Detection of a white dwarf orbiting a carbon-oxygen-depleted blue straggler in 47 Tucanae

We employed deep far-UV observations secured with the Solar Blind Channel of the Advanced Camera for Surveys onboard the Hubble Space Telescope to search for hot companions to five blue straggler stars (BSSs) showing significant surface depletion of carbon (C) and oxygen (O), in the Galactic globular cluster 47 Tucanae. Such a chemical pattern has been interpreted as the chemical signature of the mass-transfer formation process for the observed blue stragglers. The mass transfer origin is also expected to leave a photometric signature’ in the form of a UV-excess, as the stripped core of the donor star should be observable as a white-dwarf (WD) companion orbiting the newborn BSS. We found strong evidence for the presence of a hot ( T > 20 000 K) WD companion to one of the investigated BSSs, indicating that it likely formed through mass transfer less than ~12 Myr ago. This is the first simultaneous evidence of the chemical and the photometric signatures of the mass-transfer formation channel. The lack of evidence for a hot companion to the other investigated blue stragglers is consistent with the expectation that the photometric signature (as well as the chemical one) is a transient phenomenon.