Accuracy of atmospheric parameters of FGK dwarfs determined by spectrum fitting

Abstract

We performed extensive tests of the accuracy of atmospheric parameter\ndetermination for FGK stars based on the spectrum fitting procedure\nSpectroscopy Made Easy (SME). Our stellar sample consists of 13 objects,\nincluding the Sun, in the temperature range 5000--6600~K and metallicity range\n-1.4 -- +0.4. The analysed stars have the advantage of having parameters\nderived by interferometry. For each star we use spectra obtained with different\nspectrographs and different signal-to-noise ratios (S/N). For the fitting we\nadopted three different sets of constraints and test how the derived parameters\ndepend upon the spectral regions (masks) used in SME. We developed and\nimplemented in SME a new method for estimating uncertainties in the resulting\nparameters based on fitting residuals, partial derivatives, and data\nuncertainties. For stars in the 5700--6600 K range the best agreement with the\neffective temperatures derived by interferometry is achieved when spectrum\nfitting includes the H$\\alpha$ and H$\\beta$ lines, while for cooler stars the\nchoice of the mask does not affect the results. The derived atmospheric\nparameters do not strongly depend on spectral resolution and S/N of the\nobservations, while the uncertainties in temperature and surface gravity\nincrease with increasing effective temperature, with minima at 50~K in Teff and\n0.1~dex in log g, for spectra with S/N=150--200. A NLTE analysis of the\nTiI/TiII and FeI/FeII ionisation equilibria and abundances determined from the\natomic CI (NLTE) and molecular CH species supports the parameters we derived\nwith SME by fitting the observed spectra including the hydrogen lines.\n