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

J. -M. Hameury (1) , J. -P. Lasota (2, 3)

1 ObAS - Observatoire astronomique de Strasbourg
2 IAP - Institut d'Astrophysique de Paris
3 UPMC - Université Pierre et Marie Curie - Paris 6

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.

Mots clés: accretion; accretion disks; stars: dwarf novae; instabilities; Astrophysics - Solar and Stellar Astrophysics

Domaines: Planète et Univers [physics]; Astrophysique [astro-ph]

J. -M. Hameury, J. -P. Lasota. Anomalous Z Cam stars: a response to mass-transfer outbursts. Astronomy and Astrophysics - A&A, 2014, 569, ⟨10.1051/0004-6361/201424535⟩. ⟨insu-03645329⟩