Evolved Stars in the Galactic Globular Cluster M55 (NGC 6809)

Abstract

We have compiled the asymptotic giant, horizontal, and upper red giant branch (AGB, HB, and RGB) stars in the globular cluster M55 (NGC 6809). Using the star counts and the R-parameter we compute the initial helium abundance Y = 0.274 ± 0.016. The R2 = NAGB/NHB ratio (0.156 ± 0.023) is unusually high for a globular cluster, being almost 2 σ away from the predicted values, and the R1 = NAGB/NRGB ratio (0.272 ± 0.047) is the highest recorded for a massive globular cluster. We argue that M55's particular HB morphology and metallicity have produced long-lived HB stars that are not too blue to avoid producing AGB stars. This result hints that we are able to map evolutionary effects on the HB. Finally, although we find no evidence of variations in HB morphology with distance from the center of the cluster, the red HB stars are significantly less concentrated than the majority of HB stars, and the bluest HB stars are more centrally concentrated.