A super-Li rich turnoff star in NGC 6397 - the puzzle persists

TL;DR

This paper discusses the discovery of an exceptionally lithium-rich turnoff star in NGC 6397, highlighting its unique properties and the challenge it poses to existing stellar enrichment theories.

Contribution

It reports the identification of the most Li-enhanced turnoff star in a Galactic globular cluster and explores potential explanations for its unusual lithium abundance.

Findings

Highest Li abundance in a Galactic GC dwarf star to date

No evidence of binarity or AGB mass transfer

Possible interaction with a red giant as enrichment source

Abstract

This presentation focuses on a recently discovered super-Li rich turnoff star in the old, metal poor globular cluster NGC 6397 (Koch et al. 2011, ApJL, 738, L29). Its unusually high NLTE lithium abundance of A(7Li) = 4.21, the highest Li enhancement found in a Galactic GC dwarf star to date, has defied any unambiguous explanation through canonical enrichment channels. Spectra of the star show no convincing evidence for binarity, and measured line strengths and chemical element abundance ratios are fully compatible with other turnoff 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.