Superoutbursts, superhumps and the tidal-thermal instability model

Abstract

Abstract. We include the tidal instability due to the 3:1 resonance in the disc instability model developed by Hameury et al. (1998) and modified by Buat-Ménard et al. (2001). We confirm earlier results by Osaki (1989) that the tidal instability can account for the SU UMa light curves. We show that in ultra-low mass ratio systems such as ER UMa stars and WZ Sge stars, the superoutburst ends while the disc is still eccentric, as proposed by Hellier (2001). However, since the disc shrinks rapidly once a cooling wave has started, the eccentricity should stop shortly after the end of a superoutburst. This result disagrees with the suggestion by Hellier that decoupling the thermal and tidal instability in the TTI model can account for late superhumps and echo outbursts in ultra-low mass ratio systems. We propose instead that ER UMa short supercycles can be explained either by the alternation of narrow and wide outbursts similar to those occurring in SS Cyg, or by the effects of irradiation (Hameury et al. 2000). In both cases, we predict that superhumps should be permanent, which is suggested by observations (Gao et al. 1999). We can also reproduce light curves similar to those of EG Cnc, varying the mass transfer rate in a TTI model including both irradiation and the presence of an inner hole in the disc.

The disk instability model for dwarf nova outbursts

“…It may be noted here that a time delay of two days between the triggering normal outburst and the start of the superoutburst was introduced in this calculation so that all of resulting superoutbursts were of precursor-main type. Full onedimensional numerical simulations (i.e., the standard form of numerical simulations for outbursts of the dwarf novae) based on the TTI model were performed by Ichikawa, Hirose, and Osaki 28) and by Buat-Ménard and Hameury, 29) basically confirming results by Osaki.…”

“…Thus, when the normal outburst starts and the disk jumps from the cold to the hot state, the disk radius expands. An expansion of the disk without any effective tidal torques thins out matter at the outer edge, immediately leading the disk to jump back to the cold state, a result demonstrated both analytically by Osaki,8) using a simplified model, and numerically by simulations 28), 29) in which the disk radius variations have been properly treated.…”

Models of AM Canum Venaticorum star outbursts

“…Buat-Ménard & Hameury 2002;Schreiber et al 2004;Hameury & Lasota 2005, and references therein). Recently, in a remarkable series of articles Smak (2009a,b,c,d) has undermined the basis of the TTI model by casting doubt on the presence of an eccentric disc in the SU UMa stars and providing strong observational arguments in favour of a different model: the enhanced masstransfer (EMT) model, originally proposed by Osaki (1985) and developed by Lasota et al (1995), Buat-Ménard & Hameury (2002), Schreiber et al (2004) and Hameury & Lasota (2005 (Smak 2008). In this article we will be testing the EMT superoutburst model for several reasons.…”

Survey of period variations of superhumps in SU UMa-type dwarf novae. VII. The seventh year (2014–2015)

“…There has been an attempt to explain WZ Sge-type rebrightenings by an enhanced mass-transfer [originally proposed by Patterson et al (2002) and modeled by Buat-Ménard and Hameury (2002). This model predicts the shrinkage of the disk radius (see figure 5 in Buat-Ménard & Hameury 2002) by an addition of matter with low-angular momentum.…”