Extension of the Calibration of Ca II Triplet as Metallicity Indicator

Abstract

The Ca II triplet lines are the strongest feature in the IR spectra of most late-type stars, and thus their equivalent width (EW) can be measured with reasonable accuracy for faint objects with moderate resolution spectroscopy. Empirical relations between the metallicities of old systems as globular clusters, and the reduced EW of the Ca II lines were derived by several authors (Rutledge et al. 1997, PASP, 109, 907). This has turned the Ca II triplet into one of the most popular stellar metallicity indicators. With the advent of 8 m class telescopes, this indicator becomes of invaluable help to study large samples of faint stars in the Local Group, and beyond. However, its application to composite stellar populations is still uncertain, due to the lack of metal rich and intermediate-age calibrators and to its still unexplored behavior in conditions of varying [Ca/Fe] ratios. We present preliminary results of our project to extend the calibration of the Ca II triplet to intermediate-age and young stellar populations and higher metallicity regimes, using red giant stars in open clusters. Also, we will explore the problematic related to its calibration against [Fe/H] and/or [Ca/H]. For this project we have four nights at the INT to observe a sample of red giants in open clusters. We expect to obtain high resolution spectra of a subsample of these stars to derive accurate elemental abundances. The main objective of this work is to apply this technique to the Local Group dwarf galaxies to derive their chemical enrichment histories. Particularly, we plan to study the LMC, where there is a wide range in age and metallicity. For this purpose, we have been granted three nights at the CTIO 4 m with Hydra. We plan to observe a large number of stars in four fields at galactocentric distances of 2.8, 4.0, 5.3 and 6.6 kpc, for which we have complementary CMDs reaching the old main sequence turnoffs.