Spectral line ratios as Teff indicators in solar-like stars

The ratios of spectral line depths are often used as indicators of the stellar effective temperature T-eff. In particular, Gray & Livingston (1997a) calibrated the temperature sensitivity of the ratios between the central depths of the line C I 538.032 nm and either the Fe I 537.958 or the Ti II 538.103, making use of observed spectra of several solar-like stars. The ultimate reason for choosing these lines was the subsequent application of their calibration to a long series of solar data, collected at Kitt Peak (in disk-integrated light) from 1978 to 1992, in order to get the T-eff variation of the Sun during its 11-yr magnetic cycle (Gray & Livingston 1997b). We propose a theoretical calibration that includes a careful treatment of convective transport and fits the stellar data very well, showing, at the same time, that the empirical calibration of Gray and Livingston incorporates in the T-eff sensitivity an undesired dependence of line ratios on the surface gravities of the individual stars they used. A possible dependence of the calibration upon stellar rotation is also explored.