The accretion discs in WZ Sge-type stars in deep quiescence. How do they outburst?

TL;DR

This study analyzes WZ Sge-type dwarf novae in deep quiescence, finding their accretion discs are too cold and low in mass to trigger outbursts via the disc instability model, challenging existing theories about superoutburst mechanisms.

Contribution

The paper provides observational evidence that contradicts the disc instability model's predictions for superoutburst triggers in WZ Sge-type stars, and suggests lower mass transfer rates than previously assumed.

Findings

Accretion discs are optically thin with low surface density.

No brightness increase observed before superoutbursts.

Mass transfer rates are an order of magnitude lower than standard assumptions.

Abstract

WZ Sge-type stars are an extreme subclass of dwarf novae characterised by very rare, large-amplitude superoutbursts. Within the disc instability model (DIM), such events are explained as being triggered by enhanced mass transfer from the donor star. We present an analysis of observations of a sample of WZ Sge-type systems in deep quiescence to assess the consistency of DIM predictions with their observed properties. We find that accretion discs in quiescent WZ Sge-type systems have very low mass-accretion rates of a few $\times$$10^{-13}$ M$_\odot$ yr$^{-1}$. The discs are entirely optically thin, and their physical conditions -- such as surface density and effective temperature -- remain well below the DIM thresholds required to trigger an outburst. Observationally, no increase in disc brightness is detected prior to the superoutburst, indicating the absence of a transition to an optically thick state, in contrast to DIM predictions of a gradual disc thickening preceding the instability. We therefore find no observational evidence that superoutbursts in WZ Sge-type systems are triggered by enhanced mass transfer from the donor. Furthermore, the inferred mass-transfer rates in these objects ($\dot{M}_{\rm tr}$~5$\times$$10^{-12}$ M$_\odot$ yr$^{-1}$) are at least an order of magnitude lower than commonly assumed. We argue that the widely adopted value of $\dot{M}_{\rm tr}$ for the prototype object WZ Sge is likely overestimated. Finally, we show that in quiescence the accretion disc radius in all systems is close to the tidal truncation radius and exceeds the 3:1 resonance radius, confirming earlier results and calling into question the standard interpretation of superhump formation.