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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents

ID: 358709.0, MPI für Astronomie / Publikationen_mpia
Lithium abundances of very low mass members of Chamaeleon I
Authors:Johnas, C. M. S.; Guenther, E. W.; Joergens, V.; Schweitzer, A.; Hauschildt, P. H.
Date of Publication (YYYY-MM-DD):2007
Title of Journal:Astronomy and Astrophysics
Start Page:667
End Page:675
Audience:Not Specified
Abstract / Description: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 Halpha 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 logɛ = 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(ɛ) = 1.55). However, the derived lithium abundances are lower than the meteoritic one (log(ɛ) = 3.31) and that of higher mass stars in Cha I (log(ɛ) = 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. Based on observations obtained at the European Southern Observatory at Paranal, Chile in program 65.L-0629(A,B) and 65.I-0011(A).
Free Keywords:stars: atmospheres; line: profiles; stars: low-mass; brown dwarfs; stars: pre-main sequence; ISM: individual objects: Chamaeleon I
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie

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