The infrared CA II triplet - A luminosity indicator for stellar population synthesis

The authors study the properties of the Ca II infrared triplet (Ca T) from a sample of Reticon spectra of 62 stars. The stars range over spectral types B though mid-M, and the sample spans some four orders of magnitude in gravity g, and almost a factor of 10 in metallicity. It is found that, over all spectral types, from F to mid-M, and over this large gravity interval, the equivalent width of Ca T correlates strongly with log g. This is an almost one-valued relation, being relatively insensitive to metallicity. Thus, since most of the light from galaxies is thought to come from G and K stars, the Ca T feature should be a useful tool for constraining the dwarf: giant ratio of the light-dominant stellar population. Existing measurements of Ca T are applied in an analysis of the stellar content in the nucleus of M31.