Multiwavelength Systematics of OB Spectra

TL;DR

This paper reviews the systematic spectral features of OB stars across optical, UV, and X-ray wavelengths, highlighting structural, evolutionary phenomena, and the importance of multiwavelength observations for understanding these massive stars.

Contribution

It provides a comprehensive survey of OB star spectral systematics across multiple wavelengths and discusses new findings from X-ray observations, emphasizing the need for improved modeling.

Findings

Identification of spectral trends correlated with stellar parameters.

Discovery of systematic morphological trends in OB X-ray spectra.

Highlighting the importance of UV and optical data for understanding magnetic and wind phenomena.

Abstract

The systematics of OB spectra are reviewed in the optical domain, dominated by photospheric lines, and in the far ultraviolet (both IUE and FUSE ranges), in which the stellar-wind profiles dominate. First, the two-dimensional (temperature, luminosity) trends in normal spectra are surveyed. Then, the normal reference frame having been established, various categories of peculiar objects can be distinguished relative to it, which reveal several phenomena of structural and/or evolutionary significance. Included are CNO anomalies at both early and late O types, three varieties of rapid rotators, hot and cool Of/WN transition objects, and the recently discovered second known magnetic O star. The importance of both optical and UV observations to understand these phenomena is emphasized; for instance, progress in understanding the structure of the new O-type magnetic oblique rotator is hampered by the current lack of a UV spectrograph. While progress in the physical interpretation of these trends and anomalies has been and is being made, increased attention to modeling the systematics would accelerate future progress in this author's opinion. Finally, preliminary results from a Chandra high-resolution survey of OB X-ray spectra (PI W. Waldron) are presented. They provide evidence that, just as emerged earlier in the UV, systematic morphological trends exist in the X-ray domain that are correlated with the optical spectral types, and hence the fundamental stellar parameters, contrary to prevailing opinion.