The nature of the high Galactic latitude O-star HD93521: new results from X-ray and optical spectroscopy

TL;DR

This study uses X-ray and optical spectroscopy to investigate the nature of the high Galactic latitude O-star HD93521, finding no evidence of a recent star formation or compact companion, and suggesting it is a runaway Population I star.

Contribution

The paper provides new X-ray and optical spectroscopic data and analysis to clarify the nature and origin of HD93521, distinguishing it from subdwarf OB stars and supporting its classification as a runaway massive star.

Findings

No signs of recent star formation around HD93521

X-ray spectrum consistent with a normal late O-type star

He and N overabundant, C and O subsolar

Abstract

Owing to its unusual location and its isolation, the nature of the high Galactic latitude O9.5Vp object HD93521 is still uncertain. We have collected X-ray and optical observations to characterize the star and its surroundings. X-ray images and spectra are analyzed to search for traces of a recent star formation event around HD93521 and to search for the signature of a possible compact companion. Optical echelle spectra are analysed with plane-parallel model atmosphere codes, assuming either a spherical star or a gravity darkened rotationally flattened star, to infer the effective temperature and surface gravity, and to derive the He, C, N and O abundances of HD93521. The X-ray images reveal no traces of a population of young low-mass stars coeval with HD93521. The X-ray spectrum of HD93521 is consistent with a normal late O-type star although with subsolar metallicity. No trace of a compact companion is found in the X-ray data. In the optical spectrum, He and N are found to be overabundant, in line with the effect of rotational mixing in this very fast rotator, whilst C and O are subsolar. A critical comparison with the properties of subdwarf OB stars, indicates that, despite some apparent similarities, HD93521 does not belong to this category. Despite some ambiguities on the runaway status of the star, the most likely explanation is that HD93521 is a Population I massive O-type star that was ejected from the Galactic plane either through dynamical interactions or a result of a supernova event in a binary system.