New IW And-type star discovered by astronomers

A team of Chinese astronomers has performed photometric observations of a dwarf nova known as Karachurin 12. As a result, they have found that Karachurin 12 is an IW And-type star. The discovery was 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 that is accreting matter from a normal star companion. They irregularly increase in brightness by a large factor, then drop back down to a quiescent state.

In CVs, mass transfer from the companion star often occurs through an 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 commonly explained by the accretion disk instability model (DIM).

Z Camelopardalis (Z Cams) are a subtype of DNe particularly distinguished by their “standstill” behavior during the decline phase of outbursts, where their brightness stabilizes approximately 0.7 magnitudes below the peak level. The DIM explains the typical standstill by describing the disk as being in a hot, stable state.

However, observations conducted in the previous decade have found that some Z Cams, like IW Andromedae (IW And for short) did not conclude by returning to the quiescent state, but instead culminated in an outburst, followed immediately by a dip and then a rapid return to standstill. This unusual behavior was identified as the “anomalous standstill phenomenon” and the objects that experience it were dubbed IW And-type systems and recognized as a subclass of Z Cams.

Now, the team of astronomers, led by Qi-Bin Sun of the Yunnan University in China, reports the detection of such unusual behavior in Karachurin 12—a system classified in 2018 as a Z Cam-type DN. The finding is based on the photometric data from the All-Sky Automated Survey for Supernovae (ASAS-SN), the Zwicky Transient Facility (ZTF), and the Transiting Exoplanet Survey Satellite (TESS).

The observations found that Karachurin 12 is a negative superhump (NSH) system with an accretion disk precession signal. In general, NSHs are signals with periods approximately 5% shorter than orbital periods exhibited by tilted accretion disks in CVs.

The collected data revealed diverse 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 outbursts and increases with weakening outbursts. The astronomers assume that this could be related to changes in the radius of the accretion disk.

All in all, 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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