The Ca II triplet lines as diagnostics of luminosity, metallicity and chromospheric activity in cool stars?

Abstract. CCD spectra of the infrared triplet lines of ionized calcium at 8498, 8542, 8662 have been obtained at a spectral resolution of 0.4 A in 146 stars brighter than V = +7:0 spanning a range in spectral types from F7 to M4 of all luminosity classes and a range in metallicity [Fe/H] from −3:0 to +1.1. These have been analysed to in-vestigate the dependence of the Ca II triplet strengths on stellar parameters like luminosity, temperature and metal-licity. A detailed study reveals a strong dependence on lu-minosity, much stronger for metal rich stars than for the metal poor ones and a milder dependence on metallicity, although much more conspicuous in supergiants than in dwarfs. All these correlations are found to be non-linear over the parameter space covered. The present study also shows chromospheric activity to be an important phe-nomenon aecting the strength and the shape of the line pro les. Stars of similar luminosity and metallicity have varying Ca II line depths owing to varying chromospheric emission lling in their Ca II absorption. The Ca II triplet strength is thus observed to be a triparametric discrimi-nant in cool stars