Blue Stragglers, Young White Dwarfs, and UV‐Excess Stars in the Core of 47 Tucanae

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Abstract
We used a set of archived Hubble Space Telescope/WFPC2 images to probe the stellar population in the core of the nearby galactic globular cluster (GGC) 47 Tuc. From the ultraviolet (UV) color magnitude diagrams (CMDs) obtained for ~4000 stars detected within the Planetary Camera (PC) field of view we have pinpointed a number of interesting objects: (1) 43 blue stragglers stars (BSSs), including 20 new candidates; (2) 12 bright (young) cooling white dwarfs (WDs) at the extreme blue region of the UV-CMD; and (3) a large population of UV-excess (UVE) stars, lying between the BSS and the WD sequences. The colors of the WD candidates identified here define a clean pattern in the CMDs, which define the WD cooling sequence. Moreover, both the location on the UV-CMDs and the number of WDs are in excellent agreement with the theoretical expectations. The UVE stars discovered here represent the largest population of anomalous blue objects ever observed in a globular cluster—if the existence of such a large population is confirmed, we have finally found the long-searched-for population of interacting binaries predicted by the theory. Finally, we have investigated the feasibility of the optical identification of the companions of the binary X-ray sources recently detected by Chandra and of binary millisecond pulsars (MSPs) residing in the core of 47 Tuc. Unfortunately, the extreme faintness expected for the MSP companions, together with the huge stellar crowding in the cluster center, prevents statistically reliable identifications based only on positional coincidences.