Search for White Dwarf companions of Blue Straggler Stars in the Globular Cluster 47 Tucanae

Blue Straggler Stars (BSSs) are an exotic population of hydrogen-burning objects not predicted by the theory of single star evolution, and are excellent tracers of internal dynamics of dense stellar systems. They can form either by direct collisions or by mass transfer (MT) in binary systems. However, many details of these scenarios are still unclear, and it is extremely difficult to distinguish between collisional and MT-BSSs. In the MT scenario, a giant star that fills its Roche lobe gradually loses mass to its companion, which becomes a BSS. The material deposited on the surface of the latter should therefore come from the innermost parts of the donor star, where it has been processed through the CNO cycle. Hence, carbon and oxygen depletion is expected on the surface of MT-BSSs, with respect to normal cluster stars: the "chemical signature" of the MT process. On the other hand, the "peeled" core of the donor star should leave a white dwarf (WD) companion to the BSS. This can be revealed by the observation of a strong excess at wavelengths below 2000 Å, where the flux of the BSS is completely negligible: the "photometric signature" of the MT process. The aim of this work was to search for the WD companions to 5 BSSs showing the "chemical signature" in the globular cluster 47 Tucanae. To this end, photometric images obtained in three far UV filters with the Solar Blind Camera (SBC/ACS) on board the HST were analysed. The combination of these data with archival catalogs has been used to build the spectral energy distribution (SED) of each BSS from the far UV to the K-band. The comparison with theoretical SEDs revealed a strong UV excess for one BSS, which can be properly reproduced by the presence of a WD of ~15000-28000 K. This is the first observational evidence for the simultaneous presence of the two signatures of the MT formation channel, and it also allowed a rough estimate of the BSS formation epoch from the WD cooling sequence.