Volume 604, August 2017
|Number of page(s)||19|
|Published online||25 August 2017|
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
2 Institute of Astronomy, Russian Academy of Sciences, 119017 Moscow, Russia
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
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 (Teff) 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 − Teff 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 Teff-colour relation at those metallicities. The results of this work provide the basis for a detailed abundance analysis presented in a companion paper.
Key words: stars: abundances / stars: atmospheres / stars: fundamental parameters / galaxies: dwarf / Local Group
Tables A.1 and A.2 are also available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/604/A129
© ESO, 2017
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