The General Assembly of Galaxy Halos: Structure, Origin and Evolution Proceedings IAU Symposium No. 317, 2015 A. Bragaglia, M. Arnaboldi, M. Rejkuba & D. Romano, eds. c  International Astronomical Union 2016 doi:10.1017/S1743921315006791 Impact of NLTE on research of early chemical enrichment of the dwarf galaxies Lyudmila Mashonkina 1 , Pascale Jablonka 2 , Pierre North 2 and Tatyana Sitnova 1 1 Institute of Astronomy, Russian Academy of Sciences Pyatnitskaya st. 48, RU-119017 Moscow, Russia email: lima@inasan.ru 2 Laboratoire d’ Astrophysique, Ecole Polytechnique F´ ed´ erale de Lausanne (EPFL) Observatoire de Sauverny, CH-1290 Versoix, Switzerland Abstract. Based on high-resolution observed spectra, the non-local thermodynamic equilibrium (NLTE) line formation, and precise stellar atmosphere parameters, we present the first complete sample of dwarf spheroidal galaxies (dSphs) with accurate chemical abundances in the very metal-poor (VMP) regime. The obtained stellar elemental ratios are compared with chemical enrichment models, and we show that NLTE is a major step forward for studies of the dSph and the Milky Way (MW) chemical evolution. Keywords. stars: abundances, galaxies: abundances, galaxies: dwarf, galaxies: evolution Studies of chemical abundances of VMP stars provide important clues for better un- derstanding the early chemical enrichment processes of the host galaxy and the onset of star formation. Our research concerns the dSphs orbiting the Milky Way, where indi- vidual stars accessible for high-resolution spectroscopy are all giants. The classical LTE assumption commonly applied to stellar abundance analyses is, in particular, question- able for such objects. We aim to revise the chemical abundances of a complete sample of VMP stars in the classical dSphs Sculptor, Sextans, and Fornax and in the ultra-faint dwarf (UFD) galaxy Bo¨ otes I based on the NLTE line formation and to test chemical enrichment models. The same methods were used to derive abundances of the MW halo comparison sample. NLTE calculations were performed for a number of chemical species, using our origi- nal model atoms. Inelastic collisions with H I atoms were treated applying accurate rate coefficients for Na I, Mg I, Al I, and Si I and the scaled Drawin formula for the remaining species. In the relevant stellar parameter range, NLTE leads to strengthened lines and negative NLTE abundance corrections for Na I, but to weakened lines and positive cor- rections for all other chemical species. The amount of NLTE correction varies according to the different species and depends on atmospheric parameters. The stellar sample includes 10 members of the Scl, Fnx, and Sex dSphs observed by Tafelmeyer et al. (2010) and Jablonka et al. (2015, JNM15) and 7 Bo¨ otes I stars from Norris et al. (2010) and Gilmore et al. (2013). For comparison with the MW halo, nine cool VMP giants were selected from Cohen et al. (2013, CCT13), and three stars from our previous studies. For Bo¨ otes I and CCT13 we rely on the published equivalent widths. Stellar atmosphere parameters. This study is based on non-spectroscopic T eff and log g. The effective temperatures were derived from photometry in the original papers. For the Scl, Fnx, and Sex stars the accurate determination of log g takes advantage of the known distance. We show that, when applying NLTE, the Fe I/Fe II and Ti I/Ti II ionisation equilibria are fulfilled using the adopted T eff /log g for all stars, except for the two most 334 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921315006791 Downloaded from https://www.cambridge.org/core. IP address: 54.163.42.124, on 22 May 2020 at 09:56:10, subject to the Cambridge Core terms of use, available at