New IW And-type star discovered by astronomers

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

Cataclysmic variables (CVs) are binary star systems consisting of a white dwarf primary accreting matter from a normal companion star. They irregularly increase in light by a large factor, then decrease back to a quiescent state.

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

Z Camelopardalis (Z Cams) are a subtype of DNe particularly distinguished by their “pausing” behavior during the decline phase of outbursts, where their luminosity stabilizes about 0.7 magnitude below peak that level. DIM explains the standard standstill by describing the disk as being in a hot, stable state.

However, observations made in the last decade found that some Z Cams, such as IW Andromedae (IW And for short) did not end by returning to the quiescent state, but instead ended in an explosion, which was immediately followed by a dip and then a rapid. back to stop. This unusual behavior was identified as the “anomalous standstill phenomenon” and objects experiencing it were called IW And-type systems and recognized as a subclass of Z Cams.

Now, a team of astronomers, led by Qi-Bin Sun of Yunnan University in China, reports the discovery of such unusual behavior in Karachurin 12—a system classified in 2018 as a Z Cam-type DN. The search is based on photometric data from the All-Sky Automated Survey for Supernovae (ASAS-SN), the Zwicky Transient 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 periods about 5% shorter than the orbital periods exhibited by tilted accretion disks in CVs.

The collected data revealed different cycle patterns in Karachurin 12, with NSH amplitude varying throughout the cycle. The IW And cycle period for Karachurin 12 was measured to be 35.69 days, while the accretion disk precession period was found to be approximately 4.96 days.

Furthermore, the study found that the NSH amplitude of Karachurin 12 decreases with increasing bursts and increases with weakening bursts. Astronomers speculate that this may be related to changes in the radius of the accretion disk.

Overall, the authors of the paper noted that the obtained results for Karachurin 12 point to a potential link between the IW And phenomenon and a tilted disk. Therefore, they suggest that the tilted thermally unstable disk model effectively explains the IW And phenomenon in this system.

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