New IW And Star Discovery Sparks Excitement

A group of Chinese astronomers has recently studied the dwarf nova known as Karachurin 12 and identified it as an IW And-type star. Their findings, published on September 4 on the preprint server arXiv, provide new insights into this type of celestial object.

Cataclysmic variables (CVs) are binary star systems where a white dwarf accumulates matter from a companion star. These systems experience dramatic fluctuations in brightness, with periodic surges followed by a return to a lower brightness.

In these systems, mass transfer from the companion star typically occurs via an accretion disk around the white dwarf. Occasionally, thermal instability in the disk leads to dwarf nova (DN) outbursts, a phenomenon explained by the accretion disk instability model (DIM).

Z Camelopardalis (Z Cams) are a specific subtype of dwarf novae known for their unique "standstill" phase, where their brightness stabilizes around 0.7 magnitudes below the peak during the decline phase. The DIM model attributes this to a stable, hot disk state.

However, recent observations have shown that some Z Cams, such as IW Andromedae (IW And), exhibit a different behavior. Instead of returning to a quiescent state after an outburst, they undergo an outburst, followed by a dip, and then quickly return to standstill. This unusual pattern, termed the "anomalous standstill phenomenon," has led to the classification of these stars as IW And-type systems.

Led by Qi-Bin Sun from Yunnan University in China, the research team has found similar behavior in Karachurin 12, which was classified as a Z Cam-type DN in 2018. The analysis utilized 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 study revealed that Karachurin 12 is a negative superhump (NSH) system with an accretion disk precession signal.

The data showed varying cycle patterns in Karachurin 12, with fluctuations in the NSH amplitude. The IW And cycle period was measured at 35.69 days, and the disk precession period was about 4.96 days. Additionally, the study found that the NSH amplitude decreases with stronger outbursts and increases with weaker ones, potentially due to changes in the disk's radius.

Overall, the researchers suggest that these findings point to a possible link between the IW And phenomenon and a tilted disk. They propose that the tilted thermally unstable disk model might effectively explain the observed behavior in Karachurin 12.