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

1. Introduction

The system V635 Cas/4U 0115+63 is a Be X-ray binary star system (BeXRB) with a 3.61s spin period and a 24.3 day orbital period (Cominsky et al. 1978; Rappaport et al. 1978). The optical counterpart, V635 Cas, was tentatively classified as a early type Be star based on its optical colours and the presence of variable H and occasional H emission (Johns et al. 1978; Kriss et al. 1983; Hutchings & Crampton 1981). The temporal evolution of V635 Cas is very different from that of other BeXRBs. The X-ray outbursts from the neutron star are varied in strength but typically last for a month. The associated optical and infrared activity is far more prolonged, lasting typically 6 months. Unlike many BeXRBs the peak in the X-ray flux is not centred on periastron passage. This suggests that it is the episodic equatorial mass loss from the companion star which is the trigger for each outburst. Mendelson & Mazeh (1991) concluded that X-ray outbursts occur when the optical outburst is relatively long (200 days) and strong (1mag). X-ray emission during weaker optical outbursts is not seen due to centrifugal inhibition of matter and the propeller effect (Kriss et al. 1983; Mendelson & Mazeh 1991).

Negueruela et al. (1997, Paper I) discuss the May-June 1994 X-ray outburst in the context of long term observations of V635 Cas. We conclude that the large variations in optical luminosity originate in the Be circumstellar envelope and not an accretion disk around the neutron star. The orbit of the neutron star is relatively close to the companion and its gravitational pull may play an important role in the evolution of the circumstellar disk.

In this paper we present two data sets that can be used to constrain the physical and geometric models for the circumstellar disk. The companion's spectral type is a critical parameter for modelling the disk. However this is hard to derive as many of the companion stars in the BeXRBs are faint and the photospheric lines are often filled in with disk emission. Many of the systems have a spectral classification based on optical colours. In Sect. 3.1 we present the first blue spectra of V635 Cas with sufficient signal to noise to derive a spectral class.

The disks in Oe/Be stars are highly variable. We need a database of high quality data in order to model the evolution of the disk size, temperature and density. In Sect. 3.2 we present the first far red optical spectra obtained for this system.

© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998
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