On the consistent treatment of the quasi-hydrostatic layers in hot star atmospheres

Abstract

Context. Spectroscopic analysis remains the most common method to derive masses of massive stars, the most fundamental stellar parameter. While binary orbits and stellar pulsations can provide much sharper constraints on the stellar mass, these methods are only rarely applicable to massive stars. Unfortunately, spectroscopic masses of massive stars heavily depend on the detailed physics of model atmospheres. Aims. We demonstrate the impact of a consistent treatment of the radiative pressure on inferred gravities and spectroscopic masses of massive stars. Specifically, we investigate the contribution of line and continuum transitions to the photospheric radiative pressure. We further explore the effect of model parameters, e.g., abundances, on the deduced spectroscopic mass. Lastly, we compare our results with the plane-parallel TLUSTY code, commonly used for the analysis of massive stars with photospheric spectra. Methods. We calculate a small set of O-star models with the Potsdam Wolf-Rayet (PoWR) code using different approaches for the quasi-hydrostatic part. These models allow us to quantify the effect of accounting for the radiative pressure consistently. We further use PoWR models to show how the Doppler widths of line profiles and abundances of elements such as iron affect the radiative pressure, and, as a consequence, the derived spectroscopic masses. Results. Our study implies that errors on the order of a factor of two in the inferred spectroscopic mass are to be expected when neglecting the contribution of line and continuum transitions to the radiative acceleration in the photosphere. Usage of implausible microturbulent velocities, or the neglect of important opacity sources such as Fe, may result in errors of approximately 50% in the spectroscopic mass. A comparison with TLUSTY model atmospheres reveals a very good agreement with PoWR at the limit of low mass-loss rates.

The detection of variable radio emission from the fast rotating magnetic hot B-star HR 7355 and evidence for its X-ray aurorae

“…the the wind, and also accounts for X-rays (Gräfener et al 2002;Hamann & Gräfener 2003;Sander et al 2015). Already the first inspection of the spectra reveals a lack of asymmetric line profiles which would be expected for spectral lines formed in a stellar wind.…”

Measuring the stellar wind parameters in IGR J17544-2619 and Vela X-1 constrains the accretion physics in supergiant fast X-ray transient and classical supergiant X-ray binaries

“…The surface gravity g and R imply the stellar mass (M ) via g = GM R −2 . Instead of g , one may specify the effective surface gravity g eff , which accurately accounts for the outward force exerted by the radiation field, as described thoroughly by Sander et al (2015).…”

The donor star of the X-ray pulsar X1908+075

“…In the quasi-hydrostatic regime we use this equation to calculate a hydrodynamically-consistent velocity field (Sander et al 2015). In the outer part we assume a so-called "beta-law",…”

“…The details are described in Sander et al (2015). In addition, the clumping factor D describes the maximum density contrast reached in the outer parts of the wind.…”