Astron. Astrophys. 336, 960-965 (1998)

Optical spectroscopy of V635 Cassiopeiae/4U 0115+63

S.J. Unger ¹, P. Roche ², I. Negueruela ³, F.A. Ringwald ⁴, C. Lloyd ⁵ and M.J. Coe <sup>6

1</sup> IPAC, M.S. 100-22, California Institute of Technology, Pasadena, CA 91125, USA
² Astronomy Centre, CPES, Sussex University, Falmer, Brighton, BN1 9QJ, UK
³ Astrophysics Group, Liverpool John Moores University, Byrom St., Liverpool, L3 3AF, UK
⁴ Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802-6305, USA
⁵ Rutherford Appleton Laboratory, Chilton, Oxon, OX11 0QX, UK
⁶ Physics Department Southampton University, Southampton SO17 1BJ, UK

Received 26 August 1997 / Accepted 21 January 1998

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 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 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.

Key words: stars: circumstellar matter stars: emission line, Be stars: neutron stars: individual: V635 Cas stars: individual: Hiltner 102

Send offprint requests to: sju@ipac.caltech.edu

SIMBAD Objects

Contents

• 1. Introduction
• 2. Observations
  • 2.1. Blue spectra
  • 2.2. Red spectra
• 3. Discussion
  • 3.1. Spectral classification
  • 3.2. Probing the circumstellar disk
    • 3.2.1. Hei lines
    • 3.2.2. Paschen lines
    • 3.2.3. Oi lines
    • 3.2.4. Other possible features
• 4. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998
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