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

TL;DR

This paper addresses the apparent non-coeval evolution in the γ Persei binary by testing coeval single-star evolution against detailed stellar models. Using MIST isochrones and a tailored grid of MESA tracks, the authors find that joint fits are either inconsistent with observed masses or require implausible metallicities, signaling a breakdown of standard coevality. They propose a triple-origin scenario in which the primary is a rejuvenated merger product (a blue straggler) formed from a MS+MS collision, while the secondary traces the true system age of ~750–900 Myr. Consequently, the merger likely occurred ~150–200 Myr after formation, within a 500–775 Myr age window from now, with progenitor masses confined to a narrow diagonal band near M1a ≈ 0.9–2.1 Msun and M1b ≈ 1.7–2.5 Msun; this framework explains the observed evolutionary states and anchors the system’s formation history.

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 $γ$ 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 $γ$ 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 $M_{1,b}\simeq2.3$-$2.5\,M_\odot$.

Figures (11)

• Figure 1: Evolutionary tracks for the $\gamma$ Persei primary component, colour--coded by stellar age. The three subpanels illustrate representative models that place the star in distinct post-main-sequence evolutionary phases. From left to right: the end of the subgiant branch; the red--clump phase; either the upper red--giant branch or the early asymptotic--giant branch. The position of the primary component is shown in cyan color with observational uncertainties.
• Figure 2: Evolutionary tracks for the $\gamma$ Persei secondary component, colour--coded by stellar age. The three subpanels illustrate representative models that place the star in distinct evolutionary stages. From left to right: subgiant phase; a model located in the vicinity of the main-sequence turn-off point; a main-sequence phase. The observed position of the secondary component is shown with its measurement uncertainties (green triangle), and the main-sequence turn-off point is marked for each model with a red circle.
• Figure 3: The four best-fitting models for the primary component of the $\gamma$ Persei system. Each model corresponds to the red-clump phase of stellar evolution. The yellow point corresponds to the best-fitting model, while the purple point (with error bars) presents the place of the primary component.
• Figure 4: The four best-fitting models for the secondary component of the $\gamma$ Persei system. The panels correspond to different evolutionary stages: the two left-hand panels show models near the main-sequence turn-off (the lower-left panel is still on the late main-sequence), while the upper-right panel represents a subgiant model. The yellow point corresponds to the best-fitting model, while the purple point (with errorbars) presents the place of the primary component.
• Figure 5: Allowed combinations of the progenitor masses $(M_{1,a},M_{1,b})$. The blue region shows the parameter space permitted by the total-mass constraint ($3.4$--$4.2\,M_\odot$) and the requirement that both stars remain below the turn-off mass ($M_{\rm TO}\simeq2.3-2.5\,M_\odot$). The light red shaded regime denotes the region where $M_{1,b}>M_{1,a}$, as assumed throughout the analysis. The red points indicate the extreme allowed configurations discussed in the text: the lower-mass limit ($M_{1,a}^{\rm lower}=0.9\,M_\odot$, $M_{1,b}^{\rm lower}=2.5\,M_\odot$), the upper-mass limit ($M_{1,a}^{\rm upper}=1.7\,M_\odot$, $M_{1,b}^{\rm upper}=2.5\,M_\odot$), and the equal-mass upper case ($M_{1,a}=M_{1,b}=2.1\,M_\odot$). Only mass pairs that lie within the diagonal band satisfy all physical constraints.