Z Cam stars: A particular response to a general phenomenon

¹ Observatoire de Strasbourg, UMR 7550 du CNRS, 11 rue de l'Université, 67000 Strasbourg, France e-mail: hameury@astro.u-strasbg.fr
² Institut d'Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France e-mail: lasota@iap.fr

Corresponding author: V. Buat-Ménard, buat@astro.u-strasbg.fr

Received: 11 October 2000
Accepted: 22 January 2001

Abstract

We show that the disc instability model can reproduce all the observed properties of Z Cam stars if the energy equation includes heating of the outer disc by the mass-transfer stream impact and by tidal torques and if the mass-transfer rate from the secondary varies by about around the value critical for stability. In particular the magnitude difference between outburst maxima and standstills corresponds to observations, all outbursts are of the inside-out type and can be divided into two classes: long (wide) and short (narrow) outbursts, as observed. Mass transfer rate fluctuations should occur in other dwarf novae but one can exclude variations similar to those observed in magnetic systems (AM Her's and some DQ Her's) and some nova-like systems (VY Scl's), in which become very small during low states; these would produce mini-outbursts which, although detectable, have never been observed.