The Ca II triplet in red giant spectra: [Fe/H] determinations and the role of [Ca/Fe]

TL;DR

This study calibrates the Ca II triplet lines in red giants to estimate metallicity ([Fe/H]) and confirms its reliability across different [Ca/Fe] ratios, enabling accurate metallicity measurements in diverse stellar populations.

Contribution

The paper provides a calibration of Ca II triplet line strength to [Fe/H] using Galactic clusters and validates its applicability to systems with varying [Ca/Fe], including the LMC.

Findings

Ca II triplet calibration agrees with high-resolution spectroscopy after small offset correction.

The method shows minimal bias across a range of [Ca/Fe] ratios.

Ca II triplet metallicity estimates are reliable for different chemical evolution histories.

Abstract

Measurements are presented and analyzed of the strength of the Ca II triplet lines in red giants in Galactic globular and open clusters, and in a sample of red giants in the LMC disk that have significantly different [Ca/Fe] abundance ratios to the Galactic objects. The Galactic objects are used to generate a calibration between Ca II triplet line strength and [Fe/H], which is then used to estimate [Fe/H]_CaT for the LMC stars. The values are then compared with the [Fe/H]_spec determinations from high dispersion spectroscopy. After allowance for a small systematic offset the two abundance determinations are in excellent agreement. Further, as found in earlier studies, e.g., Battaglia et al. (2008), the difference is only a very weak function of the [Ca/Fe] ratio. For example, changing [Ca/Fe] from +0.3 to -0.2 causes the Ca II based abundance to underestimate [Fe/H]_spec by only ~0.15 dex, assuming a Galactic calibration. Consequently, the Ca II triplet approach to metallicity determinations can be used without significant bias to study stellar systems that have substantially different chemical evolution histories.