The formation of the Milky Way halo and its dwarf satellites; a NLTE-1D abundance analysis

Abstract

We present a homogeneous set of accurate atmospheric parameters for a complete sample of very and extremely metal-poor stars in the dwarf spheroidal galaxies (dSphs) Sculptor, Ursa Minor, Sextans, Fornax, Botes I, Ursa Major II, and Leo IV. We also deliver a Milky Way (MW) comparison sample of giant stars covering the -4 < [Fe/H] < -1.7 metallicity range. We show that, in the [Fe/H] -3.7 regime, the non-local thermodynamic equilibrium (NLTE) calculations with non-spectroscopic effective temperature (T eff ) and surface gravity (log g) based on the photometric methods and known distance provide consistent abundances of the Fe i and Fe ii lines. This justifies the Fe i/Fe ii ionisation equilibrium method to determine log g for the MW halo giants with unknown distance. The atmospheric parameters of the dSphs and MW stars were checked with independent methods. In the [Fe/H] > -3.5 regime, the Ti i/Ti ii ionisation equilibrium is fulfilled in the NLTE calculations. In the log g -T eff plane, all the stars sit on the giant branch of the evolutionary tracks corresponding to [Fe/H] = -2 to -4, in line with their metallicities. For some of the most metal-poor stars of our sample, we achieve relatively inconsistent NLTE abundances from the two ionisation stages for both iron and titanium. We suggest that this is a consequence of the uncertainty in the T eff -colour relation at those metallicities. The results of this work provide the basis for a detailed abundance analysis presented in a companion paper.