Thermal-viscous instability in tilted accretion disks: A possible application to IW Andromeda-type dwarf novae

Abstract

IW And stars are a subgroup of dwarf novae characterized by repetitive light variations of the intermediate-brightness state with oscillations, which is terminated by brightening. This group of dwarf novae is also known to exhibit a wide variety even within one system in long-term light curves including usual dwarf-nova outbursts, Z Cam-type standstills, and so on, besides the typical IW And-type variations mentioned above. Following the recent observations suggesting that some IW And stars seem to have tilted disks, we have investigated how the thermalviscous instability works in tilted accretion disks in dwarf novae and whether it could reproduce the essential features of the light curves in IW And stars. By adopting various simplifying assumptions for tilted disks, we have performed time-dependent one-dimensional numerical simulations of a viscous disk by taking into account various mass supply patterns to the disk; that is, the gas stream from the secondary star flows not only to the outer edge of the disk but also to the inner portions of the disk. We find that tilted disks can achieve a new kind of accretion cycle, in which the inner disk almost always stays in the hot state while the outer disk repeats outbursts, thereby reproducing alternating mid-brightness interval sometimes with dips and brightening, which are quite reminiscent of the most characteristic observational light variations of IW And stars. Further, we have found that our simulations produce diverse light variations, depending on different mass supply patterns even without time variations in mass transfer rates. This could explain the wide variety in long-term light curves of IW And stars.

Bow shocks, nova shells, disc winds and tilted discs: the nova-like V341 Ara has it all

“…What about the anomalously small "superhump" period deficit we have found in V341 Ara? The super-orbital variations in the NL simulations presented in Kimura et al (2020) are driven entirely by the behaviour of the outer disc. Moreover, Kimura et al (2020) note that a gap may form in the accretion disc near the radius of the stream-disc impact point.…”

“…The light curves produced by their simulations depend strongly on the adopted system parameters and tilt angles, and some of them show promise as models for the IW And phenomenon. Kimura et al (2020) lie in a range where ordinary dwarf nova eruptions would be expected for non-tilted discs. However, the highest accretion rates they consider would produce steady accretion in the absence of any disc tilt.…”

“…Building on this idea, and noting that several systems exhibiting the IW And phenomenon also display negative superhumps, Kimura et al (2020) suggested that spatially confined disc instabilities may occur naturally in tilted accretion discs. The accretion stream from the donor star will impact a tilted accretion disc at a distance from the accretor that depends on both the tilt angle and the precession phase.…”

“…The accretion stream from the donor star will impact a tilted accretion disc at a distance from the accretor that depends on both the tilt angle and the precession phase. Kimura et al (2020) therefore carried out 1-dimensional disc instability simulations in which mass is added in this timeand radius-dependent fashion. The light curves produced by their simulations depend strongly on the adopted system parameters and tilt angles, and some of them show promise as models for the IW And phenomenon.…”

“…Systems accreting at these rates would normally be NL variables. Figure 8 in Kimura et al (2020) shows the predicted light curves for such systems, from simulations designed to mimic tilt angle between 0 • and 15 • . Remarkably, the light curves for moderate to high tilt angles exhibit large-amplitude variations on the precession period, reaching ∆ 2 mag peak-to-peak.…”