TY - JOUR
T1 - Coupling hydrodynamics with comoving frame radiative transfer
T2 - II. Stellar wind stratification in the high-mass X-ray binary Vela X-1
AU - Sander, A. A.C.
AU - Fürst, F.
AU - Kretschmar, P.
AU - Oskinova, L. M.
AU - Todt, H.
AU - Hainich, R.
AU - Shenar, T.
AU - Hamann, W. R.
N1 - Publisher Copyright:
© ESO 2018.
PY - 2018/2/1
Y1 - 2018/2/1
N2 - 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 constructed 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 calculated additional models for different X-ray luminosity regimes. Methods. We used the recently updated version of the Potsdam Wolf-Rayet 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 NS in the wind. The terminal wind velocity is confirmed at v∞≈ 600 km s-1. On the other hand, the wind velocity in the inner region where the NS is located is only ≈ 100 km s-1, 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 because the ionization increases. Conclusions. Accounting for radiation hydrodynamics, our Vela X-1 donor atmosphere model reveals a low wind speed at the NS location, and it provides quantitative information on wind driving in this important HMXB.
AB - 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 constructed 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 calculated additional models for different X-ray luminosity regimes. Methods. We used the recently updated version of the Potsdam Wolf-Rayet 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 NS in the wind. The terminal wind velocity is confirmed at v∞≈ 600 km s-1. On the other hand, the wind velocity in the inner region where the NS is located is only ≈ 100 km s-1, 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 because the ionization increases. Conclusions. Accounting for radiation hydrodynamics, our Vela X-1 donor atmosphere model reveals a low wind speed at the NS location, and it provides quantitative information on wind driving in this important HMXB.
KW - Stars: Atmospheres
KW - Stars: Early-type
KW - Stars: Mass-loss
KW - Stars: Massive
KW - Stars: winds, outflows
KW - X-rays: Binaries
UR - http://www.scopus.com/inward/record.url?scp=85042769869&partnerID=8YFLogxK
U2 - 10.1051/0004-6361/201731575
DO - 10.1051/0004-6361/201731575
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AN - SCOPUS:85042769869
SN - 0004-6361
VL - 610
JO - Astronomy and Astrophysics
JF - Astronomy and Astrophysics
M1 - A60
ER -