Patrick de Laverny — Page 2

arXiv2014

The AMBRE Project: Parameterisation of FGK-type stars from the ESO:HARPS archived spectra

The AMBRE project is a collaboration between the European Southern Observatory (ESO) and the Observatoire de la Cote d'Azur (OCA). It has been established to determine the stellar atmospheric parameters (effective temperature, surface gravity, global metallicities and abundance of alpha-elements over iron) of the archived spectra of four ESO spectrographs. The analysis of the ESO:HARPS archived spectra is presented. The sample being analysed (AMBRE:HARPS) covers the period from 2003 to 2010 and is comprised of 126688 scientific spectra corresponding to 17218 different stars. For the analysis of the spectral sample, the automated pipeline developed for the analysis of the AMBRE:FEROS archived spectra has been adapted to the characteristics of the HARPS spectra. Within the pipeline, the stellar parameters are determined by the MATISSE algorithm, developed at OCA for the analysis of large samples of stellar spectra in the framework of galactic archaeology. In the present application, MATISSE uses the AMBRE grid of synthetic spectra, which covers FGKM-type stars for a range of gravities and metallicities. We first determined the radial velocity and its associated error for the ~15% of the AMBRE:HARPS spectra, for which this velocity had not been derived by the ESO:HARPS reduction pipeline. The stellar atmospheric parameters and the associated chemical index [alpha/Fe] with their associated errors have then been estimated for all the spectra of the AMBRE:HARPS archived sample. Based on quality criteria, we accepted and delivered the parameterisation of ~71% of the total sample to ESO. These spectra correspond to ~10706 stars; each are observed between one and several hundred times. This automatic parameterisation of the AMBRE:HARPS spectra shows that the large majority of these stars are cool main-sequence dwarfs with metallicities greater than -0.5 dex.

astro-ph.IM↗

arXiv1999

The formation of lithium lines in the atmospheres of super Li-rich AGB stars

The formation of lithium lines in the atmosphere of C-rich giants is discussed. LTE and NLTE approximations are used to model lithium lines in the spectra of super Li-rich AGB stars. The system of equations of the statistical balance of neutral Li in plane-parallel model atmospheres is solved for a 20-level atom model. JOLA and line-by-line models of molecular absorption are used to compute synthetic spectra as well as the opacity in the frequencies of bound-bound and bound-free transitions of the lithium lines. Curves of growth and synthetic spectra are computed in LTE and NLTE for several model atmospheres of different Teff and C/O ratios for four lithium absorptions, namely: the $λ4603$, $λ6104$, $λ6708$ and $λ8126$ Å Li I lines. The sensitivity of the NLTE effects on Teff and the C/O ratio is discussed. We found that NLTE mainly affects the resonance line doublet ($λ6708$) while the impact of NLTE effects on the lithium subordinate lines, formed in the inner regions of C-rich giant atmospheres, is rather weak. Therefore the use of these lines is recommended for Li determination in AGB stars. However, in no case can we properly account for the formation of Li lines in AGB stars until sphericity, velocity stratifications, dust, chromospheres and other related phenomena, which are in fact present in AGB star atmospheres, are considered. Our results are used to derive Li abundances in three super Li-rich C-stars taking into account NLTE effects. Finally, the net Li yield from this class of stars into the interstellar medium is reconsidered.

astro-ph↗