Atmospheric NLTE models for the spectroscopic analysis of blue stars with winds

III. X-ray emission from wind-embedded shocks

¹ LMU Munich, Universitätssternwarte, Scheinerstr. 1, 81679 München, Germany
e-mail: luiz@usm.uni-muenchen.de
² Centro de Astrobiología, CSIC-INTA, Ctra. Torrejón a Ajalvir km.4, 28850 Madrid, Spain
³ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Received: 9 November 2015
Accepted: 1 March 2016

Abstract

Context. Extreme ultraviolet (EUV) and X-ray radiation emitted from wind-embedded shocks in hot, massive stars can affect the ionization balance in their outer atmospheres and can be the mechanism responsible for producing highly ionized atomic species detected in stellar wind UV spectra.

Aims. To allow for these processes in the context of spectral analysis, we have implemented the emission from wind-embedded shocks and related physics into our unified, NLTE model atmosphere/spectrum synthesis code FASTWIND.

Methods. The shock structure and corresponding emission is calculated as a function of user-supplied parameters (volume filling factor, radial stratification of shock strength, and radial onset of emission). We account for a temperature and density stratification inside the postshock cooling zones, calculated for radiative and adiabatic cooling in the inner and outer wind, respectively. The high-energy absorption of the cool wind is considered by adding important K-shell opacities, and corresponding Auger ionization rates have been included in the NLTE network. To test our implementation and to check the resulting effects, we calculated a comprehensive model grid with a variety of X-ray emission parameters.

Results. We tested and verified our implementation carefully against corresponding results from various alternative model atmosphere codes, and studied the effects from shock emission for important ions from He, C, N, O, Si, and P. Surprisingly, dielectronic recombination turned out to play an essential role for the ionization balance of O iv/O v (particularly in dwarfs with T_eff~ 45 000 K). Finally, we investigated the frequency dependence and radial behavior of the mass absorption coefficient, κ_ν(r), which is important in the context of X-ray line formation in massive star winds.

Conclusions. In almost all of the cases considered, direct ionization is of major influence because of the enhanced EUV radiation field, and Auger ionization only affects N vi and O vi significantly. The approximation of a radially constant κ_ν is justified for r ≳ 1.2 R_∗ and λ ≲ 18 Å and also for many models at longer wavelengths. To estimate the actual value of this quantity, however, the He ii opacities need to be calculated from detailed NLTE modeling, at least for wavelengths longer than 18 to 20 Å, and information on the individual CNO abundances has to be present.