Spectral line ratios as ${\vec T}_{\bf \sf eff}$ indicators in solar-like stars

Abstract

The ratios of spectral line depths are often used as indicators of the stellar effective temperature Teff. In particular, Gray & Livingston ([CITE]) 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 Teff variation of the Sun during its 11-yr magnetic cycle (Gray & Livingston [CITE]). 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 Teff 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.