# Detection of a white dwarf orbiting a Carbon-Oxygen depleted blue straggler in 47 Tucanae

## TL;DR

This study used deep UV observations from the Hubble Space Telescope to identify a white dwarf companion orbiting a blue straggler star in 47 Tucanae, providing direct evidence of a mass transfer formation process.

## Contribution

First to simultaneously detect chemical signatures and a hot white dwarf companion indicating mass transfer origin in a blue straggler.

## Key findings

- Detected a hot white dwarf companion to one blue straggler.
- Confirmed chemical depletion as a signature of mass transfer.
- Found no hot companions for other blue stragglers, suggesting transient signatures.

## Abstract

We have 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 stragglers 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 > 20000 K) WD companion to one of the investigated BSS, 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.