Mass transfer variation in the outburst model of dwarf novae and soft X-ray transients

Abstract

Context. In the standard formulation of the disc instability model for soft X-ray transient and dwarf nova outbursts, the mass transfer rate from the secondary is assumed to be constant. This may seem natural since the L1 point remains shielded from the accretion luminosity by the accretion disc. However an indirect heating could take place and could lead to an enhancement of the mass transfer rate. It is still debated whether such an enhancement of the mass transfer rate during outbursts is a missing ingredient of the current model. Aims. We discuss two mechanisms that could result in an enhancement of the mass transfer rate during outbursts: the hot outer disc rim itself could significantly heat the L1 point; scattered radiation by optically thin outflowing matter could also heat L1 significantly. We determine quantitatively the increase of the mass transfer rate resulting from an extra heating. Methods. We estimate the heating of the L1 point by the disc rim by analytical and numerical arguments. The fraction of the luminosity scattered by matter above the disc, here modeled as a spherically symmetric outflow, is determined analytically. We finally solve in a numerical way the vertical structure equations for the secondary star and calculate the mass transfer enhancement. Results. During outbursts, the temperature at the outer edge of the disc reaches 10 4 K. The disc edge heats up the upper layer of the secondary with a flux of the order of the intrinsic stellar flux. This probably has no large effect on the mass transfer rate. In soft X-ray transients, the environing medium of the disc (corona+wind) could back-scatter a certain fraction of the accretion luminosity toward L1. In dwarf novae, the same effect could be due to the wind present during outburst. Since soft X-ray transients reach high luminosities, even a low efficiency of this effect could yield a significant heating of L1, whereas we show that in dwarf novae this effect is negligible. Initially the incoming radiation does not penetrate below the photosphere of the secondary. Depending on the heating efficiency, which has to be determined, the mass transfer rate could be significantly increased.