The equatorial disc of the Be star X Persei

Telting, J. H.; Waters, L. B. F. M.; Roche, P.; Boogert, A. C. A.; Clark, James; de Martino, D. and Persi, P. (1998). The equatorial disc of the Be star X Persei. Monthly Notices of the Royal Astronomical Society, 296(4) pp. 785–799.

DOI: https://doi.org/10.1046/j.1365-8711.1998.01433.x

Abstract

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: ?0=(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.

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