Merger of Noncompact Binaries: Triggering Mechanisms and Merger Timescales of W UMa Contact Binaries

Abstract The merger of noncompact binaries can produce a variety of significant celestial objects, including blue stragglers, luminous red novae, and others. However, several key details of the merger of noncompact binaries remain unclear. Using the MESA stellar evolution code, we investigate the triggering mechanisms and merger timescales of observed W UMa contact binaries (a type of noncompact binary that is closest to merging). Our results show that for the majority of observed W UMa contact binaries (89%), the merger-triggering mechanism is tidal instability, while thermal instability accounts for the remaining cases (11%). We suggest that tidal instability is the dominant mechanism for triggering the mergers of W UMa contact binaries. In addition, our results indicate that the distribution of merger timescales for observed W UMa contact binaries ranges from ∼2.5 Myr to ∼1.0 Gyr, peaking at 0.26 Gyr, with an average merger timescale of 0.36 Gyr. We suggest that the merger timescale of each W UMa contact binary may be a more comprehensive criterion for identifying potential merger candidates than the mass ratio or other individual parameters. Finally, we present some promising merger candidates of W UMa contact binaries (e.g., those with the shortest or undetermined merger timescales) for future studies on noncompact binary mergers.