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

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.

Figures (10)

• Figure 1: Normalised spectra of ordinary SU UMa-type stars (HT Cas, BZ UMa, V436 Cen, and T Leo -- four upper spectra) and WZ Sge-type objects (SSS J122221.7-311525, EG Cnc, WZ Sge, and BW Scl -- four lower spectra). The orbital periods of all the objects are indicated in the plot legend. The plot emphasises the Balmer absorption features and continuum, thus, the Balmer emission lines in some objects do not fit in the plot. The spectra of the WZ Sge-type objects clearly exhibit the presence of the broad Balmer absorption lines from a WD, whereas the ordinary SU UMa-type stars do not.
• Figure 2: Simulated spectra of a CV calculated as a sum of a WD model spectrum and a power law with added double-peaked emission lines of arbitrary strength, mimicking an accretion disc spectrum. The left panel shows the WD and disc spectra with the same flux at 4600 Å, along with their combination. The right panel shows three combined spectra with different flux ratios.
• Figure 3: Left: the spectrum of BW Scl, shown together with the best-fitting WD model spectrum. The WD emission (grey dashed line) contributes 90 cent of the total system flux at 4600 Å. Right: The accretion disc spectrum obtained after a subtraction of the WD spectrum from the spectrum of BW Scl. Its colour temperature is 10580$\pm$40 K.
• Figure 4: Left: Accretion disc spectra of EG Cnc with (red) and without (blue) hotspot contribution to the Balmer lines. Right: Accretion disc spectrum of BW Scl (black solid line) and an example (not the best-fit) of a hydrogen slab model (blue dashed line) calculated with the code of V. Suleimanov.
• Figure 5: Profiles of the H$\alpha$ emission lines shown together with the corresponding model fits.