Young LMC clusters: the role of red supergiants and multiple stellar populations in their integrated light and CMDs

TL;DR

This study investigates the optical spectra of young LMC clusters, revealing the importance of red supergiants and potential age-spreads, by combining integrated spectra, UV data, and CMD analysis to improve stellar population models.

Contribution

The paper introduces new young stellar population models incorporating red supergiants and compares multiple methods to better understand cluster stellar populations.

Findings

Red supergiants significantly influence optical spectra.

Models require about 20% more RSG luminosity than canonical predictions.

Possible presence of age-spreads in young clusters cannot be ruled out.

Abstract

The optical integrated spectra of three LMC young stellar clusters (NGC 1984, NGC 1994 and NGC 2011) exhibit concave continua and prominent molecular bands which deviate significantly from the predictions of single stellar population (SSP) models. In order to understand the appearance of these spectra, we create a set of young stellar population (MILES) models, which we make available to the community. We use archival International Ultraviolet Explorer integrated UV spectra to independently constrain the cluster masses and extinction, and rule out strong stochastic effects in the optical spectra. In addition, we also analyze deep colour-magnitude diagrams of the clusters to provide independent age determinations based on isochrone fitting. We explore hypotheses including age-spreads in the clusters, a top-heavy initial mass function, different SSP models and the role of red supergiant stars (RSG). We find that the strong molecular features in the optical spectra can only be reproduced by modeling an increased fraction of about 20 per cent by luminosity of RSG above what is predicted by canonical stellar evolution models. Given the uncertainties in stellar evolution at Myr ages, we cannot presently rule-out the presence of Myr age-spreads in these clusters. Our work combines different wavelengths as well as different approaches (resolved data as well as integrated spectra for the same sample) in order to reveal the complete picture. We show that each approach provides important information but in combination can we better understand the cluster stellar populations.