Letter to the Editor

IRAS 12556–7731: a “chamaeleonic” lithium-rich M-giant^⋆,⋆⋆

¹ INAF – Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
e-mail: alcala@oacn.inaf.it
² INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
³ Space Telescope Science Institute, 3800 San Martin Drive, Baltimore, MD 21218, USA

Received: 2 May 2011
Accepted: 15 June 2011

Abstract

Aims. In this letter we characterise IRAS 12556–7731 as the first lithium-rich M-type giant. Based on its late spectral type and high lithium content, and because of its proximity in angular distance to the Chamaeleon II star-forming region, the star was misclassified as a young low-mass star in a previous work.

Methods. Based on HARPS data, synthetic spectral modelling, and proper motions, we derive the astrophysical parameters and kinematics of the star and discuss its evolutionary status.

Results. This solar-mass red giant (T_eff = 3460  ±  60 K and log g = 0.6  ±  0.2) is characterised by a relatively fast rotation (vsini ~ 8 km s^-1), slightly subsolar metallicity and a high-lithium abundance, A(Li)  =  2.4  ±  0.2 dex. We discuss IRAS 12556–7731 within the context of other known lithium-rich K-type giants. Because it is close to the tip of the red giant branch, IRAS 12556–7731 is the coolest lithium-rich giant known so far, and it is among the least massive and most luminous giants where enhancement of lithium has been detected. Among several possible explanations, we cannot preclude the possibility that the lithium enhancement and rapid rotation of the star were triggered by the engulfment of a brown dwarf or a planet.