Analysis of X-Ray Emission from OB Stars. IV. About X-Ray Emission from Inhomogeneous Winds of OB Stars

We study the origin of X-ray emission from OB stars due to collisions of stellar winds and/or inhomogeneities in the winds. The low-resolution X-ray spectra of a big sample of OB stars were fitted by both the stationary APEC/MEKAL models and by this model with an additional PSHOCK component describing the nonstationary X-ray emission. These spectra were also described by two-temperature PSHOCK models. More than ~50% of considered spectra can be described by the above-mentioned model combinations including the PSHOCK model and the quality of the fits appears to be better for O stars. The plasma temperature of the PSHOCK component is about 1–5 keV with the ionization timescale τ_(u)~ 10^(8)–10^(13)s cm^(-3). The temperature of the PSHOCK component increases with the momentum and kinetic energy of the stellar wind by a power law with an index ~0.12–0.14.Such dependencies were not revealed through modeling by the stationary APEC/MEKAL models only. At the same time the X-ray luminosity of OB stars depends on momentum and kinetic energy of their winds similarly either for stationary or for nonstationary models. We conclude that many O stars and some B stars can be sources of the nonstationary X-rays formed in their inhomogeneous stellar wind.

Analysis of the X-ray emission of OB stars:O stars

We investigate the global properties of X-ray emission from O stars, analyzing the X-ray spectra of 32 O stars from archival data of the XMM-Newton space observatory. We examine two hypotheses about of the origin of X-ray emission from O stars. The first is a paradigm proposed by Pollock, that was revealed from an analysis of the ζ Ori X-ray observation. The second is the magnetically confined wind-shock(MCWS) model. For checking Pollock's hypothesis, we determine the distribution of the ratio of half width at half maximum(HWHM) to the wind terminal velocity for lines in spectra of all examined stars. In addition, we check three probable consequences from the MCWS model. We analyze if a correlation exists between the spectral hardness and such stellar parameters as the wind terminal velocity, stellar magnetic field and mass loss rate. The result showed that Pollock's hypothesis is not correct. We also established that not all consequences of the MCWS model considered by us are confirmed. In addition, our spectral analysis method indicated that O stars probably have clumped stellar winds with spherical clumps.

Analysis of the X-ray emission of OB stars II: B stars

This paper is the second part of an investigation into the mechanism for the origin of X-rays in early-type stars. Archival X-ray observations of 25 B stars, obtained by the XMM-Newton satellite, are analysed. We check two hypotheses on the origin of X-ray emission: the Magnetically Confined Wind Shock Model(MCWS) and Pollock’s paradigm. For all studied stars, the mean ratio of the half widths at half maximum to the terminal velocities appears to be R ≈ 0.15-0.20 in contradiction to Pollock’s hypothesis that R ≈ 0.5. We checked three possible consequences of the MCWS model: correlations between the hardness of the X-ray spectra for B stars and terminal wind velocities, mass loss rates and magnetic fields.It was shown that such correlations are marginal or even absent both for magnetic and non-magnetic B stars.

Analysis of the X-ray emission from OB stars Ⅲ: low-resolution spectra of OB stars

This paper is the third in our series of papers devoted to the investigation of X-ray emission from OB stars.In our two previous papers,we study the high-resolution X-ray spectra of 32 O stars and 25 B stars to investigate the correlations between the properties of X-ray emission and stellar parameters.We checked if the X-ray hardness and post-shock plasma temperature grow with increasing stellar magnetic field,mass loss rate and terminal wind velocity.Our previous analysis of high-resolution spectra showed that the correlations are weak or even absent.In the present paper,we analyzed low-resolution X-ray spectra,using model-independent X-ray hardness values for checking the above mentioned dependencies.We establish that X-ray luminosities LX weakly depend on the stellar magnetic field.At the same time,Lx ∝ M0.5 and LX ∝ Ekin0.5,where M is the mass loss rate and Ekin is the kinetic energy of the wind.The X-ray luminosities decrease with growing magnetic confinement parameter η.We also argue that there is an additional(probably non-thermal) component contributed to the stellar X-ray emission.