On the accretion disc and evolutionary stage of β Lyrae

We modelled the V-band light curve of β Lyrae (Lyr) with two stellar components plus an optically thick accretion disc around the gainer assuming a semidetached configuration. We present the results of this calculation, giving physical parameters for the stars and the disc, along with general system dimensions. We discuss the evolutionary stage of the system finding the best match with one of the evolutionary models of Van Rensbergen et al. According to this model, the system is found at age 2.30 × 107 yr, in the phase of rapid mass transfer, the second one in the life of this binary, in a Case-B mass-exchange stage with M yr−1. This result, along with the reported rate of orbital period change and observational evidence of mass-loss, suggests that the mass transfer in β Lyr is quasi-conservative. The best model indicates that β Lyr finished a relatively large mass-loss episode 31 400 yr ago. The light-curve model that best fit the observations has inclination angle i = 81°, M 1 = 13.2 M, M 2 = 3.0 M, R 1 = 6.0 R and R 2 = 15.2 R. The disc contributes 22 per cent to the total V-band light curve at quadrature, has a radius of 28.3 R and the outer edge thickness is 11.2 R. The light-curve model is significantly better with two bright regions in the disc rim with temperatures 10 and 20 per cent higher than the disc outer edge temperature. We compare our results with earlier studies of this interacting binary.