Spectral line-depth ratios as temperature indicators for cool stars

The use of spectral line-depth ratios as a stellar thermometer in G and K dwarfs is developed and refined beyond an earlier study (Gray and Johnson 1991). Ratios incorporating a line with any degree of saturation, as with the lambda-6252 V I to lambda-6253 Fe I ratio used in the 1991 work, produce metallicity dependent results. This dependence is investigated here, and a correction derived. Ten line-depth ratios using only weak lines are shown to have negligible metallicty dependence and resolve temperature differences as small as 6 K for early K dwarfs from a single exposure having a signal-to-noise ratio of 500. Precision deteriorates badly toward G0 for these particular spectral lines. Smaller temperature differences can be resolved by combining exposures. Relative temperatures of 65 dwarfs are given, a few having errors near 1 K. Inconsistencies ~50 K between temperatures derived from color indices and from spectral lines are most likely a result of interstellar reddening affecting the photometry.