Chemical and kinematical properties of BSSs and HB stars in NGC 6397

TL;DR

This study investigates the chemical and rotational properties of HB stars, BSSs, and unevolved stars in NGC 6397, revealing temperature-dependent abundance anomalies likely caused by radiative levitation.

Contribution

First evidence of iron abundance deviations in BSSs, linking chemical anomalies to radiative levitation in hot stars within a globular cluster.

Findings

Unevolved stars have low rotational velocities (<10 km/s).

HB and BSSs show broad rotational velocity distribution (0-70 km/s).

Hot HB and BSSs exhibit iron abundance deviations due to radiative levitation.

Abstract

We used three sets of high-resolution spectra acquired with the multifiber facility FLAMES at the Very Large Telescope of the European Southern Observatory to investigate the chemical and kinematical properties of a sample of 42 horizontal branch (HB) stars, 18 Blue Straggler Stars (BSSs) and 86 main sequence turn-off and sub-giant branch stars in the nearby globular cluster NGC 6397. We measured rotational velocities and Fe, O and Mg abundances. All the unevolved stars in our sample turn out to have low rotational velocites (v sin i< 10\kms), while HB stars and BSSs show a broad distribution, with values ranging from 0 to 70 \kms. For HB stars with T<10500 K there is a clear temperature-oxygen anti-correlation, that can be understood if the star position along the HB is mainly determined by the He content. The hottest BSSs and HB stars (with temperatures T>8200 K and T> 10500 K, respectively) also show significant deviations in their iron abundance with respect to the cluster metallicity (as traced by the unevolved stars, [Fe/H]=-2.12). While similar chemical patterns have been already observed in other hot HB stars, this is the first evidence ever collected for BSSs. We interprete these abundance anomalies as due to the metal radiative levitation, occurring in stars with shallow or no convective envelopes.