ASASSN-22ak: La Belle au bois dormant in a hydrogen-depleted dwarf nova?

TL;DR

This paper reports on the 2022 outburst of ASASSN-22ak, a hydrogen-depleted dwarf nova, revealing its similarities to AM CVn stars and suggesting it is an evolved cataclysmic variable with low hydrogen content.

Contribution

It presents detailed observations of ASASSN-22ak's outburst, analyzes its superhumps, and proposes its evolutionary link to AM CVn stars based on its low hydrogen and high helium content.

Findings

ASASSN-22ak shows low-amplitude superhumps with a period of 0.042876 days.

It is similar to CRTS J112253.3-111037, an object near AM CVn stars.

The object likely has an evolved core with depleted hydrogen in the secondary.

Abstract

ASASSN-22ak is a transient discovered by the All-Sky Automated Survey for Supernovae and by Gaia in 2022 January. Although this object had been in deep quiescence at least for seven years before this outburst, it has been showing relatively regular long (35-40 d) outbursts with intervals of 132-188 d after the 2022 January outburst. This "waking up" phenomenon appears similar to the very unusual (hydrogen-rich) WZ Sge star V3101 Cyg. Time-resolved photometry during the 2023 outburst detected low-amplitude (0.05 mag) superhumps with a period of 0.042876(3) d. ASASSN-22ak appears to be very similar to CRTS J112253.3-111037, which is known to have a very low mass ratio and is considered to be an object evolving close to AM CVn stars as inferred from the low hydrogen and high helium content. ASASSN-22ak is likely yet another object having an evolved core and strongly depleted hydrogen in the secondary. The case of ASASSN-22ak strengthens the idea that a considerable fraction of AM CVn stars are formed from evolved cataclysmic variables. Both ASASSN-22ak and V3101 Cyg before the initial outbursts were probably in dormant states with low quiescent viscosity or low mass-transfer rates. The current "high" states of ASASSN-22ak and V3101 Cyg may have been induced by radiation during the initial outburst or these objects are simply returning to ordinary states, either in terms of quiescent viscosity or mass-transfer rates. We also provide updated superhump period and estimated mass ratio for CRTS J112253.3-111037.