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Abstract. Brown dwarf atmospheres form molecules, then high temperature condensates (corundum, titanates, silicates, and iron compounds), and then low temperature condensates (ices) as they cool down over time. These produce large opacities which govern entirely their spectral energy distribution. Just as it is important to know molecular opacities (TiO, H20, CH 4 , etc.) with accuracy, it is imperative to understand the interplay of processes (e.g. condensation, sedimentation, coagulation, convection) that determines the radial and size distribution of grains. Limiting case models have shown that young, hot brown (L) dwarfs form dust mostly in equilibrium, while at much cooler stages (late T dwarfs) all high temperature condensates have sedimented out of their photospheres. But this process is gradual and all intermediate classes of brown dwarfs can partly be understood in terms of partial sedimentation of dust. With new models accounting for these processes, we describe the effects they may have upon brown dwarf spectral properties.

Context. The Gaia-ESO Public Spectroscopic Survey is an ambitious project designed to obtain astrophysical parameters and elemental abundances for 100,000 stars, including large representative samples of the stellar populations in the Galaxy, and a well-defined sample of 60 (plus 20 archive) open clusters. We provide internally consistent results calibrated on benchmark stars and star clusters, extending across a very wide range of abundances and ages. This provides a legacy data set of intrinsic value, and equally a large wide-ranging dataset that is of value for homogenisation of other and future stellar surveys and Gaia's astrophysical parameters. Aims. This article provides an overview of the survey methodology, the scientific aims, and the implementation, including a description of the data processing for the GIRAFFE spectra. A companion paper introduces the survey results. Methods. Gaia-ESO aspires to quantify both random and systematic contributions to measurement uncertainties. Thus all available spectroscopic analysis techniques are utilised, each spectrum being analysed by up to several different analysis pipelines, with considerable effort being made to homogenise and calibrate the resulting parameters. We describe here the sequence of activities up to delivery of processed data products to the ESO Science Archive Facility for open use. Results. The Gaia-ESO Survey obtained 202,000 spectra of 115,000 stars using 340 allocated VLT nights between December 2011 and January 2018 from GIRAFFE and UVES. Conclusions. The full consistently reduced final data set of spectra was released through the ESO Science Archive Facility in late 2020, with the full astrophysical parameters sets following in 2022. A companion article reviews the survey implementation, scientific highlights, the open cluster survey, and data products.

Two problems are discussed here. The first one is the 0.4 dex discrepancy between the 7 Li abundance derived from the spectra of metal-poor halo stars on the one hand, and from Big Bang nucleosynthesis, based on the cosmological parameters constrained by the Wilkinson Microwave Anisotropy Probe (WMAP) measurements, on the other hand. Lithium, indeed, can be depleted in the convection zone of unevolved stars, by a combination of diffusion and slow mixing with the hotter layers below the convection zone, where 7 Li is destroyed by the 7 Li(p,α) 4 He reaction. The understanding of the hydrodynamics of this crucial zone near the bottom of the convective envelope in dwarfs or turn-off stars of solar metallicity has recently made enormous progress with the inclusion of internal gravity waves. However, similar work for metal-poor stars is still lacking. So in spite of several investigations claiming to have partly explained the above mentioned 7 Li discrepancy, it is not yet clear whether or not the depletion occurring in the metal-poor stars themselves is adequate to produce a 7 Li plateau. The second problem concerns the difficulties met in accounting for the large amount of 6 Li recently found in metal-poor halo stars (Asplund et al. 2006 [15]). It has already been suggested (Cayrel et al. 2007 [24]) that the convection-related asymmetry of the 7 Li line could mimic the signal attributed so far to the weak blend of 6 Li in the red wing of the 7 Li line. However, this suggestion was only based on a hydrodynamical simulation for a single set of atmospheric parameters, representing the halo turn-off star HD 74000. Now the theoretical line asymmetry has been computed for an extended range in effective temperature, gravity and metallicity, covering the stars of the Asplund et al. sample. The computed asymmetry is about two per cent, a value which is also the mean of the 6 Li/ 7 Li ratio of the full sample determined with symmetric profiles by Asplund et al. 2006. Our conclusion is that these observations can be reinterpreted in terms of intrinsic line asymmetry, plus the amount of 6 Li/ 7 Li given by Asplund et al. reduced by 0.02. This drastically reduces the number of certain 6 Li detections, from 9 to 1 or 2.10th Symposium on Nuclei in the Cosmos

Molecular features such as the G-band, CN-band and NH-band are important diagnostics for measuring a star's carbon and nitrogen abundances, especially in metal-poor stars where atomic lines are no longer visible in stellar spectra. Unlike atomic transitions, molecular features tend to form in bands, which cover large wavelength regions in a spectrum. While it is a trivial matter to compute carbon and nitrogen molecular bands under the assumption of 1D, it is extremely time consuming in 3D. In this contribution to the 2016 CO 5 BOLD workshop we review the improvements made to the 3D spectral synthesis code Linfor3D, and discuss the new challenges found when computing molecular features in 3D.