Line-depth ratios: Temperature indices for giant stars

Ratios of the depths of appropriately chosen spectral lines are shown to be excellent indicators of stellar temperatures for giant stars in the G3 to K3 spectral type range. We calibrate five line-depth ratios against B-V and R-I color indices and then translate these into temperatures. Our goal is to set up line-depth ratios to (1) accurately monitor any temperature variations of a few degrees or less that may occur during magnetic cycles or oscillations and (2) rank giants precisely on a temperature coordinate. This is not an absolute calibration of stellar temperatures. We show how giant spectra can be misleading because of the complex dependences of spectral lines on metallicity and absolute magnitude as well as temperature, and it is essential to make corrections to accommodate these complications. The five line-depth ratios we use yield precision for monitoring, i.e., detecting temperature variations, of 4 K from a single exposure. Ranking giants by temperature can be done with errors of similar to 25 K but could be improved with better determinations of the metallicity and absolute-magnitude corrections.