Lithium abundances of very low mass members of Chamaeleon I ^*

¹ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: cjohnas@hs.uni-hamburg.de
² Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 27 March 2007
Accepted: 6 August 2007

Abstract

Aims.We present the first study of the lithium abundances of very low mass objects in Chamaeleon I close to the hydrogen burning mass limit based on atmospheric models and high-resolution spectroscopic observations. The studied objects, Cha Hα 2, 3, 4, 5, 6 and 8, are very young brown dwarf candidates and very low mass stars on the verge of lithium depletion.

Methods.For this analysis, we have computed a new “GAIA-cond” class model grid over effective temperatures from 2600 K to 3100 K, surface gravities from log(g) = 3.5 to 5.5, and lithium abundances from logϵ = 0.0 to 3.7, for two different line profile setups introduced in previous work. Calculated synthetic spectra are compared with high-resolution UVES / VLT echelle spectra of the objects.

Results.We find good descriptions of the lithium resonance doublet lines at 6708 Å and of the surrounding pseudo-continuum and determine a consistent set of lithium abundances (log(ϵ) = 1.55). However, the derived lithium abundances are lower than the meteoritic one (log(ϵ) = 3.31) and that of higher mass stars in Cha I (log(ϵ) = 3.1/3.4 for LTE–/non-LTE–calculations). By modeling the TiO-line, we demonstrate that veiling does not make the lithium lines appear weaker. We can also rule out that the results are spoiled by the presence of spots.

Conclusions.A possible explanation for these results would be that the objects are either more massive, or much older, than previously thought, so that the lithium depletion has already started. Although the uncertainties of the masses and ages are large, they are not large enough as to explain the observed lithium depletion. Therefore, the most likely explanation is either a lack of understanding of the details of the formation of the lithium line, or a lack of understanding of the internal structure of the very young low-mass objects.