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

I. Homogeneous set of atmospheric parameters^⋆

¹ Universitäts-Sternwarte München, Scheinerstr. 1, 81679 München, Germany
e-mail: lyuda@usm.lmu.de
² Institute of Astronomy, Russian Academy of Sciences, 119017 Moscow, Russia
e-mail: lima@inasan.ru
³ Laboratoire d’Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
⁴ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 92125 Meudon Cedex, France

Received: 14 March 2017
Accepted: 19 April 2017

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, Boötes 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.