Interacting winds in massive binaries

Massive stars feature highly energetic stellar winds that interact whenever two such stars are bound in a binary system. The signatures of these interactions are nowadays found over a wide range of wavelengths, including the radio domain, the optical band, as well as X-rays and even γ-rays. A proper understanding of these effects is thus important to derive the fundamental parameters of the components of massive binaries from spectroscopic and photometric observations

A multi-epoch XMM-Newton campaign on the core of the massive Cygnus OB2 association

peer reviewedContext. Cyg OB2 is one of the most massive associations of O-type stars in our Galaxy. Despite the large interstellar reddening towards Cyg OB2, many studies, spanning a wide range of wavelengths, have been conducted to more clearly understand this association. X-ray observations provide a powerful tool to overcome the effect of interstellar absorption and study the most energetic processes associated with the stars in Cyg OB2. Aims: We analyse XMM-Newton data to investigate the X-ray and UV properties of massive O-type stars as well as low-mass pre-main sequence stars in Cyg OB2. Methods: We obtained six XMM-Newton observations of the core of Cyg OB2. In our analysis, we pay particular attention to the variability of the X-ray bright OB stars, especially the luminous blue variable candidate Cyg OB2 #12. Results: We find that X-ray variability is quite common among the stars in Cyg OB2. Whilst short-term variations are restricted mostly to low-mass pre-main sequence stars, one third of the OB stars display long-term variations. The X-ray flux of Cyg OB2 #12 varies by 37%, over timescales from days to years, whilst its mean log L[SUB]X[/SUB]/L[SUB]bol[/SUB] amounts to - 6.10. Conclusions: These properties suggest that Cyg OB2 #12 is either an interacting-wind system or displays a magnetically confined wind. Two other X-ray bright O-type stars (MT91 516 and CPR2002 A11) display variations that suggest they are interacting wind binary systems. Based on observations collected with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA).Full Table 2 and Table 4 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/536/A31</A

La malédiction X a encore frappé!

Not Availabl

X-ray emission from massive stars: lessons learned and remaining mysteries

Nearly all types of massive stars are known to display X-ray emission at some level. Tremendous insight into this phenomenon has been gained thanks to the sensitivity of the current generation of X-ray satellites and X-ray observations are nowadays an integral part of multi-wavelength studies of massive stars and their stellar winds. In this contribution, we review the overall picture and highlight a few recent results related to the temporal modulation of the X-ray emission of single massive stars, the orbital modulation of the emission due to colliding winds in massive binary systems, and the properties of the enigmatic gamma Cas stars, Be stars that display a very hard and strong X-ray emission

Spectral Modelling of Massive Binary Systems: The Example of LZ Cep

peer reviewedDespite their importance for many astrophysical processes, massive stars are still not fully understood. Massive binaries offer an attractive way to improve our knowledge of the fundamental properties of these objects. However, some secondary effects are known to generate variations in the spectra of massive binaries, rendering their analyses more difficult. We present here a new approach to the computation of synthetic spectra of massive binaries at different phases of their orbital cycle. Our model starts with the Roche potential modified by radiation pressure and accounts for the influence of the companion star on the shape and physical properties of the stellar surface. We further account for gravity darkening and reflection effects to compute the surface temperature. Once the local gravity and temperature are determined, we interpolate in a grid of NLTE plan-parallel atmosphere model spectra to obtain the local contribution to the spectrum at each surface points. Then we sum all the contributions, accounting for the Doppler shift, and limb-darkening to obtain the total spectrum. The computation is repeated for different orbital phases and can be compared to the observations to determine the best parameters. We illustrate our method through the example of the LZ Cep system (O9III + ON9.7V)

Modeling Fe-Kalpha Fluorescence in the X-ray Spectra of gamma-Cas Stars

Named after their prototype, gamma Cas stars are a subset of Oe/Be stars emitting hard, bright and variable X-rays. The origin of these peculiar X-ray emissions is still in debate. Proposed scenarios include accretion onto a compact object, magnetic interaction of the Be star with its circumstellar disk, and collision between the wind of a stripped helium star companion and the Be disk. In this work, we model fluorescent Fe-K line emission from the rotating circumstellar disk and the photosphere of the Be star. The fluorescent Fe-K line is very sensitive to the physical conditions near the X-ray source. Thus, this line will help to better understand the geometry of the ionizing source and the X-ray emission mechanism in gamma Cas stars. Athena/X-IFU will unveil the morphology of these lines, thereby offering robust diagnostics to distinguish between the various scenarios.PRODEX HERMe

Chemical abundances of fast-rotating OB stars

Fast rotation in massive stars is predicted to induce mixing in their interior, but recent observations have challenged this concept by revealing a population of fast-rotating stars with normal nitrogen abundances at their surface (Hunter et al. 2009, A&A, 496, 841; Brott et al. 2011, A&A, 530, A116, but see Maeder et al. 2014, A&A, 565, A39). However, as the binary fraction of these stars is unknown, the importance of mass-transfer processes cannot be quantified. As a result, no definitive statements about the ability of single-star evolutionary models including rotation to reproduce these observations can be made. Our work combines for the first time a detailed surface abundance analysis with a radial-velocity monitoring for a sample of bright, fast-rotating Galactic OB stars to put strong constraints on stellar evolutionary and interior models.Abundances of fast-rotating massive star

A quantitative study of the O stars in NGC 2244

peer reviewedAbstract. NGC2244 located in the Rosette Nebula is a young open cluster composed of seven O-type stars. A first paper focused on the multiplicity of these stars, revealed only one binary system out of the seven studied stars. The minimum binary fraction of this cluster (~ 14%) diff ers to the average fraction measured on the nearby clusters (~ 44%). In order to better constrain this discrepancy, an analysis based on the determination of the stellar and wind parameters of these stars with the CMFGEN atmosphere code was performed. The main results con rfim that all the stars have an age between 0 and 5 Myr, and that the N surface abundance appears to be consistent with the evolutionary models for a population of stars of the same age. Moreover, this investigation exhibits the existence of dynamical interactions inside this young open cluster sufficiently strong to eject the hottest component from its centre