Carbon Stars in the Satellites and Halo of M31

TL;DR

This study spectroscopically identifies and analyzes the properties of 41 carbon stars in M31's satellites and halo, revealing their photometric and spectral characteristics and potential origins.

Contribution

It provides the first detailed spectroscopic and photometric analysis of carbon stars in M31's satellites and halo, including their temperature, metallicity, and possible extrinsic origin.

Findings

Identification of two populations of carbon stars based on brightness and color.

Spectroscopic analysis links spectral features to temperature and metallicity.

Evidence suggests some sub-TRGB stars may be extrinsic or faint AGB stars.

Abstract

We spectroscopically identify a sample of carbon stars in the satellites and halo of M31 using moderate-resolution optical spectroscopy from the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo survey. We present the photometric properties of our sample of 41 stars, including their brightness with respect to the tip of the red giant branch (TRGB) and their distributions in various color-color spaces. This analysis reveals a bluer population of carbon stars fainter than the TRGB and a redder population of carbon stars brighter than the TRGB. We then apply principal component analysis to determine the sample's eigenspectra and eigencoefficients. Correlating the eigencoefficients with various observable properties reveals the spectral features that trace effective temperature and metallicity. Putting the spectroscopic and photometric information together, we find the carbon stars in the satellites and halo of M31 to be minimally impacted by dust and internal dynamics. We also find that while there is evidence to suggest that the sub-TRGB stars are extrinsic in origin, it is also possible that they are are particularly faint members of the asymptotic giant branch.