Oxygen- and carbon-rich variable red giant populations in the Magellanic Clouds from EROS, OGLE, MACHO, and 2MASS photometry

TL;DR

This paper introduces a new photometric method to classify C-rich and O-rich AGB stars in the Magellanic Clouds by analyzing the slopes of their tracks in the colour-magnitude diagram, avoiding the need for period data.

Contribution

The authors develop a novel, period-independent photometric classification method for AGB stars based on colour-magnitude diagram slopes, applicable across multiple surveys and filter combinations.

Findings

Successfully separates C-rich and O-rich AGB stars with minimal confusion.

Works effectively across various photometric surveys and filter sets.

Produces comprehensive C/O maps of the Magellanic Clouds.

Abstract

The carbon-to-oxygen (C/O) ratio of asymptotic giant branch (AGB) stars constitutes an important index of evolutionary and environment/metallicity factor. We develop a method for mass C/O classification of AGBs in photometric surveys without using periods. For this purpose we rely on the slopes in the tracks of individual stars in the colour-magnitude diagram. We demonstrate that our method enables the separation of C-rich and O-rich AGB stars with little confusion. For the Magellanic Clouds we demonstrate that this method works for several photometric surveys and filter combinations. As we rely on no period identification, our results are relatively insensitive to the phase coverage, aliasing, and time-sampling problems that plague period analyses. For a subsample of our stars, we verify our C/O classification against published C/O catalogues. With our method we are able to produce C/O maps of the entire Magellanic Clouds. Our purely photometric method for classification of C- and O-rich AGBs constitutes a method of choice for large, near-infrared photometric surveys. Because our method depends on the slope of colour-magnitude variation but not on magnitude zero point, it remains applicable to objects with unknown distances.