Binary coalescence from case A evolution -- mergers and blue stragglers

TL;DR

This study models main-sequence mergers from binary evolution, analyzing their properties, and compares the results with observations of star clusters, revealing insights into blue straggler formation.

Contribution

It provides a new set of models for binary mergers from case A evolution and fits their observable properties, enhancing understanding of blue straggler origins.

Findings

Mergers can appear on the left of the ZAMS due to helium enrichment.

Central hydrogen content of mergers is independent of total mass.

Models match observations of NGC 2660 with about 0.5 solar masses lost during merging.

Abstract

We constructed some main-sequence mergers from case A binary evolution and studied their characteristics via Eggleton's stellar evolution code. Both total mass and orbital angular momentum are conservative in our binary evolutions. Some mergers might be on the left of the ZAMS as defined by normal surface composition on a CMD because of enhanced surface helium content. The study also shows that central hydrogen content of the mergers is independent of mass. As a consequence, we fit the formula of magnitude and B-V of the mergers when they return back to thermal equilibrium with maximum error 0.29 and 0.037, respectively. Employing the consequences above, we performed Monte Carlo simulations to examine our models in NGC 2682 and NGC 2660. In NGC 2682, binary mergers from our models cover the region with high luminosity, but its importance is much less than that of AML. Our results are well-matched to the observations of NGC2660 if there is about 0.5Mo of mass loss in the merger process.