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

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.

Accepted for publication on A&A

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• Lithium abundances as a tracer of AGB stars pollution in the globular cluster NGC 6752
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