Optical Spectroscopy of V635 Cassiopeiae/4U 0115+63

TL;DR

This study provides the first direct spectral classification of V635 Cas as an O9e star, revealing extreme variability and disk-loss events in its circumstellar environment, and reclassifies a flux standard star based on spectral analysis.

Contribution

It offers the first direct spectral type determination of V635 Cas and documents its extreme variability and disk-loss phenomena through optical spectroscopy.

Findings

V635 Cas is classified as an O9e star.

First observation of a disk-loss event in this system.

Reclassification of the flux standard star to O9.7 II.

Abstract

V635 Cas is the optical counterpart of the X-ray binary system 4U 0115+63. It was previously tentatively identified as a Be star based on its optical colours and the presence of H alpha emission. Our observations indicate that it is an O9e star. This is the first direct determination of this star's optical spectral type. The presence of a hotter companion star may in part explain the large temporal variation observed in this system. Extreme variability was observed in 1992 February when both the H alpha and a series of Paschen lines changed from emission to absorption. This was interpreted as a disk-loss event and it is the first time that it has been observed in this system. We use far red spectra of V635 Cas to probe the circumstellar disk, discussing the various line formation regions. The lines observed are consistent with a late type Oe star. The flux standard Hiltner 102 was also observed. Although it is classified as a B0 III star, we re-classify it as a O9.7 II star with a slight nitrogen enhancement.