Abstract
We present VLT-UVES Li abundances for 28 halo dwarf stars between [Fe/H]=-2.5 and -3.5, 10 of which have [Fe/H]<-3. Four different T_eff scales have been used. Direct Infrared Flux Method (IRFM) has been used on the basis of 2MASS infrared photometry. H_alpha wings have been fitted against synthetic grids computed by means of 1D LTE atmosphere models, assuming different self-broadening theories. Finally, a grid of H_alpha profiles has been 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. A(Li) does not exhibit a plateau below [Fe/H]=-3. A strong positive correlation with [Fe/H] appears, not influenced by the choice of the T_eff estimator. From a linear fit, we obtain a strong slope of about 0.30 dex in A(Li) per dex in [Fe/H], significant to 2-3 sigma, and consistent among all the four T_eff estimators. A significant slope is also detected in the A(Li)--T_eff plane, driven mainly by the coolest stars in the sample which appear Li-poor. Removing such stars does not alter the behavior in the A(Li)-[Fe/H] plane. 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. The meltdown of the Spite plateau below [Fe/H]\sim-3 is established, but its cause is unclear. If the primordial A(Li) is the one derived from WMAP, 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. The fact that no star below [Fe/H]=-3 lies above the plateau suggests that they formed at plateau level and underwent subsequent depletion.
Highlights
Spite & Spite (1982a,b) first noted that metal-poor (−2.4 ≤ [Fe/H] ≤ −1.4), warm (5700 K ≤ Teff ≤ 6250 K), dwarf stars exhibit a remarkably constant Li abundance, irrespective of metallicity and effective temperature, and interpreted this plateau in Li abundance as being representative of the abundance of Li synthesized during the Article published by EDP SciencesA&A 522, A26 (2010)primordial hot and dense phase of the Universe (Big Bang, Wagoner et al 1967; see Iocco et al 2009, for a review)
We decided to adopt the 3D temperature scale together with the 3D NLTE Li abundance set as our preferred values, and when not otherwise specified, we will refer to these
We have presented the largest sample to date of Li abundances for EMP halo dwarf stars (27 abundances and one upper limit), including the largest sample to date below [Fe/H] = −3 (10 abundances)
Summary
A more effective approach might be to start with a properly crafted function, possibly non-linear, and having fitting parameters “within” the terms as in A exp(B + Cx). In this case, an iterative process is necessary to determine these parameters. This iteration requires starting values, which will affect the convergence of the process and the solution if one is found: in this case, the scheme might diverge or converge to different local minima, depending on the starting point This last issue is hardly a problem for our application – we are content with a good fit, and do not necessarily need to achieve the closest one. To have the problem of constructing the fitting function and choosing the initial values for the parameters
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