Anomalous Z Cam stars: a response to mass-transfer outbursts

¹ Observatoire Astronomique de Strasbourg, CNRS UMR 7550, 67000 Strasbourg, France
e-mail: jean-marie.hameury@astro.unistra.fr
² CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98bis Bd Arago, 75014 Paris, France
³ Nicolaus Copernicus Astronomical Center, Bartycka 18, 00716 Warsaw, Poland
⁴ Sorbonne Universités, UPMC Univ. Paris 06, UMR 7095, 98bis Bd Arago, 75014 Paris, France

Received: 4 July 2014
Accepted: 21 July 2014

Abstract

Context. Recent observations of two unusual Z Cam systems, V513 Cas and IW And have shown light curves that seem to contradict the disc-instability model for dwarf novae: outbursts are appearing during standstills of the system when according to the model, the disc is supposed to be in a hot quasi-equilibrium state.

Aims. We investigate what additional physical processes need to be included in the model to reconcile it with observations of such anomalous Z Cam systems.

Methods. We used our code for modeling thermal-viscous outbursts of the accretion discs and determined what types of mass-transfer variations reproduce the observed light curves.

Results. Outbursts of mass transfer (with a duration of a few days, with a short rise time and an exponential decay) from the stellar companion will account for the observed properties of V513 Cas and IW And, provided they are followed by a short but significant mass-transfer dip. The total mass involved in outbursts is of the order of 10²³g.

Conclusions. We studied the possible origins of these mass transfer outbursts and showed that they most probably result from a giant flare near the secondary star surface, possibly due to the absence of star spots in the L₁ region.