The metal-poor end of the Spite plateau

I. Stellar parameters, metallicities, and lithium abundances^*,**,***

¹ CIFIST Marie Curie Excellence Team, France
² GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
³ Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
e-mail: lsbordone@mpa-garching.mpg.de
⁴ INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
⁵ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
⁶ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP 226, boulevard du Triomphe, 1050 Bruxelles, Belgium
⁷ Dpto. de Astrofísica y Ciencias de la Atmósfera, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁸ Astrophysikalisches Institut Potsdam An der Sternwarte 16, 14482 Potsdam, Germany
⁹ Centre de Recherche Astrophysique de Lyon, UMR 5574: Université de Lyon, École Normale Supérieure de Lyon, 46 allée d’Italie, 69364 Lyon Cedex 07, France
¹⁰ Université Montpellier 2, CNRS, GRAAL, 34095 Montpellier, France
¹¹ Indian Institute of Astrophysiscs, II Block, Koramangala, Bangalore 560 034, India
¹² Dept. if Physics & Astronomy, and JINA: Joint Insrtitute for Nuclear Astrophysics, Michigan State University, E. Lansing, MI 48824, USA
¹³ Cassiopée – Observatoire de la Côte d’Azur, Boulevard de l’Observatoire, BP 4229, 06304 Nice Cedex 4, France

Received: 11 September 2009
Accepted: 17 March 2010

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 .

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.