[Paper Review] The NIR Ca II triplet at low metallicity - Searching for extremely low-metallicity stars in classical dwarf galaxies

Else Starkenburg, V. Hill+10 more|University of Groningen research database (University of Groningen / Centre for Information Technology)|Feb 15, 2010

Stellar, planetary, and galactic studiesPhysics and Astronomy86 references136 citations

TL;DR

This paper re-evaluates the near-infrared Ca II triplet (CaT) method for measuring metallicities in low-metallicity red giant stars, showing that standard empirical calibrations break down below [Fe/H] = –2. The authors present a new, non-linear calibration valid from [Fe/H] = –4 to –0.5, which, when applied to classical dwarf galaxies, suggests a higher abundance of extremely metal-poor stars ([Fe/H] ≤ –3) than previously thought, resolving a long-standing discrepancy in the observed metallicity distribution.

ABSTRACT

The NIR Ca II triplet absorption lines have proven to be an important tool for quantitative spectroscopy of individual red giant branch stars in the Local Group, providing a better understanding of metallicities of stars in the Milky Way and dwarf galaxies and thereby an opportunity to constrain their chemical evolution processes. An interesting puzzle in this field is the significant lack of extremely metal-poor stars, below [Fe/H]=-3, found in classical dwarf galaxies around the Milky Way using this technique. The question arises whether these stars are really absent, or if the empirical Ca II triplet method used to study these systems is biased in the low-metallicity regime. Here we present results of synthetic spectral analysis of the Ca II triplet, that is focused on a better understanding of spectroscopic measurements of low-metallicity giant stars. Our results start to deviate strongly from the widely-used and linear empirical calibrations at [Fe/H]

Motivation & Objective

• To resolve the apparent lack of extremely metal-poor stars ([Fe/H] ≤ –3) in classical dwarf galaxies, as inferred from standard Ca II triplet (CaT) spectroscopy.
• To investigate whether the widely used linear empirical calibrations of the CaT are biased in the low-metallicity regime.
• To develop a new, physically motivated calibration for CaT equivalent widths that remains accurate at [Fe/H] < –2.
• To re-analyze existing low-resolution data using the new calibration and reassess the true metallicity distribution of red giant stars in classical dSphs.

Proposed method

• Synthetic spectral analysis of the Ca II triplet lines using detailed model atmospheres and radiative transfer calculations across a range of metallicities ([Fe/H] from –4 to –0.5).
• Incorporation of non-LTE effects on Ca II line strengths using empirical corrections derived from model grids.
• Development of a new non-linear calibration formula: [Fe/H] or [Ca/H] = a + b×(Abs. mag.) + c×EW(2+3) + d×EW(2+3)^–1.5 + e×EW(2+3)×(Abs. mag.), where Abs. mag. is V–V_HB, M_V, or M_I.
• Calibration of the new relation using high-resolution spectroscopic data from the DART survey as a reference.
• Validation of the new calibration by comparing synthetic CaT line ratios (8542Å/8662Å) with observed values and assessing sensitivity to luminosity and metallicity.

Experimental results

Research questions

• RQ1 Is the observed lack of extremely metal-poor stars in classical dwarf galaxies real, or is it an artifact of flawed CaT-based metallicity calibrations?
• RQ2 How do standard linear CaT calibrations perform at [Fe/H] < –2, and what causes their failure?
• RQ3 Can a new, non-linear calibration be derived from synthetic spectra that accurately measures metallicities down to [Fe/H] = –4?
• RQ4 What is the true abundance of [Fe/H] ≤ –3 stars in classical dSphs when corrected with the new calibration?

Key findings

• The widely used linear CaT calibrations significantly overestimate metallicity at [Fe/H] < –2, leading to an underestimation of the number of extremely metal-poor stars.
• The new non-linear calibration is valid for [Fe/H] between –4 and –0.5 and provides a much better match to high-resolution reference data than previous methods.
• The ratio of the 8542Å to 8662Å CaT lines is 1.27 on average for [Fe/H] between –2.5 and –0.5, with only small dependence on luminosity, confirming consistency with observed data.
• When applied to existing low-resolution data, the new calibration suggests that classical dwarf galaxies contain significantly more extremely metal-poor stars than previously inferred.
• The new calibration reduces the discrepancy between the metallicity distribution of classical dSphs and that of the Galactic halo, particularly in the [Fe/H] ≤ –3 regime.