M supergiants in Local Group irregular galaxies: metallicities and distances

JHK photometry of confirmed and suspected red supergiants in NGC 3109, NGC 6822, IC 1613, Sextans A, and DDO 210 together with CO and H2O indices of some of the brighter stars has been done. The color and spectral type distributions in NGC 6822 and IC 1613 indicate that NGC 6822 is slightly more metal-rich than the Small Magellanic Cloud, while IC 1613 is more metal-poor. Sextans A is apparently comparable in metallicity to IC 1613, while NGC 3109 is probably comparable to the SMC. The absolute K magnitudes of the brightest red supergiants become brighter as the parent galaxy luminosity increases. The scatter about the mean relation is about 0.1 mag, allowing an estimte of 25.34 for the true distance modulus of NGC 3109. Comparison with data from Humphreys, Jones, and Sitko for the spiral galaxy M33 shows that it fits the irregular calibration less well.