Hydrodynamic simulations of irradiated secondaries in dwarf novae

Observatoire Astronomique, Université Louis Pasteur and CNRS, 11 rue de l'Université, 67000 Strasbourg, France e-mail: [viallet;hameury]@astro.u-strasbg.fr

Received: 4 May 2007
Accepted: 21 August 2007

Abstract

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 shaded from the primary irradiation by the accretion disc. Previous investigations of the possibility of a circulation flow transporting heat from hot regions to L₁ have given opposite answers.

Aims.We investigate the surface flow of irradiated secondaries numerically. We consider the full time-dependent problem and take the two-dimensional nature of the flow into account.

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 the 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 shaded from irradiation. Under the assumption of hydrostatic equilibrium, the Coriolis force could lead to a moderate increase in 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 possible, however, that this assumption breaks up due to the strong surface velocity of the flow transiting by L₁, close to 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 in the mass transfer rate is moderate, so unlikely to be able to account for the duration of long outbursts.