Yonsei Evolutionary Population Synthesis (YEPS) Model. I. Spectroscopic Evolution of Simple Stellar Populations

TL;DR

The paper introduces the spectroscopic features of the updated Yonsei Evolutionary Population Synthesis (YEPS) model, emphasizing the impact of horizontal-branch stars on age and metallicity estimations of old stellar populations.

Contribution

It presents the first detailed spectroscopic analysis of the 2012 version of the YEPS model, incorporating horizontal-branch effects on spectral line indices.

Findings

Horizontal-branch stars significantly affect Balmer and metallicity-sensitive lines.

Inclusion of horizontal-branch stars can lead to up to 5 Gyr differences in age estimates.

The model explains globular cluster index bimodality and improves metallicity determinations.

Abstract

We present a series of papers on the year-2012 version of Yonsei Evolutionary Population Synthesis (YEPS) model which is constructed on over 20 years of heritage. This first paper delineates the spectroscopic aspect of integrated light from stellar populations older than 1 Gyr. The standard YEPS is based on the most up-to-date Yonsei-Yale stellar evolutionary tracks and BaSel 3.1 flux libraries, and provides absorption line indices of the Lick/IDS system and high-order Balmer lines for simple stellar populations as functions of stellar parameters, such as metallicity, age and {\alpha}-element mixture. Special care has been taken to incorporate systematic contribution from horizontal-branch stars which alters the temperature-sensitive Balmer lines significantly, resulting in up to 5 Gyr difference in age estimation of old, metal-poor stellar populations. We also find that the horizontal branches exert an appreciable effect not only on the Balmer lines but also on the metallicity-sensitive lines including the magnesium index. This is critical to explain the intriguing bimodality found in index distributions of globular clusters in massive galaxies and to derive spectroscopic metallicities accurately from various indices. A full set of the spectroscopic and photometric YEPS model data of the entire parameter space is currently downloadable at http://web.yonsei.ac.kr/cosmic/data/YEPS.htm