Population Synthesis at the Crossroads

TL;DR

This paper reviews the current state of population synthesis, highlighting recent revisions that incorporate stochastic effects, stellar multiplicity, rotation, and complex evolutionary phases to improve model accuracy.

Contribution

It identifies key processes such as stochastic effects, stellar multiplicity, and rotation as critical ingredients that are reshaping population synthesis models.

Findings

Stochastic effects can bias initial mass function estimates.

Stellar multiplicity significantly impacts population evolution.

Rotation alters stellar temperatures and luminosities, affecting mass-to-light ratios.

Abstract

The current state-of-the-art of population synthesis is reviewed. The field is currently undergoing major revisions with the recognition of several key processes as new critical ingredients. Stochastic effects can artificially enhance or suppress certain evolutionary phases and/or stellar mass regimes and introduce systematic biases in, e.g., the determination of the stellar initial mass function. Post-main-sequence evolution is often associated with irregular variations of stellar properties on ultra-short time-scales. Examples are asymptotic giant branch stars and luminous blue variables, both of which are poorly treated in the models. Stars rarely form in isolation, and the fraction of truly single stars may be very small. Therefore, stellar multiplicity must be accounted for since many systems will develop tidal interaction over the course of their evolution. Last but not least, stellar rotation can drastically increase stellar temperatures and luminosities, which in turn leads to revised mass-to-light ratios in population synthesis models.