Red Supergiants as Cosmic Abundance Probes: The Magellanic Clouds

TL;DR

This study demonstrates a new efficient method using moderate resolution near-infrared spectra of red supergiants to accurately determine metallicities in the Magellanic Clouds, enabling extragalactic chemical abundance research.

Contribution

The paper introduces a novel spectral technique for measuring metallicities of RSGs from a narrow 1μm window, suitable for distant galaxy studies, validated on the Magellanic Clouds.

Findings

Average metallicity of LMC: -0.37 +/- 0.14

Average metallicity of SMC: -0.53 +/- 0.16

Method consistent with other stellar abundance measurements

Abstract

Red Supergiants (RSGs) are cool (~4000K), highly luminous stars (L - 10^5 Lsun), and are among the brightest near-infrared (NIR) sources in star-forming galaxies. This makes them powerful probes of the properties of their host galaxies, such as kinematics and chemical abundances. We have developed a technique whereby metallicities of RSGs may be extracted from a narrow spectral window around 1{\mu}m from only moderate resolution data. The method is therefore extremely efficient, allowing stars at large distances to be studied, and so has tremendous potential for extragalactic abundance work. Here, we present an abundance study of the Large and Small Magellanic Clouds (LMC and SMC respectively) using samples of 9-10 RSGs in each. We find average abundances for the two galaxies of [Z]LMC = -0.37 +/- 0.14 and [Z]SMC = -0.53 +/- 0.16 (with respect to a Solar metallicity of Zsun=0.012). These values are consistent with other studies of young stars in these galaxies, and though our result for the SMC may appear high it is consistent with recent studies of hot stars which find 0.5-0.8dex below Solar. Our best-fit temperatures are on the whole consistent with those from fits to the optical-infrared spectral energy distributions, which is remarkable considering the narrow spectral range being studied. Combined with our recent study of RSGs in the Galactic cluster Per OB1, these results indicate that this technique performs well over a range of metallicities, paving the way for forthcoming studies of more distant galaxies beyond the Local Group.