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The analysis of stellar spectra depends upon the effective temperature (Teff)and the surface gravity (log g). However, the estimation of log g with highaccuracy is challenging. A classical approach is to search for log g thatsatisfies the ionization balance, i.e., the abundances from neutral and ionizedmetallic lines being in agreement. We propose a method of using empiricalrelations between Teff, log g and line-depth ratios, for which we meticulouslyselect pairs of FeI and FeII lines and pairs of CaI and CaII lines. Based onYJ-band (0.97-1.32 micron) high-resolution spectra of 42 FGK stars (dwarfs tosupergiants), we selected five FeI-FeII and four CaI-CaII line pairs togetherwith 13 FeI-FeI pairs (for estimating Teff, and derived the empiricalrelations. Using such relations does not require complex numerical models andtools for estimating chemical abundances. The relations we present allows oneto derive Teff and log g with a precision of around 50 K and 0.2 dex,respectively, but the achievable accuracy depends on the accuracy of thecalibrators' stellar parameters. It is essential to revise the calibration byobserving stars with accurate stellar parameters available, e.g., stars withasteroseismic log g and stars analyzed with complete stellar models taking intoaccount the effects of non-local thermodynamic equilibrium and convection. Inaddition, the calibrators we used have a limited metallicity range, [Fe/H]between -0.2 and +0.2 dex, and our relations need to be tested andre-calibrated based on a calibrating dataset for a wider range of metallicity.

Line-depth ratios as indicators of effective temperature and surface gravity