The X-ray catalog of spectroscopically identified Galactic O stars: Investigating the dependence of X-ray luminosity on stellar and wind parameters

TL;DR

This study presents a comprehensive X-ray catalog of Galactic O stars, analyzing how their X-ray luminosity relates to stellar and wind parameters, revealing correlations and differences across spectral types and luminosity classes.

Contribution

It provides the first detailed analysis of X-ray properties of a large, spectroscopically identified sample of Galactic O stars, linking X-ray luminosity to stellar and wind parameters.

Findings

X-ray luminosities correlate with bolometric luminosity for dwarfs and giants.

X-ray luminosities also correlate with wind parameters in dwarfs and giants.

Supergiants are systematically harder in X-rays and show breakdown of correlations.

Abstract

The X-ray emission of O-type stars was first discovered in the early days of the Einstein satellite. Since then many different surveys have confirmed that the ratio of X-ray to bolometric luminosity in O-type stars is roughly constant, but there is a paucity of studies that account for detailed information on spectral and wind properties of O-stars. Recently a significant sample of O stars within our Galaxy was spectroscopically identified and presented in the Galactic O-Star Spectroscopic Survey (GOSS). At the same time, a large high-fidelity catalog of X-ray sources detected by the XMM-Newton X-ray telescope was released. Here we present the X-ray catalog of O stars with known spectral types and investigate the dependence of their X-ray properties on spectral type as well as stellar and wind parameters. We find that, among the GOSS sample, 127 O-stars have a unique XMM-Newton source counterpart and a Gaia data release 2 (DR2) association. Terminal velocities are known for a subsample of 35 of these stars. We confirm that the X-ray luminosities of dwarf and giant O stars correlate with their bolometric luminosity. For the subsample of O stars with measure terminal velocities we find that the X-ray luminosities of dwarf and giant O stars also correlate with wind parameters. However, we find that these correlations break down for supergiant stars. Moreover, we show that supergiant stars are systematically harder in X-rays compared to giant and dwarf O-type stars. We find that the X-ray luminosity depends on spectral type, but seems to be independent of whether the stars are single or in a binary system. Finally, we show that the distribution of log(Lx/Lbol) in our sample stars is non-Gaussian, with the peak of the distribution at log(Lx/Lbol) around -6.6.