Reevaluating Old Stellar Populations (1805.08784v1)

Abstract: Determining the properties of old stellar populations (those with age >1 Gyr) has long involved the comparison of their integrated light, either in the form of photometry or spectroscopic indexes, with empirical or synthetic templates. Here we reevaluate the properties of old stellar populations using a new set of stellar population synthesis models, designed to incorporate the effects of binary stellar evolution pathways as a function of stellar mass and age. We find that single-aged stellar population models incorporating binary stars, as well as new stellar evolution and atmosphere models, can reproduce the colours and spectral indices observed in both globular clusters and quiescent galaxies. The best fitting model populations are often younger than those derived from older spectral synthesis models, and may also lie at slightly higher metallicities.

• The paper introduces BPASS v2.2, a model that integrates binary evolution pathways to challenge traditional single-star assumptions.
• The updated models accurately reproduce photometric colors and spectral indices in globular clusters and quiescent galaxies, indicating revised ages and metallicities.
• The research implies that incorporating binary interactions is crucial for refining stellar evolution theories and cosmic evolutionary timelines.

Reevaluating Old Stellar Populations: A New Insight into Stellar Evolution Models

The paper titled "Reevaluating Old Stellar Populations" by Stanway and Eldridge presents a compelling revision of our understanding of old stellar populations through enhanced stellar population synthesis models. By incorporating binary stellar evolution pathways, the authors significantly advance the analysis of integrated light from stellar populations that are older than 1 Gyr.

Summary of Findings

The paper reexamines longstanding assumptions in stellar population models by introducing the BPASS v2.2 models, which feature an expanded grid of stellar models and a refined treatment of binaries. Notably, this version accounts for stellar interactions such as mass transfer and rejuvenation, which are particularly relevant for stars in binary systems. The results from BPASS v2.2 suggest that previous models may have underestimated the complexity involved in old stellar systems, primarily by omitting interactions between stars.

Key results include the ability of the new models to match photometric colors and spectral indices observed in both globular clusters and quiescent galaxies across various environments, suggesting that these populations might be younger and at slightly higher metallicity than previously estimated. This insight aligns the model outputs closer to empirical observations and provides a new perspective on the ages and characteristics of these ancient systems.

Numerical Results and Bold Claims

The results from the BPASS v2.2 models highlight deviations from older models, particularly in the predicted ages and metallicities of stellar populations. For instance, they predict smaller $K$-band mass-to-light ratios at late ages, particularly in comparison to models with a Salpeter IMF, due to the contribution of low mass, less luminous stars not considered in older models. This leads to a consistent inference that resolved assumptions about isolated stellar evolution inadequately captured the dynamics of star populations.

Implications for Stellar Population Analysis

The implications of adopting binary-interactions in modeling are profound, both theoretically and practically. Theoretically, the paper challenges the traditional single-star evolution approach, impacting the characterization of stellar population ages and contributing to an improved understanding of stellar dynamics. Practically, this research suggests that current constraints on star formation history in globular clusters and galaxies need reconsideration, potentially leading to revisions in the timeline of cosmic evolution.

Moving forward, the authors point towards incorporating additional complexities such as $\alpha$-element enhancement and improved post-main-sequence stellar wind treatments. The paper also opens avenues for using transient event rates as empirical constraints for stellar population models, offering a new approach to verify these theoretical frameworks through observations.

In conclusion, this paper significantly advances the field by providing a more accurate and nuanced understanding of old stellar populations. By considering binary interactions, it suggests necessary paradigm shifts in modeling approaches and sets a new benchmark for future research on star formation and evolution in the Universe. As these models are integrated into broader astrophysical research, we can expect a more precise characterization of the cosmos' oldest stars and the history they reveal.