Coupling hydrodynamics with comoving frame radiative transfer:
II. Stellar wind stratification in the high-mass X-ray binary Vela X-1
Andreas A.C. Sander, Felix Fürst, Peter Kretschmar, Lidia M. Oskinova, Helge Todt, Rainer Hainich, Tomer Shenar, Wolf-Rainer Hamann
CONTEXT: Vela X-1, a prototypical high mass X-ray binary (HMXB), hosts a
neutron star (NS) in a close orbit around an early B-supergiant donor
star. Accretion of the donor star's wind onto the NS powers its strong
X-ray luminosity. To understand the physics of HMXBs, detailed knowledge
about the donor star winds is required.
AIMS: To gain a realistic
picture of the donor star in Vela X-1, we construct a hydrodynamically
consistent atmosphere model describing the wind stratification while
properly reproducing the observed donor spectrum. To investigate how
X-ray illumination affects the stellar wind, we calculate additional
models for different X-ray luminosity regimes.
METHODS: We use the
recently updated version of the PoWR code to consistently solve the
hydrodynamic equation together with the statistical equations and the
radiative transfer.
RESULTS: The wind flow in Vela X-1 is driven by ions
from various elements with Fe III and S III leading in the outer wind.
The model-predicted mass-loss rate is in line with earlier empirical
studies. The mass-loss rate is almost unaffected by the presence of the
accreting neutron star in the wind. The terminal wind velocity is
confirmed at v_\infty ~ 600 km/s. On the other hand, the wind
velocity in the inner region where the NS is located is only
~ 100 km/s, which is not expected on the basis of a standard
β-velocity law. In models with an enhanced level of X-rays, the
velocity field in the outer wind can be altered. If the X-ray flux is
too high, the acceleration breaks down due increased ionization.
CONCLUSIONS: Accounting for radiation hydrodynamics, our Vela X-1 donor
atmosphere model reveals a low wind speed at the NS location, and
provides quantitative information on wind driving in this important
HMXB.
Preprint (sander-vela-x1-2017.pdf, 541kB)
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