Letter to the Editor

Line shift, line asymmetry, and the Li/Li isotopic ratio determination ^*,**

¹ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 61 Av. de l'Observatoire, 75014 Paris, France
² Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
³ Institut d'Astrophysique de Paris, 98bis bd Arago, 75014 Paris, France
⁴ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France e-mail: Piercarlo.Bonifacio@obspm.fr
⁵ CIFIST, Marie Curie Excellence Team
⁶ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy

Received: 24 July 2007
Accepted: 27 August 2007

Abstract

Context.Line asymmetries are generated by convective Doppler shifts in stellar atmospheres, especially in metal-poor stars, where convective motions penetrate to higher atmospheric levels. Such asymmetries are usually neglected in abundance analyses. The determination of the isotopic ratio is prone to suffering from such asymmetries, as the contribution of ⁶Li is a slight blending reinforcement of the red wing of each component of the corresponding ⁷Li line, with respect to its blue wing.

Aims.The present paper studies the halo star HD 74000 and estimates the impact of convection-related asymmetries on the Li isotopic ratio determination.

Methods.Two methods are used to meet this aim. The first, which is purely empirical, consists in deriving a template profile from another element that can be assumed to originate in the same stellar atmospheric layers as Li I, producing absorption lines of approximately the same equivalent width as individual components of the ⁷Li I resonance line. The second method consists in conducting the abundance analysis based on NLTE line formation in a 3D hydrodynamical model atmosphere, taking into account the effects of photospheric convection.

Results.The results of the first method show that the convective asymmetry generates an excess absorption in the red wing of the ⁷Li absorption feature that mimics the presence of ⁶Li at a level comparable to the hitherto published values. This opens the possibility that only an upper limit on has thus far been derived. The second method confirms these findings.

Conclusions.From this work, it appears that a systematic reappraisal of former determinations of ⁶Li abundances in halo stars is warranted.