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]<-2. We provide a new calibration for Ca II triplet studies which is valid for -0.5<[Fe/H]<-4. We subsequently apply this new calibration to current data sets and suggest that the classical dwarf galaxies are not so devoid of extremely low-metallicity stars as was previously thought.
Highlights
Introduction and outlineTo understand galaxy evolution it is critical to understand how the metallicities of stars in physically different environments develop with time
One star from the Boötes I dwarf galaxy as studied by Norris et al (2008, 2010) using medium- and highresolution spectroscopy is plotted. For this star only one of the two strongest Ca ii NIR triplet (CaT) lines could be measured by Norris et al (2008) with confidence, and the total equivalent width for both lines was inferred using this single line and the observed ratio between the two lines from Norris et al (2008, 1996)2 Norris et al (2008) find [Fe/H] = −3.45 from medium-resolution spectroscopy using the Ca II H and K lines, which is very close to the value we deduce from the EW of the CaT line in the same spectrum, [Fe/H] = −3.32
For the errors in the sum of equivalent widths we use the results of Battaglia et al (2008b) who found that the random error from repeated measurements of the low-resolution sample in the sum of the two broadest lines is well represented by σEW2+3 ≈ 6/(S/N)
Summary
To understand galaxy evolution it is critical to understand how the metallicities of stars in physically different environments develop with time. A direct detailed comparison between the low-resolution CaT metallicities and high-resolution measurements for large samples of RGB stars in the nearby dwarf galaxies Fornax and Sculptor is given by Battaglia et al (2008b) They concluded that the CaT − [Fe/H] relation (calibrated on globular clusters) can be applied with confidence to RGB stars in composite stellar populations over the range −2.5 < [Fe/H] < −0.5. The change in relative strength of the core and wings of the line changes the sensitivity of the line to the Ca abundance and to the overall metallicity in general This understanding of the physical process motivates our re-calibration of the relation between CaT equivalent width and [Fe/H] at lower metallicities. Because of the broad wings, the area of the core of the line that is missing is a negligible fraction of the total equivalent width
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