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

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.

7 pages, 3 figures, to appear in the proceedings of 'Lithium in the Cosmos', Paris, Feb. 27-29, 2012, Memorie della Societa' Astronomica Italiana Supplementi