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

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.

MNRAS accepted; 12 pages, 8 figures; this version incorporates the content of an erratum which corrects two figures, but leaves the results and conclusions unchanged

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• The IACOB project VIII. Searching for empirical signatures of binarity in fast-rotating O-type stars
• The X-ray emission of gamma Cassiopeiae during the 2020-2021 disc eruption
• X-ray emission of massive stars and their winds
• Discovery of extraordinary X-ray emission from magnetospheric interaction in the unique binary stellar system $\epsilon$ Lupi
• Helium-like X-ray line complexes show that the hottest plasma on the O supergiant zeta Puppis is in its wind
• Unveiling the soft X-ray source population towards the inner Galactic disk with XMM-Newton