Curiouser and curiouser: the peculiar chemical composition of the Li/Na-rich star in Omega Centauri

TL;DR

This study analyzes a unique Li/Na-rich giant star in Omega Centauri, revealing its chemical peculiarities and suggesting it may have accreted material from an evolved companion or formed from super-AGB star ejecta.

Contribution

It provides detailed spectroscopic analysis of a peculiar star, challenging existing models of red giant evolution and chemical enrichment in globular clusters.

Findings

Star has high Li and Na abundances, not explained by standard mixing.

Star shows high C and N, inconsistent with typical globular cluster patterns.

Evidence suggests the star may be in a binary system with accreted material.

Abstract

We present a multi-instrument spectroscopic analysis of the unique Li/Na-rich giant star 25664 in Omega Centauri using spectra acquired with FLAMES-GIRAFFE, X-SHOOTER, UVES and HARPS. Li and Na abundances have been derived from the UVES spectrum using transitions weakly sensitive to non-local thermodynamic equilibrium and assumed isotopic ratio. This new analysis confirms the surprising Li and Na abundances of this star (A(Li) =+2.71+-0.07 dex, [Na/Fe]=+1.00+-0.05 dex). Additionally, we provide new pieces of evidence for its chemical characterisation. The 12C/13C isotopic ratio (15+-2) shows that this star has not yet undergone the extra-mixing episode usually associated with the red giant branch bump. Therefore, we can rule out the scenario of efficient deep extra-mixing during the red giant branch phase envisaged to explain the high Li and Na abundances. Also, the star exhibits high abundances of both C and N ([C/Fe]=+0.45+-0.16 dex and [N/Fe]=+0.99+-0.20 dex), not compatible with the typical C-N anticorrelation observed in globular cluster stars. We found evidence of a radial velocity variability in 25664, suggesting that the star could be part of a binary system, likely having accreted material from a more massive companion when the latter was evolving in the AGB phase. Viable candidates for the donor star are AGB stars with 3-4 Msun and super-AGB stars (~7-8 Msun), both able to produce Li- and Na-rich material. Alternatively, the star could be formed from the pure ejecta of a super-AGB stars, before the dilution with primordial gas occurs.