# Yonsei evolutionary population synthesis (YEPS). II. Spectro-photometric evolution of helium-enhanced stellar populations

## TL;DR

This paper develops spectro-photometric evolution models for stellar populations with varying initial helium abundance, revealing significant effects on their observable properties and implications for understanding multiple stellar populations in globular clusters.

## Contribution

It introduces new evolutionary population synthesis models accounting for helium enhancement, highlighting their impact on spectro-photometric properties and their relation to second-generation globular cluster stars.

## Key findings

- Helium enhancement causes dramatic changes in spectral and photometric properties.
- The evolution of helium-enhanced populations depends on metallicity and age.
- Models provide insights into second-generation stellar populations in globular clusters.

## Abstract

The discovery of multiple stellar populations in Milky Way globular clusters (GCs) has stimulated various follow-up studies on helium-enhanced stellar populations. Here we present the evolutionary population synthesis models for the spectro-photometric evolution of simple stellar populations (SSPs) with varying initial helium abundance ($Y_{\rm ini}$). We show that $Y_{\rm ini}$ brings about {dramatic} changes in spectro-photometric properties of SSPs. Like the normal-helium SSPs, the integrated spectro-photometric evolution of helium-enhanced SSPs is also dependent on metallicity and age for a given $Y_{\rm ini}$. {We discuss the implications and prospects for the helium-enhanced populations in relation to the second-generation populations found in the Milky Way GCs.} All of the models are available at \url{http://web.yonsei.ac.kr/cosmic/data/YEPS.htm}.