A new technique of discovering red supergiants in nearby galaxies with mid-IR photometry

The role of episodic mass loss in massive star evolution is one of the outstanding questions of current stellar evolution theory. Episodic mass loss produces dust and therefore causes evolved massive stars to be very luminous in the mid-infrared. Currently, the number of investigated massive stars at these late stages is too small to quantitatively constrain stellar mass-loss theories. We present a new technique for identifying evolved massive stars in the Local Group with the final goal of providing a census of dusty stars. We aim to increase the number of investigated luminous mid-IR sources to shed light on the late stages of life of these objects. The method is based on mid-infrared color selection criteria and archival Spitzer Space Telescope images of nearby galaxies. We apply our method to two star-forming dwarf-irregular galaxies in the Local Group: Sextans A and IC 1613. From these galaxies we select 8 targets, which we follow up with spectroscopy. Our spectral classification and analysis yielded the discovery of 5 red supergiants, 1 candidate F-type giant in these galaxies and 2 foreground M giants. We show that our technique is a successful and powerful selection method that can be extended to other galaxies of the Local Group and beyond.