2015ApJ...808..148S

We present atmospheric parameters for 51 nearby F and G dwarf and subgiant stars uniformly distributed over the -2.60< [Fe/H] < +0.20 metallicity range that is suitable for the Galactic chemical evolution research. Lines of iron in the two ionization stages, Fe i and Fe ii, were used to derive a homogeneous set of effective temperatures, surface gravities, iron abundances, and microturbulence velocities. Our spectroscopic analyses took advantage of employing high-resolution (R ≥ 60,000) Shane/Hamilton and Canada-France-Hawaii Telescope/ESPaDOnS observed spectra and non-LTE (NLTE) line formation for Fe i and Fe ii in the classical one-dimensional model atmospheres. The spectroscopic method was tested in advance with the 20 benchmark stars, for which there are multiple measurements of the infrared flux method effective temperature and their Hipparcos parallax error is less than 10%. We found NLTE abundances from lines of Fe i and Fe ii to be consistent within 0.06 dex for every benchmark star, when applying a scaling factor of S_H = 0.5 to the Drawinian rates of inelastic Fe+H collisions. The obtained atmospheric parameters were checked for each program star by comparing its position in the log g-T_eff plane with the theoretical evolutionary track of given metallicity and α-enhancement in the Yi et al. grid. Our final effective temperatures lie exactly in between the T_IRFM scales of Alonso et al. and Casagrande et al., with a mean difference of +46 and -51 K, respectively. NLTE leads to higher surface gravity compared with that for LTE. The shift in log g is smaller than 0.1 dex for stars with [Fe/H] ≥ -0.75, T_eff ≤ 5750 K, or log g ≥ 4.20. NLTE analysis is crucial for the very metal-poor turnoff and subgiant stars, for which the shift in log g between NLTE and LTE can be up to 0.5 dex. The obtained accurate atmospheric parameters will be used in the forthcoming papers to determine NLTE abundances of important astrophysical elements from lithium to europium and to improve observational constraints on the chemodynamical models of the Galaxy evolution.