The calcium infrared triplet lines in stellar spectra.

Observations of the infrared triplet lines of ionized calcium are often used as diagnostics of surface gravity among the stars contributing to the integrated light of distant galaxies. We have calculated the equivalent widths of these lines for a series of models with a broad range of input parameters to test their sensitivity to surface gravity, temperature, and calcium abundance. Over a larger range of parameter space than in earlier investigations, we find the sensitivity to be more complex than previously thought. We derive theoretical relations between equivalent width and log(g) for different values of T_eff and metallicity and compare our results with observations. By actual NLTE calculations, we show that departures from LTE are small. Comparison of our results with observations of stars in the Galaxy suggests that the Ca/Fe ratio is an increasing function of metallicity.