Spectral modelling of massive binary systems

Abstract

Context. The spectra of massive binaries may be affected by interactions between the stars in the system. These are believed to produce observational phenomena such as the Struve-Sahade effect. Aims. We simulate the spectra of massive binaries at different phases of the orbital cycle, accounting for the gravitational influence of the companion star on the shape and physical properties of the stellar surface. Methods. We used the Roche potential modified to account for radiation pressure to compute the stellar surface of close circular systems. We furthermore used the tidal interactions with dissipation of energy through shear code for surface computations of eccentric systems. In both cases, we accounted for gravity darkening and mutual heating generated by irradiation to compute the surface temperature. We then interpolated non-local thermodynamic equilibrium (NLTE) plane-parallel atmosphere model spectra in a grid to obtain the local spectrum at each surface point. We finally summed all contributions, accounting for the Doppler shift, limb-darkening, and visibility to obtain the total synthetic spectrum. We computed different orbital phases and different sets of physical and orbital parameters. Results. Our models predict line strength variations through the orbital cycle, but fail to completely reproduce the Struve-Sahade effect. Including radiation pressure allows us to reproduce a surface temperature distribution that is consistent with observations of semi-detached binary systems. Conclusions. Radiation pressure effects on the stellar surface are weak in (over)contact binaries and well-detached systems but can become very significant in semi-detached systems. The classical von Zeipel theorem is sufficient for the spectral computation. Broad-band light curves derived from the spectral computation are different from those computed with a model in which the stellar surfaces are equipotentials of the Roche potential scaled by the instantaneous orbital separation. In many cases, the fit of two Gaussian/Lorentzian profiles fails to properly measure the equivalent width of the lines and leads to apparent variations that could explain some of the effects reported in the literature.

Chemical abundances of fast-rotating massive stars

“…This gravity darkening effect implies that the lines of a fast rotator can be created from different regions around the star; thus, He ii lines are preferentially formed near the poles, while He i lines originate from a larger area of the stellar surface. We used the Code of Massive Binary Spectral Computation (CoMBISpeC; Palate et al 2013) to examine the effect of gravity darkening and stellar rotational flattening on the determination of stellar parameters. To this aim, two stars representing the extreme v sin i values encountered in our sample (HD 149757, v sin i ∼ 378 km s −1 and HD 163892, v sin i = 205 km s −1 ) were considered.…”

Short-term spectroscopic variability of Plaskett’s star

“…We have thus tried to model Plaskett's star with the TIDES code (see Moreno et al 1999Moreno et al , 2005Moreno et al , 2011 combined to CoMBiSpeC 4 (see Rauw 2012 andPalate et al 2013b). The TIDES code computes the time-dependent shape of the stellar surface for eccentric and/or asynchronous systems accounting for centrifugal and Coriolis forces, gas pressure, viscous effects and gravitational interactions.…”

Spectral modelling of theαVirginis (Spica) binary system

“…In a first stage, the code was designed for circular massive binary systems (Palate & Rauw 2012) in which the distorted shape of the stellar surfaces due to the gravitational interaction is calculated A&A 556, A49 (2013) and then the emergent spectra at different orbital phases are computed. In a second paper the method was extended to include eccentric binaries and to incorporate radiation pressure effects (Palate et al 2013). This new version of the CoMBiSpeC (Code of Massive Binary Spectral Computation) now allows spectral computation of massive binary systems in general.…”