Common Envelope Mergers: A Possible Channel for Forming Single sdB Stars

TL;DR

This paper explores a new evolutionary pathway for single sdB stars through mergers of red giants with lower mass companions during the common envelope phase, leading to rapidly rotating stars that may shed mass and become sdB stars.

Contribution

It introduces a population synthesis model demonstrating that mergers during the common envelope phase can produce single sdB stars with specific core mass distributions and high rotation rates.

Findings

Mergers produce rapidly rotating horizontal branch stars.

Core mass distribution peaks between 0.47 and 0.54 solar masses.

Rapid rotation may facilitate mass loss, forming single sdB stars.

Abstract

We quantify an evolutionary channel for single sdB stars based on mergers of binaries containing a red giant star and a lower mass main sequence or brown dwarf companion in our Galaxy. Population synthesis calculations that follow mergers during the common envelope phase of evolution of such systems reveal a population of rapidly rotating horizontal branch stars with a distribution of core masses between 0.32 Mo - 0.7 Mo that is strongly peaked between 0.47 Mo - 0.54 Mo. The high rotation rates in these stars are a natural consequence of the orbital angular momentum deposition during the merger and the subsequent stellar contraction of the merged object from the tip of the red giant branch. We suggest that centrifugally enhanced mass loss facilitated by the rapid rotation of these stars may lead to the formation of single sdB stars for some of these objects.