A group of Chinese astronomers conducted photometric observations of the dwarf nova known as Karachurin 12. As a result, they discovered that Karachurin 12 is an IW And-type star. The discovery was detailed in a research paper published on the preprint server on September 4. arXiv.

Cataclysmic variables (CVs) are binary star systems that form from a primary white dwarf accreting material from a partner star. Their brightness increases dramatically in an unstable manner and then returns to a dormant state.

In CVs, mass transfer from the companion star usually occurs through the accretion disk around the white dwarf, and in some cases thermal instability in the disk results in explosions known as dwarf novae (DNe). This behavior is often explained by the accretion disk instability model (DIM).

Z Camelopardalis (Z Camelopardalis) is a DNe subspecies that is particularly notable for its “dimming” behavior during the decay phase of flares, when their brightness stabilizes at about 0.7 magnitudes below maximum. DIM describes the driver as a warm and stable state, explaining the typical stall.

However, observations made in the previous decade revealed that some Z-cells, such as IW Andromedae (abbreviated as IW And), do not terminate in a dormant state, but instead terminate with a burst, immediately followed by a drop, and then a rapid return to real estate. This unusual behavior was described as the “anomalous stationarity phenomenon”, and objects that experience it were called IW And-type systems and were considered a subclass of Z-cells.

Now, a team of astronomers led by Qi-Bin Sun from Yunnan University in China has reported the discovery of such unusual behavior in Karachurin 12, a system classified as Z Cam-type DN in 2018. The discovery is based on photometric data from the Automated Earthwide Supernova Search (ASAS-SN), the Zwicky Transit Facility (ZTF), and the Transiting Exoplanet Survey Satellite (TESS).

Observations revealed that Karachurin 12 is a negative superhump (NSH) system with an accretion disk precession signal. In general, 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 cycle patterns in Karachurin 12, where the NSH amplitude varies throughout the cycle. The IW Andean cycle period for Karachurin 12 was determined to be 35.69 days, while the precession period of the accretion disk was determined to be 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 due to a change in the radius of the accretion disk.

Overall, the authors of the paper stated that the results obtained for Karachurin 12 point to 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.