The equatorial disc of the Be star X Persei

• Published in: Monthly notices of the Royal Astronomical Society Vol. 296; no. 4; pp. 785 - 799
• Main Authors: Telting, J.H., Waters, L.B.F.M., Roche, P., Boogert, A.C.A., Clark, J.S., de Martino, D., Persi, P.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 01-06-1998
• Subjects: circumstellar matter; infrared: stars; stars: emission-line; stars: emission‐line, Be; stars: individual: X Per; stars: mass-loss; ultraviolet: stars
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1046/j.1365-8711.1998.01433.x

We study the long-term behaviour of the equatorial disc of the Be/X-ray binary X Persei (X Per), combining new low-resolution IUE spectra and IR photometry with UV, optical and IR observations collected from the literature. We find that the near-UV continuum level of X Per varies along with the optical brightness. From the UV observations, we also find that during optical high states the flux excess due to the intrinsic stellar variability and/or electron scattering in the disc is at most 15–20 per cent of the photospheric flux. From the data taken in discless and near-discless states (optical low states) we show that the stellar photosphere can be modelled with Teff=31000 K and log g=4. With this model we derive E(B−V)=0.39, and estimate the distance to X Per as 950±200 pc (assuming R*=9 R⊙). We fit the (quasi-)simultaneous optical and IR photometry with a simple disc model including free–bound and free–free radiation. We find that the density of the disc at the photosphere of the star varies along with the brightness of X Per, and that in optical high states the disc in X Per is among the densest of all Be stars: ρ0=(1.5±0.3)×10−10 g cm−3. The disc density at the photosphere varies by a factor of at least 20 from optical high to low states. During disc build-up and break-down phases, and also in phases when the disc is relatively stable, we find a very steep radial density gradient of the disc of X Per. This may reflect the limitations of some of the assumptions in our model. We find that in a disc-loss event the disc loses mass at a rate of about 5×10−9 M⊙ yr−1. For a disc build-up phase we find a disc-mass growth rate of about 4×10−9 M⊙ yr−1. This growth rate is consistent with a model that feeds the disc from the ‘ordinary’ mass-loss of the star, but we cannot exclude the possibility that other phenomena contribute to the disc growth as well.