The Correlation between X-Ray Line Ionization and Optical Spectral Types of the OB Stars

TL;DR

This study finds clear correlations between X-ray ionization levels and optical spectral types in OB stars, revealing trends that were not predicted by existing models and suggesting a link to fundamental stellar parameters.

Contribution

It uncovers new empirical correlations between X-ray ionization states and optical spectral types in OB stars, guiding future astrophysical modeling.

Findings

Higher ionization lines weaken with later spectral types

H-like to He-like line ratios decrease with spectral type

Correlations are consistent across a small sample of normal OB stars

Abstract

Marked correlations are reported between the ionization of the X-ray line spectra of normal OB stars, as observed by the Chandra X-Ray Observatory, and their optical spectral types. These correlations include the progressive weakening of the higher ionization relative to the lower ionization X-ray lines with advancing spectral type, and the similarly decreasing intensity ratios of the H-like to He-like lines of the alpha ions. These relationships were not predicted by models, nor have they been clearly evident in astrophysical studies of a few objects; rather, they have emerged from morphological analysis of an adequate (albeit still small) sample, from which known peculiar objects such as magnetic stars and very rapid rotators have been isolated to reveal the normal trends. This process is analogous to that which first demonstrated the strong relationships between the UV wind profiles and the optical spectral types of normal OB stars, which likely bear a physical as well as a historical connection to the present X-ray results. Since the optical spectral types are calibrated in terms of fundamental stellar parameters, it follows that the winds and X-ray spectra are determined by the latter. These observations provide strong guidance for further astrophysical modeling of these phenomena.