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The Formation of the Milky Way Halo and Its Dwarf Satellites, a NLTE-1D Abundance Analysis. I. Homogeneous Set of Atmospheric Parameters

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The Formation of the Milky Way Halo and Its Dwarf Satellites, a NLTE-1D Abundance Analysis. I. Homogeneous Set of Atmospheric Parameters Astronomy & Astrophysics manuscript no. dsph_parameters_final c ESO 2021 July 2, 2021 The formation of the Milky Way halo and its dwarf satellites, a NLTE-1D abundance analysis. I. Homogeneous set of atmospheric parameters L. Mashonkina1, 2, P. Jablonka3, 4, Yu. Pakhomov2, T. Sitnova2, and P. North3 1 Universitäts-Sternwarte München, Scheinerstr. 1, D-81679 München, Germany e-mail: [email protected] 2 Institute of Astronomy, Russian Academy of Sciences, RU-119017 Moscow, Russia e-mail: [email protected] 3 Laboratoire d’ Astrophysique, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, CH-1290 Versoix, Switzerland 4 GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, F-92125 Meudon Cedex, France Received / Accepted 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 (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 logg - 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 hardly achieve consistent 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 these work provide the base for a detailed abundance analysis presented in a companion paper. Key words. Stars: abundances – Stars: atmospheres – Stars: fundamental parameters – Local Group – galaxies: dwarf 1. Introduction [Fe/H]1 < 2) stars in the Draco, Sextans, and Ursa Mi- nor dSphs (Shetroneet− al. 2001) much of the observational How do the first stages of star formation proceed in galaxies? efforts were invested to obtain detailed chemical abundances Do galaxies follow a universal path, independant of their final of stars in the Milky Way satellites. The largest samples of masses? What is the level of homogeneity of the interstellar the VMP and extremely metal-poor (EMP, [Fe/H] < 3) medium from which stars form? How does it evolve? What is stars, which were observed with a spectral resolving power− the stellar initial mass function of the first stars? of R > 20 000, are available in the literature for the classi- These questions can essentially only be addressed in depth cal dSphs in Sculptor (Tafelmeyer et al. 2010, Kirby & Cohen in the Local Group. Only there can we analyse individual stars 2012, Jablonka et al. 2015, Simon et al. 2015) and Ursa Minor in sufficient detail to guide our understanding of the physics of (Sadakane et al. 2004, Cohen & Huang 2010, Kirby & Cohen arXiv:1704.07656v2 [astro-ph.SR] 2 May 2017 star formation, supernovae feeback, and the early build-up of 2012, Uraletal. 2015). As for the UFDs, the most studied galaxies. The comparison between ultra-faint, classical dwarf cases are Boötes I (Feltzinget al. 2009, Norrisetal. 2010, spheroidal galaxies (UFDs, dSphs), and the Milky Way pop- Gilmore et al. 2013, Ishigakiet al. 2014, Frebel et al. 2016), ulation offers a fantastic opportunity to probe different galaxy Segue 1 (Frebel et al. 2014), Coma Berenices and Ursa Major II masses, star formation histories, levels of chemical enrichement. (Frebel et al. 2010). However, the total number of new stars in Both nucleosynthetic processes and galaxy formation models each individual paper nowhere exceeds 7. Therefore, it is com- largely benefit from the diversity of the populations sampled that mon to combine these samples altogether. However, they were way. gathered with different spectroscopic setups and analysed in dif- We are, however, facing two limitations: ferent ways, with different methods of the determination of at- a) Heterogeneity in the samples. Since the first high- mospheric parameters, different model atmospheres, radiation resolution spectroscopic study of the very metal-poor (VMP, 1 In the classical notation, where [X/H] = log(N /N ) X H star − Send offprint requests to: L. Mashonkina; e-mail: [email protected] log(NX/NH)Sun. Article number, page 1 of 20 A&A proofs: manuscript no. dsph_parameters_final transfer and line formation codes, and line atomic data. This can and the ultra-faint dwarfs Boötes I, Ursa Major II (UMa II), and lead easily to inaccurate conclusions. Leo IV (Table1). This sample covers the 4 [Fe/H] < 1.5 b) Heterogeneity arises also, when applying the LTE as- metallicity range. It is assembled from the− following≤ papers:− sumption for determination of the chemical abundances of the stellar samples with various effective temperatures, surface grav- – Sculptor: Jablonka et al. (2015), Kirby & Cohen (2012), ities, and metallicities. Individual stars in the dSphs that are ac- Simon et al. (2015), and Tafelmeyer et al. (2010), cessible to high-resolution spectroscopy are all giants, and line – Ursa Minor: Cohen&Huang (2010), Kirby&Cohen formation, in particular in the metal-poor atmospheres, is sub- (2012), and Ural et al. (2015), ject to the departures from LTE because of low electron number – Fornax and Sextans: Tafelmeyer et al. (2010), density and low ultra-violet (UV) opacity. For each galaxy, the Milky Way or its satellites, the sampled range of metallicity can – Boötes I: Gilmore et al. (2013), Norris et al. (2010), and be large (Tolstoy et al. 2009). Similarly, the position of the stars Frebel et al. (2016, Boo-980), along the red giant branch (i.e., their effective temperatures and – UMa II: Frebel et al. (2010), surface gravities) can vary between samples. – Leo IV-S1: Simon et al. (2010). In the literature, determinations of atmospheric parameters and chemical abundances based on the non-local thermodynamic The comparison sample in the Milky Way halo was selected equilibrium (NLTE) line formation were reported for Milky Way from the literature based on the following criteria. stars spanning a large interval of metallicities (Hansen et al. 2013, Ruchti et al. 2013, Bensby et al. 2014, Sitnova et al. 2015, 1. The MW and dSph stellar samples should have similar tem- Zhao et al. 2016), however, none has yet treated both the Milky peratures, luminosities, and metallicity range: cool giants Way and the dSph stellar samples. with T ff 5250 K and [Fe/H] < 2. In this context, our project aims at providing a homogeneous e ≤ − 2. High spectral resolution (R > 30 000) observational material set of atmospheric parameters and elemental abundances for the should be accessible VMP and EMP stars in aset of dSphsas well as for a Milky Way halo comparison sample. By employing high-resolution spectral 3. Photometry in the V, I, J, K bands was available to derive observations and treating the NLTE line formation, our desire photometric Teff. is to push the accuracy of the abundance analysis to the point where the trends of the stellar abundance ratios with metallicity Binaries, variables, carbon-enhancedstars, and Ca-poor stars can be robustly discussed. were ignored. In the following, we present the determination of accurate at- As a result, the MW comparison sample includes 12 stars mospheric parameters: effective temperatures, Teff, surface grav- from Cohen et al. (2013, hereafter, CCT13), two stars from ities, log g, iron abundances (metallicity, [Fe/H]), and micro- Mashonkina et al. (2010, 2014), and nine stars from Burris et al. turbulence velocities, ξt. We rely on the photometric methods, (2000). For the latter subsample we used spectra from the VLT2 when deriving the effective temperatures. The surface gravities / UVES2 and CFHT/ESPaDOnS3 archives. are based on the known distance for the dSph stars and estab- The characteristics of the stellar spectra, which were used in lishing the NLTE ionisation equilibrium between Fe i and Fe ii this analysis, are summarized in Table1. Details on the observa- for the Milky Way stars. The metallicities and microturbulence tions and the data reduction can be found in the original papers. velocities were determined from the NLTE calculations for Fe i- We based our study on the published equivalent widths (Wobss) ii. A companion paper focuses on the NLTE abundances of a and line profile fitting, where the observed spectra are available, large set of chemical elements, spanning from Na to Ba, and the as indicated in Table1. analysis of the galaxy abundance trends. The paper is structured as follows: Section 2 describes the stellar sample and the observational material. Effective tempera- 3. Effective temperatures tures are determined in Sect. 3. In Sect. 4, we demonstrate that the Fe i/Fe ii ionisation equilibrium method is working in NLTE This study is based on photometric effective temperatures. We down to extremely low metallicities and we derive spectroscopic could adopt the published data for about half of our sample, surface gravities for the Milky Way (MW) giant sample. The namely: stellar atmosphere parameters are checked with the Ti i/Ti ii ion- isation equilibrium and a set of theoretical evolutionary tracks in – stars in the Sculptor, Fornax, and Sextans dSphs, for Sect. 5. Comparison with the literature is conducted in Sect.
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