Observational discovery of the asymptotic giant branch bump in densely populated color-magnitude diagrams of galaxies and star clusters

The huge photometric databases that are being created for stars in the Magellanic Clouds and in Local Group galaxies not only allow a detailed study of the star formation history of these systems but also provide important information on stellar evolutionary theory. Two low-level features above the fed clump in the LMC color-magnitude diagram (CMD) have been discussed by Zaritsky & Lin and Alcock et al. as the possible signature of an intervening population. I conclude that one of the features, which has also been associated with the red giant branch (RGB) bump predicted by theory and observed in globular clusters, is instead produced during asymptotic giant branch (AGE) evolution. I will call it the AGE bump. Ln this Letter, it will be shown that the stellar evolution predictions about the position and strength of the AGE bump are in very good agreement with the observed structures in the LMC, M31, and globular clusters. The position in the CMD of the other feature is consistent with the location of the blue loops of stars that are a few hundred megayears old, and it has also been observed in the CMD of other galaxies with a young stellar population.