FLAMES and XSHOOTER spectroscopy along the two BSS sequences of M30

TL;DR

This study uses FLAMES and XSHOOTER spectroscopy to analyze 15 Blue Straggler Stars in M30, revealing their rotational velocities, chemical abundances, and formation mechanisms, with findings consistent across two BSS sequences and comparable to other globular clusters.

Contribution

First spectroscopic comparison of BSSs along two sequences in M30, highlighting rotational and chemical properties linked to their formation processes.

Findings

Most BSSs have low rotational velocities (~30 km/s)

Stars hotter than 7800 K show radiative levitation effects

Red sequence BSSs exhibit oxygen depletion consistent with mass transfer formation

Abstract

We present spectroscopic observations acquired with FLAMES and XSHOOTER at the Very Large Telescope for a sample of 15 Blue Straggler Stars (BSSs) in the globular cluster (GC) M30. The targets have been selected to sample the two BSS sequences discovered, with 7 BSSs along the blue sequence and 8 along the red one. No difference in the kinematical properties of the two groups of BSSs has been found. In particular, almost all the observed BSSs have projected rotational velocity lower than ~30 km/s, with only one (blue) fast rotating BSS (>90 km/s), identified as a W UMa binary. This rotational velocity distribution is similar to those obtained in 47 Tucanae and NGC 6397, while M4 remains the only GC studied so far harboring a large fraction of fast rotating BSSs. All stars hotter than ~7800 K (regardless of the parent BSS sequence) show iron abundances larger than those measured from normal cluster stars, with a clearcut trend with the effective temperature. This behaviour suggests that particle trasport mechanisms driven by radiative levitation occur in the photosphere of these stars, as already observed for the BSSs in NGC 6397. Finally, 4 BSSs belonging to the red sequence (not affected by radiative levitation) show a strong depletion of [O/Fe], with respect to the abundance measured in Red Giant Branch and Horizontal Branch stars. This O-depletion is compatible with the chemical signature expected in BSSs formed by mass transfer processes in binary systems, in agreement with the mechanism proposed for the formation of BSSs in the red sequence.