A Globular Cluster Metallicity Scale Based on the Abundance of Feii

By R. P. Kraft, I. Ivans. 2002

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Assuming that in the atmospheres of low‐mass, metal‐poor red giant stars, one‐dimensional models based on local thermodynamic equilibrium accurately predict the abundance of iron from Feii, we derive a globular cluster metallicity scale based on the equivalent widths of Feii lines measured from high‐resolution spectra of giants in 16 key clusters lying in the abundance range −2.4 < [Fe/H]II < −0.7. We base the scale largely on the analysis of spectra of 149 giant stars in 11 clusters by the Lick‐Texas group supplemented by high‐resolution studies of giants in five other clusters. We also derive ab initio the true distance moduli for certain key clusters (M5, M3, M13, M92, and M15) as a means of setting stellar surface gravities. Allowances are made for changes in the abundance scale if one employs (1) Kurucz models with and without convective overshooting to represent giant star atmospheres in place of MARCS models and (2) the Houdashelt et al. color‐temperature scale in place of the Alonso et al. scale. We find that [Fe/H]II is correlated linearly with W′, the reduced strength of the near‐infrared Caii triplet defined by Rutledge et al., although the actual correlation coefficients depend on the atmospheric model employed. The correlations, limited to the range −2.4 < [Fe/H]II < −0.7, are as follows: 1. [Fe/H]II = 0.531W′ - 3.279 (MARCS), 2. [Fe/H]II = 0.537W′ - 3.225 (Kurucz with convective overshooting), 3. [Fe/H]II = 0.562W′ - 3.329 (Kurucz without convective overshooting). We also discuss how to estimate [X/Fe] ratios. We suggest that C, N, and O, as well as elements appearing in the spectrum in the singly ionized state, e.g., Ti, Sc, Ba, La, and Eu, should be normalized to the abundance of Feii. Other elements, which appear mostly in the neutral state, but for which the dominant species is nevertheless the ionized state, are probably best normalized to Fei, but uncertainties remain.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/f10be085dded7b95eeb55d341a3f82dff75284c8

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THE SEGUE STELLAR PARAMETER PIPELINE. IV. VALIDATION WITH AN EXTENDED SAMPLE OF GALACTIC GLOBULAR AND OPEN CLUSTERS (2010, background): “Finally, Kraft & Ivans (2003) also report consistent abundances from Fe i and Fe ii lines of −0.82 and −0.81, respectively.”
Intrinsic iron spread and a new metallicity scale for globular clusters (2009, methodology): “10 we show the comparison of our [Fe/H] values from UVES with those of the metallicity scale by Kraft & Ivans (2003), not much used in spite of being based on the homogeneous re-analysis of EWs of Fe II lines from high resolution spectra.”
Abundances in giant stars of the globular cluster NGC 6752 (2005, unknown): “Kraft & Ivans (2003) found [Fe/H]I = −1.50 and [Fe/H]II = −1.42 using Kurucz models and [Fe/H]I = −1.51 and [Fe/H]II = −1.50 using MARCS models.”
RINGS AND RADIAL WAVES IN THE DISK OF THE MILKY WAY (2015, methodology): “These model atmospheres are compared with fiducial sequences fit to open clusters and globular clusters observed with the SDSS and corrected for reddening using values from Kraft & Ivans (2003).”
A COMPLETE SPECTROSCOPIC SURVEY OF THE MILKY WAY SATELLITE SEGUE 1: THE DARKEST GALAXY (2010, background): “The lowest metallicity Segue 1 giants are also at least a factor of four more metal-poor than any known star in a globular cluster (e.g., King et al. 1998; Kraft & Ivans 2003; Preston et al. 2006; Carretta et al. 2009).”
Abundance patterns of multiple populations in globular clusters: a chemical evolution model based on yields from AGB ejecta (2010, result): “This is consistent with previous determinations, e.g. for NGC 6752 (σ=0.02 Yong et al. 2005), and for the clusters examined by Kraft & Ivans (2003) (0.03 – 0.10 dex).”
The Effective Temperature Scale of FGK Stars. I. Determination of Temperatures and Angular Diameters with the Infrared Flux Method (2005, unknown): “The surface gravities of the cluster giants were calculated from their distance moduli (Kraft & Ivans 2003) and the bolometric corrections of AAM99a, taking 0.85M⊙ as an estimate of the stellar masses.”
The two metallicity groups of the globular cluster M 22: a chemical perspective (2011, background): “Indeed, while there are good reasons to base [Fe/H] on the lines of Fe II (Kraft & Ivans 2003) our resolution limits the number of unblended Fe II lines so that it is necessary to base [Fe/H] on lines of Fe I.”
A Multicolor and Fourier Study of RR Lyrae Variables in the Globular Cluster NGC 5272 (M3) (2004, background): “…hereafter ZW), has been revised using high resolution spectroscopic data of giant stars and appears to be somewhat higher, i.e. –1.47 (Kraft et al. 1992) or –1.50 (Kraft & Ivans 2003, hereafter KI) in the FeII metallicity scale, and –1.34 (Carretta & Gratton 1997 in their own metallicity scale).”
THE ABUNDANCES OF NEUTRON-CAPTURE SPECIES IN THE VERY METAL-POOR GLOBULAR CLUSTER M15: A UNIFORM ANALYSIS OF RED GIANT BRANCH AND RED HORIZONTAL BRANCH STARS (2011, methodology): “This is done in order to minimize ionization equilibrium uncertainties as described in detail by Kraft & Ivans (2003)(54).”
Chemical abundance analysis of the open clusters Cr 110, NGC 2099 (M 37), NGC 2420, NGC 7789, and M 67 (NGC 2682) (2009, methodology): “Low resolution spectroscopy using the infrared calcium triplet by Carrera et al. (2007) gave: [Fe/H]=–0.01±0.07 dex in the Carretta & Gratton (1997) scale, [Fe/H]=0.0±0.3 dex in the Zinn & West (1984) scale and [Fe/H]=–0.19±0.21 dex in the Kraft & Ivans (2003) scale.”
ON THE SHOULDERS OF GIANTS: PROPERTIES OF THE STELLAR HALO AND THE MILKY WAY MASS DISTRIBUTION (2014, background): “The [Fe/H] values for M92, M13 and M71 are taken to be -2.38, -1.60 and -0.81 (Kraft & Ivans 2003;An et al. 2008) respectively whereas for NGC6791 it is assumed to be +0.40 (An et al. 2008).”
The formation and assembly history of the Milky Way revealed by its globular cluster population (2018, background): “There exist systematic differences in metallicity and abundance scales adopted by various studies reporting GC ages (e.g. Carretta & Gratton 1997; Kraft & Ivans 2003; Carretta et al. 2009), which in turn affect the GC age measurements (e.g. VandenBerg et al. 2016).21 These may be improved by better…”