The Metal-Poor End of the Lithium Plateau
Résumé
We present our current sample of Lithium abundances in 28 low metallicity dwarf and Turn Off (TO) stars ([Fe/H] between -2.5 and -3.5), based on high resolution, high signal to noise echelle spectra. Nine new stars have been added to the Bonifacio et al. [1] sample, and the full sample has been reanalyzed in order to take into account the effect of two different possible temperature scales. In fact, the Li abundance measurement based on the 670.8 nm line is highly sensitive to temperature, and Teff scales are still poorly calibrated at low metallicities. First, the effective temperature has been derived from Halpha profile fitting, and second, directly from the star's infrared flux. The two methods offer similar precision but are affected by different uncertainties and systematics. The infrared flux method (IRFM) leads to a larger Teff dispersion than the Halpha profile fitting, while also producing an offset of about 150 K towards hotter temperatures. This leads to a contraction of the metallicity scale of the sample, which encompasses [Fe/H] = -3.7 to -2.5 when using Halpha calibrated temperatures, and [Fe/H] = -3.4 to -2.5 when using IRFM. The higher average IRFM temperature increases somewhat the mean Li abundance, changing from A(Li)Halpha = 2.10 to A(Li)IRFM = 2.18