Distortion of secondaries in semi-detached binaries and the cataclysmic variable period minimum

¹ C.R.A.L (UMR 5574 CNRS), École Normale Supérieure, 69364 Lyon Cedex 07, France e-mail: vrenvoiz, ibaraffe@ens-lyon.fr
² Max-Planck Institut für Astrophysik, Karl-Schwarzschild-str.1, 85741 Garching, Germany e-mail: hsr@mpa-garching.mpg.de
³ Department of Physics & Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK e-mail: u.c.kolb@open.ac.uk

Corresponding author: V. Renvoizé, vrenvoiz@ens-lyon.fr

Received: 27 December 2001
Accepted: 15 April 2002

Abstract

Based on SPH simulations, we quantify the geometrical distortion effect due to tidal and rotational forces on polytropic secondaries in semi-detached binaries. The main effect is an expansion of the polytropic star, with an effect on the radius of ~5%–12%, depending on the polytropic index and the mass ratio. We apply such distortion effects to the secular evolution of secondaries in cataclysmic variable systems. We focus on systems below the 2–3 h period gap and that approach the minimum period. We find a significant increase of the predicted minimum period (~4% if changes in the secondary's thermal relaxation are approximately taken into account). Though an improvement, the effect is not big enough to solve the mismatch between predicted and observed minimum period at 80 min.