Population synthesis for double white dwarfs

II. Semi-detached systems: AM CVn stars

¹ Astronomical Institute "Anton Pannekoek", Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
² Massachusetts Institute of Technology, Massachusetts Ave. 77, Cambridge MA 02139, USA
³ Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
⁴ Institute of Astronomy of the Russian Academy of Sciences, 48 Pyatnitskaya Str., 109017 Moscow, Russia

Corresponding author: G. Nelemans, GijsN@astro.uva.nl

Received: 17 November 2000
Accepted: 2 January 2001

Abstract

We study two models for AM CVn stars: white dwarfs accreting (i) from a helium white dwarf companion and (ii) from a helium-star donor. We show that in the first model possibly no accretion disk forms at the onset of the mass transfer. The stability and the rate of mass transfer then depend on the tidal coupling between the accretor and the orbital motion. In the second model the formation of AM CVn stars may be prevented by detonation of the CO white dwarf accretor and the disruption of the system. With the most favourable conditions for the formation of AM CVn stars we find a current Galactic birth rate of . Unfavourable conditions give . The expected total number of the systems in the Galaxy is 9.4 10⁷ and 1.6 10⁷, respectively. We model very simple selection effects to get some idea about the currently expected observable population and discuss the (quite good) agreement with the observed systems.