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 Johanson 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 metallicity 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 GO 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 of approximate to 50 K between temperatures derived from color indices and from spectral lines are most likely a result of interstellar reddening affecting the photometry.