggesting different episodes of star formation in the complex. The youngest stars are concentrated in the inner part of the complex, while the older OB stars are scattered over outer regions. We derived the present day mass function for the entire N206 complex as well as for the cluster NGC2018. The total ionizing photon flux produced by all massive stars in the N206 complex is Q(0) approximate to 5 x 10(50) s(-1), and the mechanical luminosity of their stellar winds amounts to L-mec = 1.7 x 10(38) erg s(-1). Three very massive Of stars are found to dominate the feedback among 164 OB stars in the sample. The two WR winds alone release about as much mechanical luminosity as the whole OB star sample. The cumulative mechanical feedback from all massive stellar winds is comparable to the combined mechanical energy of the supernova explosions that likely occurred in the complex. Accounting also for the WR wind and supernovae, the mechanical input over the last five Myr is approximate to 2.3 x 10(52) erg. Conclusions. The N206 complex in the LMC has undergone star formation episodes since more than 30 Myr ago. From the spectral analyses of its massive star population, we derive a current star formation rate of 2.2 x 10(-3) M-circle dot yr(-1). From the combined input of mechanical energy from all stellar winds, only a minor fraction is emitted in the form of X-rays. The corresponding input accumulated over a long time also exceeds the current energy content of the complex by more than a factor of five. The morphology of the complex suggests a leakage of hot gas from the superbubble.},
  language  = {en}
}
@article{SanderHamannTodtetal.2017,
  author    = {Sander, Andreas Alexander Christoph and Hamann, Wolf-Rainer and Todt, Helge Tobias and Hainich, Rainer and Shenar, Tomer},
  title     = {Coupling hydrodynamics with comoving frame radiative transfer I. A unified approach for OB and WR stars},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {603},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201730642},
  pages     = {14},
  year      = {2017},
  abstract  = {Context. For more than two decades, stellar atmosphere codes have been used to derive the stellar and wind parameters of massive stars. Although they have become a powerful tool and sufficiently reproduce the observed spectral appearance, they can hardly be used for more than measuring parameters. One major obstacle is their inconsistency between the calculated radiation field and the wind stratification due to the usage of prescribed mass-loss rates and wind-velocity fields. Aims. We present the concepts for a new generation of hydrodynamically consistent non-local thermodynamical equilibrium (nonLTE) stellar atmosphere models that allow for detailed studies of radiation-driven stellar winds. As a first demonstration, this new kind of model is applied to a massive O star. Methods. Based on earlier works, the PoWR code has been extended with the option to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer in order to obtain a hydrodynamically consistent atmosphere stratification. In these models, the whole velocity field is iteratively updated together with an adjustment of the mass-loss rate. Results. The concepts for obtaining hydrodynamically consistent models using a comoving-frame radiative transfer are outlined. To provide a useful benchmark, we present a demonstration model, which was motivated to describe the well-studied O4 supergiant zeta Pup. The obtained stellar and wind parameters are within the current range of literature values. Conclusions. For the first time, the PoWR code has been used to obtain a hydrodynamically consistent model for a massive O star. This has been achieved by a profound revision of earlier concepts used for Wolf-Rayet stars. The velocity field is shaped by various elements contributing to the radiative acceleration, especially in the outer wind. The results further indicate that for more dense winds deviations from a standard beta-law occur.},
  language  = {en}
}