Chandra grating spectroscopy of embedded wind shock X-ray emission from O stars shows low plasma temperatures and significant wind absorption

TL;DR

This study analyzes high-resolution X-ray spectra of six O stars to understand their wind shock temperatures and absorption, revealing low plasma temperatures and significant wind absorption effects on observed X-ray properties.

Contribution

It provides a uniform analysis of multiple O stars, quantifies wind absorption effects, and derives wind mass-loss rates consistent with other methods, highlighting the importance of absorption in X-ray observations.

Findings

Hot plasma temperatures are around 10^7 K with a power-law distribution.

Wind absorption significantly affects X-ray spectra, especially at longer wavelengths.

Empirical X-ray hardness trends are mainly due to absorption effects.

Abstract

We present a uniform analysis of six examples of embedded wind shock (EWS) O star X-ray sources observed at high resolution with the Chandra grating spectrometers. By modeling both the hot plasma emission and the continuum absorption of the soft X-rays by the cool, partially ionized bulk of the wind we derive the temperature distribution of the shock-heated plasma and the wind mass-loss rate of each star. We find a similar temperature distribution for each star's hot wind plasma, consistent with a power-law differential emission measure, $\frac{d\log EM}{d\log T}$, with a slope a little steeper than -2, up to temperatures of only about $10^7$ K. The wind mass-loss rates, which are derived from the broadband X-ray absorption signatures in the spectra, are consistent with those found from other diagnostics. The most notable conclusion of this study is that wind absorption is a very important effect, especially at longer wavelengths. More than 90 per cent of the X-rays between 18 and 25 Angstrom units produced by shocks in the wind of $\zeta$ Puppis are absorbed, for example. It appears that the empirical trend of X-ray hardness with spectral subtype among O stars is primarily an absorption effect.