Deep and low mass-ratio contact binaries and their third bodies

TL;DR

This paper reviews over 100 deep, low mass-ratio contact binaries, highlighting their potential to reveal stellar merger processes and the frequent indication of third bodies influencing their period variations.

Contribution

It provides a comprehensive catalog of DLMCBs, analyzes their period changes, and discusses the significance of third bodies in their evolution and merger processes.

Findings

Over 100 DLMCBs identified since 2004.

Approximately half show period variations, including cyclic changes.

Most DLMCBs exhibit signs of third bodies affecting their dynamics.

Abstract

Deep and low mass-ratio contact binaries (DLMCBs) are believed to be in the final stage of their contact phase, potentially leading to the formation of fast-rotating single stars such as FK Com-type stars and blue stragglers, as well as luminous red novae. These systems serve as an excellent laboratory for studying stellar coalescence and merging processes. Our search for DLMCBs began in 2004 and has since identified a group of such systems. Together with that collected from the literature, more than 100 DLMCBs have been detected so far. Half of them have had their periods investigated based on O-C curves. Some have shown period increases, while others have exhibited period decreases. Among them, more than half DLMCBs have cyclic variations, suggesting the possibility of the existence of a third body orbiting around the DLMCBs. Furthermore, with more data obtained extending the span of the O-C curve, more cyclic variations could be detected. The high proportion of signs of the presence of third bodies makes them an essential factor to consider when studying the merger of contact binaries.