SDSS J183131.63+420220.2: AM CVn star showing ER UMa-type behavior and long standstill

TL;DR

This paper reports that SDSS J183131.63+420220.2 is an AM CVn star exhibiting ER UMa-type behavior with a long standstill, providing insights into the thermal stability of accretion disks in helium dwarf novae.

Contribution

It identifies a rare AM CVn star with ER UMa and Z Cam characteristics and analyzes its long standstill and orbital period, advancing understanding of disk stability.

Findings

Long standstill of about 6 years observed.

Orbital period estimated at approximately 0.016 d.

Accretion disk near thermal stability threshold.

Abstract

SDSS J183131.63+420220.2 is an AM CVn-type cataclysmic variable. Using Asteroid Terrestrial-impact Last Alert System (ATLAS) and Zwicky Transient Facility (ZTF) data, I found that this object is actually a helium dwarf nova, which experienced a long (~6 yr) standstill (2017 to 2022). The object is currently (in 2023) in ER UMa-type state with supercycles of 20-30 d and large duty cycles exceeding 0.5. This object is the second known star among AM CVn stars having characteristics of ER UMa and Z Cam types. The long duration of the standstill phase is outstanding among helium dwarf novae. These observations indicate that the accretion disk in SDSS J183131.63+420220.2 is very close to thermal stability. I detected a period of 0.01602343(1) d in the ZTF data, which can be the orbital one. Combined with the case of MGAB-V240, the limit of thermal stability of the disks in AM CVn stars appears to be located around the orbital period of 0.0158-0.0160 d.