Fast rotating Blue Stragglers in the globular cluster M4

TL;DR

This study used high-resolution spectroscopy to analyze 20 blue straggler stars in M4, revealing a high fraction of fast rotators and no evidence of chemical depletion, suggesting diverse formation processes.

Contribution

First detection of the largest fraction of fast-rotating blue stragglers in a globular cluster, with detailed kinematic and abundance analysis in M4.

Findings

~40% of BSSs are fast rotators (>50 km/s)

No carbon/oxygen depletion observed in BSSs

High rotation fraction differs from other clusters like 47 Tuc

Abstract

We have used high resolution spectra obtained with the spectrograph FLAMES at the ESO Very Large Telescope to determine the kinematical properties and the abundance patterns of 20 blue straggler stars (BSSs) in the globular cluster M4. We found that ~ 40% of the measured BSSs are fast rotators (with rotational velocities > 50 km/s). This is the largest frequency of rapidly rotating BSSs ever detected in a globular cluster. In addition, at odds with what has been found in 47 Tucanae, no evidence of carbon and/or oxygen depletion has been revealed in the sample of 11 BSSs for which we were able to measure the abundances. This could be due either to low statistics, or to a different BSS formation process acting in M4.