X-Rays From Massive OB Stars: Thermal Emission From Radiative Shocks

TL;DR

This study models X-ray emission from massive OB stars' stellar winds, linking shock structures to observed spectral features and distinguishing between different X-ray production mechanisms.

Contribution

It introduces a simple radiative shock model that explains X-ray emission and absorption features, correlating spectral line shifts with shock structures in OB star winds.

Findings

High correlation between cold clouds and X-ray absorption.

Two plasma temperature groups indicating different shock origins.

Spectral line blue-shifts linked to shock coverage.

Abstract

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.