Iwona Kotko, Jean-Pierre Lasota, Guillaume Dubus, Jean-Marie Hameury
Outbursting AM CVn stars exhibit outbursts similar to those observed in different types of dwarf novae. Their light-curves combine the characteristic features of SU UMa, ER UMa, Z Cam and WZ Sge-type systems but also show a variety of properties never observed in dwarf novae. The compactness of AM CVn orbits and their unusual chemical composition make these systems valuable test-beds for outburst models. Understanding the role of helium in the accretion disc instability mechanism, testing the model for dwarf novae outbursts in the case of AM CVn stars, and the explanation of the outburst light-curves of these ultra-compact binaries. We calculate the properties of the hydrogen-free AM CVn stars using the Hameury et al. (1998) numerical code adapted to the different chemical composition of these systems and supplemented with formulae accounting for mass transfer rate variations, additional sources of the disc heating and the primary's magnetic field. We find how helium-dominated discs react to the thermal-viscous instability and reproduce various features of the outburst cycles in the light-curves of AM CVn stars. The AM CVn outbursts can be explained by the suitably adapted dwarf-nova disc instability model but, as in case of its application to hydrogen-dominated cataclysmic variables, one has to resort to additional mechanisms to account for the observed superoutbursts, dips, cycling states and standstills. We show that enhanced mass-transfer rate, due presumably to variable irradiation of the secondary, must not only be taken into account but is a determinant factor that shapes AM CVn star outbursts. The cause of variable secondary's irradiation has still to be understood; the best candidate being the precession of a tilted/warped disc.
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http://arxiv.org/abs/1205.5999
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