The metal-poor end of the Spite plateau

Abstract

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] ∼ −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. Hα wings were fitted with two synthetic grids computed by means of 1D LTE atmosphere models, assuming two different self-broadening theories. A grid of Hα 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 2-3σ. The slopes derived using the four T eff estimators are consistent to within 1σ. 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] ∼ −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.

TOPoS

“…The upper limit on the Li abundance was derived from the fitting formula of Sbordone et al (2010) providing the 3D NLTE Li-abundance. The 1D LTE or NLTE estimate would differ by only 0.03 dex, thus it is irrelevant in the present context.…”

“…(a) We list the strongest line of the doublet. (b) Derived from the EW and the fitting formula of Sbordone et al (2010).…”

“…The Spite plateau meltdown, highlighted by Aoki et al (2009a) and Sbordone et al (2010), is the large number of stars at metallicity below −3.0 solar, which have lithium abundances lower than the Spite plateau, but still measurable (see Fig. 3 in Spite et al 2012b).…”

THE PRIMORDIAL DEUTERIUM ABUNDANCE OF THE MOST METAL-POOR DAMPED Lyα SYSTEM∗

“…The primordial 7 Li/H ratio is deduced from the most metal-poor stars in the halo of the Milky Way. The latest determination (Asplund et al 2006;Aoki et al 2009;Meléndez et al 2010;Sbordone et al 2010;Spite et al 2015), ( ) =  A Li 2.199 0.086 7 , implies a 6σ deviation from the Standard Model value (see Fields 2011 for a review). The source of this discrepancy is currently unknown.…”

The New BBN Model with the Photon Cooling, X Particle, and the Primordial Magnetic Field

“…The big bang nucleosynthesis (BBN) model can accurately reproduce the primordial abundances of light elements except for 7 Li with  fixed by the CMB. A primordial abundance of 7 Li from this BBN theory, however, differs from 7 Li abundances on the surface of metal-poor halo stars inferred from astronomical observations [1]. It is called ``Li problem'' [2].…”