A multiwavelength approach to BD+53$^\circ$2790: the O9.5V counterpart to the X-ray binary system 4U~2206+54

TL;DR

This study provides a comprehensive multiwavelength analysis of the star BD+53$^\u2192$2790 in the X-ray binary 4U 2206+54, revealing complex stellar properties and challenging previous classifications, with implications for stellar evolution theories.

Contribution

It offers the first detailed multiwavelength spectral analysis of BD+53$^$2790, highlighting its unique features and potential classification as a He-rich supergiant, thus advancing understanding of stellar wind and magnetic phenomena.

Findings

BD+53$^$2790 shows a circumstellar envelope differing from typical Be stars.

Spectral analysis suggests a possible He over-abundance, linking it to He-rich stars.

UV spectra indicate an unusually slow, dense stellar wind for an O9.5V star.

Abstract

The X-ray binary system 4U 2206+54 hides many mysteries. Among them, the surprising behavior of both of its components: the O9.5 dwarf star BD+53$^\circ$2790 and a slowly rotating neutron star. BD+53$^\circ$2790 misled the astronomers showing itself very likely as a Be star. However, a deeper spectral analysis and more intense monitoring, revealed that the real picture was a bit more complicated: a) Although it shows evidence of a circumstellar envelope, its observable properties differ from those typical envelopes in Be stars. b) Comparison with spectral standards and models indicates a possible over-abundance in He. This would open the possibility to link the behavior of BD+53$^\circ$2790 to the He-rich class of stars. c) UV spectra shows an abnormally slow and dense wind for an O9.5V star. d) Spectral classification in the IR wavelength region suggest a more likely supergiant nature of the source, in contradiction with the optical classification. e) The presence of an intense magnetic field is under investigation. BD+53$^\circ$2790 stands as a perfect laboratory for testing stellar structure, as well as wind and evolutionary theories. The observable properties of this source in a wide range of spectral bands are discussed, and some interpretations outlined.