Supra-Horizontal-Branch Stars and Population Gradients in the Galactic Center Globular Cluster NGC 6522

Abstract

We have imaged the core of NGC 6522 and an adjacent field 1' north in the U and B passbands using the (pre-COSTAR) HST Planetary Camera. We find 13 stars located 1.5 mag above the horizontal branch. These supra-horizontal-branch (SHB) stars have not been noted previously in NGC 6522, although they occur in other globular clusters. The SHB stars' radial distribution appears to be similar to those of the red giants. There are indications that the NGC 6522 core red giants are depleted relative to the horizontal branch stars. The SHB stars may be evolved blue horizontal-branch stars, the tidally stripped cores of red giants, or evolved blue stragglers. There is also a well-populated horizontal branch with a large number of very blue stars; in these passbands the overall color-magnitude diagram resembles that of the core collapse cluster M15.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA for NASA under contract NAS 5-26555