What's Their Age Again? A Blue-Straggler Merger Scenario in the $\gamma$ Persei Binary System

TL;DR

This study investigates the age and evolutionary state of the $b3$ Persei binary system, proposing a merger scenario to explain observed discrepancies in stellar evolution models.

Contribution

It introduces a merger hypothesis to reconcile the conflicting ages and evolutionary stages of the binary components, supported by stellar evolution modeling with MESA.

Findings

The primary is a post-merger star in a red giant or red clump phase.

The secondary appears near the main-sequence turn-off or early subgiant phase.

A merger event within a few hundred Myr explains the observed stellar properties.

Abstract

We used MIST isochrone fitting and a dedicated grid of stellar evolution models computed with MESA to constrain the ages of the components of the $\gamma$ Persei binary system. While individual stars can be matched to the models at specific metallicities, no joint isochrone solution reproduces both the observed masses and evolutionary states. The stellar evolutionary tracks calculated by \texttt{MESA} reveal a clear evolutionary mismatch. The primary component of the system is in a post-main-sequence phase consistent with the red giant branch or red clump. In contrast, the lighter secondary component lies near the turn-off point of the main-sequence or is in the early phase of the subgiant branch. This discrepancy can be overcome by assuming that the $\gamma$ Persei system was born as a triple and the primary component is a rejuvenated star formed through a merger of a close-by pair of main-sequence stars. We show that the merger must have occurred no later than a few hundred Myr after system formation, and the progenitor masses of the merging stars are restricted by a combination of stars that fall within a narrow band in the $(M_{1,a},M_{1,b})$ plane, corresponding to $M_{1,a}\simeq0.9$-$2.1\,M_\odot$ and $M_{1,b}\simeq2.3$-$2.5\,M_\odot$.