Chinese astronomers have discovered that the new dwarf star Karachurin 12 is an IW And-type star that exhibits unusual behavior during outbursts. The discovery was made based on photometric observations with two cameras of the All-Sky Automated Supernova Survey (ASAS-SN) mounted on telescopes in Chile and Hawaii, a 576-megapixel camera on the 1.2-meter telescope of the Palomar Observatory at the Zwicky Transient Facility (ZTF) in California, and the Transiting Exoplanet Survey Satellite (TESS).

Cataclysmic variables (CVs) are binary star systems consisting of a white dwarf and a common companion star. They rapidly increase in brightness and then return to rest. In CVs, mass transfer from the companion star usually occurs via an accretion disk around the white dwarf, and in some cases thermal instability in the disk results in explosions known as dwarf novae (DNe).

Z Camelopardalis (Z Cams) is a DNe subtype particularly notable for its “frozen” behavior during the declining phase of the flares, with their brightness stabilizing at about 0.7 magnitudes below their peak level. However, some Z Cams, such as IW Andromedae (IW And), do not end with a return to quiescence, but instead peak abruptly, followed by a decline and a rapid return to quiescence. This unusual behavior has been described as an “anomalous quiescence phenomenon”, and the objects in which it has been observed have been referred to as IW And-type systems and are considered a subclass of Z Cams.

A team of astronomers led by Qi-Bing San from Yunnan University in China reported the discovery of this unusual behavior in Karachurin 12, a system classified as Z Cam type DN, in 2018. The discovery is based on photometric data obtained with ASAS-SN, ZTF and TESS.

Observations have shown that Karachurin 12 has a negative ridge (NSH) where the accretion disk is precessing. NSHs are signals with orbital periods approximately 5% shorter than those exhibited by the inclined accretion disks in the CV.

The collected data revealed various cyclical patterns of Karachurin 12, where the NSH amplitude varies throughout the cycle. The cycle period of IW and Karachurin 12 was measured as 35.69 days, while the precession period of the accretion disk was measured as approximately 4.96 days.

The study also showed that the amplitude of NSH Karachurin 12 decreases with increasing flares and increases with weakening flares. Astronomers suggest that this may be related to a change in the radius of the accretion disk.

The authors of the paper noted that the results obtained for Karachurin 12 indicate a potential connection between the IW And phenomenon and the inclined disk. Therefore, they suggest that the thermally unstable inclined disk model effectively explains the IW And phenomenon in this system.