Hydrodynamic simulations of irradiated secondaries in dwarf novae

Context. Secondary stars in dwarf novae are strongly irradiated during outbursts. It has been argued that this could result in an enhancement of the mass transfer rate even though the L1 region is shadowed from the primary irradiation by the accretion disc. Previous investigations of the possibility of a circulation flow transporting heat from hot regions to L1 gave opposite answers. Aims. We investigate numerically the surface flow of irradiated secondaries. We consider the full time dependent problem and we account for the two-dimensional nature of the flow. Methods. We use a simple model for the irradiation and the geometry of the secondary star: the irradiation temperature is treated as a free parameter and the secondary is replaced by a spherical star with a space-dependent Coriolis force that mimics the effect of the Roche geometry. The Euler equations are solved in spherical coordinates with the TVD-MacCormack scheme. Results. We show that the Coriolis force leads to the formation of a circulation flow from high latitude region to the close vicinity of the L1 point. However no heat can be efficiently transported to the L1 region due to the rapid radiative cooling of the hot material as it enters the equatorial belt shadowed from irradiation. Under the assumption of hydrostatic equilibrium, the Coriolis force could lead to a moderate increase of the mass transfer by pushing the gas in the vertical direction in the vicinity of L1, but only during the initial phases of the outburst (about 15 – 20 orbital periods). It remains however possible that this assumption breaks up due to the strong surface velocity of the flow transiting by L1, of the order of the sound speed. In this case however, a three-dimensional approach would then be needed to determine the mass flux leaving the secondary. Conclusions. We therefore conclude that the Coriolis force does not prevent a flow from the heated regions of the secondary towards the L1 region, at least during the initial phases of an outburst, but the resulting increase of the mass transfer rate is moderate, and it is unlikely to be able to account for the duration of long outbursts.