The metal-poor end of the Spite plateau I. Stellar parameters, metallicities, and lithium abundances

Context. The primordial nature of the Spite plateau is at odds with the WMAP satellite measurements, implying a primordial Li production at least three times higher than observed. It has also been suggested that A(Li) might exhibit a positive correlation with metallicity below [Fe/H] similar to-2.5. Previous samples studied comprised few stars below [Fe/H] = -3. Aims. We present VLT-UVES Li abundances of 28 halo dwarf stars between [Fe/H] = -2.5 and -3.5, ten of which have [Fe/H] < -3. Methods. We determined stellar parameters and abundances using four different T-eff scales. The direct infrared flux method was applied to infrared photometry. H alpha wings were fitted with two synthetic grids computed by means of 1D LTE atmosphere models, assuming two different self-broadening theories. A grid of H alpha profiles was finally computed by means of 3D hydrodynamical atmosphere models. The Li I doublet at 670.8 nm has been used to measure A(Li) by means of 3D hydrodynamical NLTE spectral syntheses. An analytical fit of A(Li)(3D,NLTE) as a function of equivalent width, T-eff, log g, and [Fe/H] has been derived and is made available. Results. We confirm previous claims that A(Li) does not exhibit a plateau below [Fe/H] = -3. We detect a strong positive correlation with [Fe/H] that is insensitive to the choice of T-eff estimator. From a linear fit, we infer a steep slope of about 0.30 dex in A(Li) per dex in [Fe/H], which has a significance of 2-3 sigma. The slopes derived using the four T-eff estimators are consistent to within 1 sigma. A significant slope is also detected in the A(Li)-T-eff plane, driven mainly by the coolest stars in the sample (T-eff < 6250), which appear to be Li-poor. However, when we remove these stars the slope detected in the A(Li)-[Fe/H] plane is not altered significantly. When the full sample is considered, the scatter in A(Li) increases by a factor of 2 towards lower metallicities, while the plateau appears very thin above [Fe/H] = -2.8. At this metallicity, the plateau lies at < A(Li)(3D,NLTE)> = 2.199 +/- 0.086. Conclusions. The meltdown of the Spite plateau below [Fe/H] similar to -3 is established, but its cause is unclear. If the primordial A(Li) were that derived from standard BBN, it appears difficult to envision a single depletion phenomenon producing a thin, metallicity independent plateau above [Fe/H] = -2.8, and a highly scattered, metallicity dependent distribution below. That no star below [Fe/H] = -3 lies above the plateau suggests that they formed at plateau level and experienced subsequent depletion.