5 Conclusions

Models for CaT index have been computed for SSP ranging in age from 1 Myr to 17 Gyr and in metallicity from 0.2 to 2.5 $Z_{\odot}$ in order to evaluate the usefulness of this index to constrain the age and metallicity of stellar populations. Two grids of models have been generated, based on different calibrations of the index as a function of the stellar parameters: effective temperature, gravity and abundance: grid I assumes the theoretical calibration given by JCJ92 and grid II a fitting to observational data (DTT89, Z91). Values from grid I are systematically lower than those from grid II. Since the abundance scale is different in the two set of models, [Ca/H] in grid I, [Fe/H] in grid II, the differences at low metallicity can be explained if an enhacement of $\alpha$ elements, like Ca, with respect to the solar partition is assumed. When cool M-type stars are present in a SSP, the different calibration assumed in both grids leads to different values for CaT, and more data and models are needed to account properly for the contribution of these stars to the index.

The evolution of EW(CaT) in SSP with time presents a primary maximum when RSG appear (around 10 Myr). The value of this peak is strong metallicity-dependent due to both, the evolutionary tracks and the intrinsic dependence of the index with the abundance for supergiants. At about 100 Myr the appearance of the AGB phase produces a sudden increase of the index (secondary maximum) followed by a decrease until 1 Gyr, remaining then almost constant for a given metallicity.

In star-forming regions, we propose the use of a diagnostic diagram, EW(CaT) versus EW($\hbox{H}_\beta$) in emission, to evaluate if CaT is produced by RSG in the region, or by an underlying old population and therefore as a tool to date the starburst. This diagnostic is a powerful indicator of the presence of RSG in high metallicity scenarios, where values larger than 4-5 Å unveil the presence of young RSG. At lower metallicities, the sole use of the diagram cannot disentangle between giant and RSG contributions, especially if the starburst is not very luminous with respect to the underlying population.

Finally, the behaviour of the index in populations older than 1 Gyr shows that the strength of CaT index is controlled by the abundance, leading us to propose another diagnostic diagram, this time using EW(CaT) versus the logarithm of the traditionally used $\hbox{H}_\beta$ (absorption) index, as a tool to break the age-metallicity degeneracy in elliptical galaxies.

More theoretical work and observed data are needed to assess the importance of CaT for abundance and age determination in stellar populations.

Acknowledgements

We thank Javier Gorgas, Enrique Pérez, Rosa González-Delgado, Eduardo Hardy and Claus Leitherer for their helpful comments. This work has been partially supported by the Spanish DGICYT project PB 93-052 and by the TMR grant ERBFMRX-CT96-0086 from the European Community. We also thank the anonymous referee for the useful comments which have contributed significatively to the improvement of the paper. Finally, we thank N. Smith for her help with English during the revision of the manuscript.