X-Rays From Massive OB Stars:
Thermal Emission From Radiative Shocks
Svetozar A. Zhekov (1) and Francesco Palla (2)
(1) Space Research Institute, Sofia, Bulgaria
(2) INAF-Osservatorio Astrofisico di Arcetri, Firenze, Italy
Chandra gratings spectra of a sample of 15 massive OB stars were analyzed under the basic assumption that the X-ray emission is produced in an ensemble of shocks formed in the winds driven by these objects. Shocks develop either as a result of radiation-driven instabilities or due to confinement of the wind by relatively strong magnetic field, and since they are radiative, a simple model of their X-ray emission was developed that allows a direct comparison with observations. According to our model, the shock structures (clumps, complete
or fractional shells) eventually become `cold' clouds in the X-ray sky of the star. As a result, it is expected that for large covering factors of the hot clumps, there is a high probability for X-ray absorption by the `cold' clouds, resulting in blue-shifted
spectral lines. Our analysis has revealed that such a correlation indeed exists for the considered sample of OB stars. As to the temperature characteristics of the X-ray emission plasma, the studied OB stars fall in two groups: (i) one with plasma temperature limited to 0.1-0.4 keV; (ii) the other wtih X-rays produced in plasmas at considerably higher temperatures. We argue that the two groups correspond to different mechanisms for the origin of X-rays: in radiative-driven instability shocks and in magnetically-confined wind shocks, respectively.
Reference: MNRAS
Status: Manuscript has been accepted
Weblink: http://arxiv.org/abs/0708.0085
Comments:
Email: zhekovs@colorado.edu