Recently, PhD student LI Xuzhi and Prof. LIU Liang, from Yunnan Observatories of the Chinese Academy of Sciences, using the Kepler data, studied the contact binaries in the open cluster NGC 6819. The study was published online in The Astronomical Journal.

A contact binary is a close binary system whose components fill their critical Roche lobe and share a common envelope. The evolution of a contact binary is extremely complex because of the strong interaction between the component stars. Meanwhile, the members of an open cluster have the same age, metal abundance, interstellar extinction conditions, etc. Therefore, there are many advantages in studying contact binaries in the same open cluster, especially for the evolution of binary systems.

In this study, the researchers analyzed seven EW-type binary systems in the field of NGC 6819 by using the Kepler data and some spectral data. They gave the system parameters of those contact binaries, and used the period-color relation to estimate the inferred physical parameters.

Basing on the parameter characteristics of the contact binary, the researchers found two deep, low mass ratio contact binaries (DLMRCBs) whose mass ratios are less than 0.25 and the fill-out factors are more than 50%. Model calculations suggested that these DLMRCBs will finally merge into single stars, such as blue stragglers or FK-Com-type stars. They also found one contact binary with a mass ratio very close to unity, which indicates that this system is in the early contact stage or a mass-ratio reverse stage.

The researchers plan to use Transiting Exoplanet Survey Satellite (TESS) data to continue searching for contact binaries in open clusters, and to study the relationship between the dynamic evolution of the open cluster and the number of contact binaries in the cluster.