A super lithium-rich red-clump star in the open cluster Trumpler 5^⋆,⋆⋆

¹ European Southern Observatory, Casilla 19001 Santiago, Chile
e-mail: lmonaco@eso.org
² GEPI, Observatoire de Paris, CNRS, Univ. Paris Diderot, place Jules Janssen, 92195 Meudon, France
³ Universidad de Concepción, Casilla 160-C, Concepción, Chile
⁴ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
⁵ Leibniz-Institut für Astrophysik Potsdam (AIP), an der Sternwarte 16, 14482 Potsdam, Germany
⁶ Observatorio Astronómico, Universidad Nacional de Córdoba, Laprida 854, 5000 Córdoba, Argentina
⁷ Las Campanas Observatory, Carnegie Institution of Washington, Colina el Pino, Casilla 601, La Serena, Chile

Received: 28 December 2013
Accepted: 14 March 2014

Abstract

Context. The existence of lithium-rich low-mass red giant stars still represents a challenge for stellar evolution models. Stellar clusters are privileged environments for this kind of investigation.

Aims. To investigate the chemical abundance pattern of the old open cluster Trumpler 5, we observed a sample of four red-clump stars with high-resolution optical spectrographs. One of them (#3416) reveals extremely strong lithium lines in its spectrum.

Methods. One-dimensional, local thermodynamic equilibrium analysis was performed on the spectra of the observed stars. A 3D-NLTE analysis was performed to derive the lithium abundance of star #3416.

Results. Star #3416 is super Li-rich with A(Li) = 3.75 dex. The lack of ⁶Li enrichment (⁶Li/⁷Li < 2%), the low carbon isotopic ratio (¹²C/¹³C = 14 ± 3), and the lack of evidence for radial velocity variation or enhanced rotational velocity (vsini = 2.8  km s^-1) all suggest that lithium production has occurred in this star through the Cameron & Fowler mechanism.

Conclusions. We identified a super Li-rich core helium-burning, red-clump star in an open cluster. Internal production is the most likely cause of the observed enrichment. Given the expected short duration of a star’s Li-rich phase, enrichment is likely to have occurred at the red clump or in the immediately preceding phases, namely during the He-flash at the tip of the red giant branch (RGB) or while ascending the brightest portion of the RGB.