Discovery of a super-Li rich turn-off star in the metal poor globular cluster NGC 6397

TL;DR

This paper reports the discovery of a super-Li rich turn-off star in the metal-poor globular cluster NGC 6397, with an unprecedented lithium abundance that challenges existing explanations and suggests possible stellar interaction mechanisms.

Contribution

It presents the first detection of an extremely lithium-rich star in a metal-poor globular cluster, highlighting the need for new models to explain such high lithium enhancement.

Findings

Star has the highest lithium abundance in a Galactic GC dwarf star to date.

High lithium abundance cannot be explained by known enhancement mechanisms.

Spectral analysis suggests possible interaction with a red giant as a cause.

Abstract

We report on the discovery of a super-Li rich turn-off star in the old (12 Gyr), metal poor ([Fe/H]=-2.1 dex) globular cluster (GC) NGC 6397, based on high-resolution MIKE/Magellan spectra. This star shows an unusually high lithium abundance of A(Li)_NLTE = 4.03 +/- 0.06 +/- 0.14 dex (or, 4.21, accounting for possible contamination from a binary companion) that lies above the canonical Li-plateau by a factor of 100. This is the highest Li enhancement found in a Galactic GC dwarf star to date. We discuss several enhancement mechanisms, but none can unambiguously explain such a high overabundance. The spectrum of the star shows a possible indication of binarity, but its line strengths and chemical element abundance ratios are fully compatible with other turn-off stars in this GC, seemingly ruling out mass transfer from an AGB companion as origin of the high A(Li). A possible cause is an interaction with a red giant that has undergone cool bottom processing.