Spectral Types of Red Supergiants in NGC 6822 and the Wolf-Lundmark-Melotte Galaxy

TL;DR

This study spectroscopically characterizes red supergiants in low-metallicity galaxies NGC 6822 and WLM, revealing spectral type shifts with metallicity and identifying potential extreme RSG variables.

Contribution

It extends the observational evidence of metallicity-dependent spectral type shifts in RSGs to lower metallicities and reports new variable RSGs in these galaxies.

Findings

Spectral types of RSGs correlate with metallicity.

Discovered two RSGs with unusually late spectral types.

Identified a RSG with spectral type variation over 11 years.

Abstract

We present moderate-resolution spectroscopic observations of red supergiants (RSGs) in the low-metallicity Local Group galaxies NGC 6822 (Z = 0.4Zsun) and Wolf-Lundmark-Melotte (WLM; Z = 0.1Zsun). By combining these observations with reduction techniques for multislit data reduction and flux calibration, we are able to analyze spectroscopic data of 16 RSGs in NGC 6822 and spectrophotometric data of 11 RSGs in WLM. Using these observations we determine spectral types for these massive stars, comparing them to Milky Way and Magellanic Clouds RSGs and thus extending observational evidence of the abundance-dependent shift of RSG spectral types to lower metallicities. In addition, we have uncovered two RSGs with unusually late spectral types (J000158.14-152332.2 in WLM, with a spectral type of M3 I, and J194453.46-144552.6 in NGC 6822, with a spectral type of M4.5 I) and a third RSG (J194449.96-144333.5 in NGC 6822) whose spectral type has varied from a M2.5 in 1997 to a K5 in 2008. All three of these stars could potentially be members of a recently-discovered class of extreme RSG variables.