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Metallicity and Alpha‐Element Abundance Measurement in Red Giant Stars from Medium‐Resolution Spectra

Kirby, Evan N. and Guhathakurta, Puragra and Sneden, Christopher (2008) Metallicity and Alpha‐Element Abundance Measurement in Red Giant Stars from Medium‐Resolution Spectra. Astrophysical Journal, 682 (2). pp. 1217-1233. ISSN 0004-637X. doi:10.1086/589627.

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We present a technique that applies spectral synthesis to medium-resolution spectroscopy (MRS; R ~ 6000) in the red (6300 Å < λ < 9100 Å) to measure [Fe/H] and [α/Fe] of individual red giant stars over a wide metallicity range. We apply our technique to 264 red giant stars in seven Galactic globular clusters and demonstrate that it reproduces the metallicities and α-enhancements derived from high-resolution spectroscopy (HRS). The MRS technique excludes the three Ca II triplet lines and instead relies on a plethora of weaker lines. Unlike empirical metallicity estimators, such as the equivalent width of the Ca II triplet, the synthetic method presented here is applicable over an arbitrarily wide metallicity range and is independent of assumptions about the α-enhancement. Estimates of cluster mean [Fe/H] from different HRS studies show typical scatter of ~0.1 dex but can be larger than 0.2 dex for metal-rich clusters. The scatter in HRS abundance estimates among individual stars in a given cluster is also comparable to 0.1 dex. By comparison, the scatter among MRS [Fe/H] estimates of individual stars in a given cluster is ~0.1 dex for most clusters but 0.17 dex for the most metal-rich cluster, M71 (〈 [ Fe/H ] 〉 = − 0.8). A star-by-star comparison of HRS versus MRS [α/Fe] estimates indicates that the precision in [ α/Fe ]_(MRS) is 0.05 dex. The errors in [ Fe/H ]_(MRS) and [ α/Fe ]_(MRS) increase beyond 0.25 dex only below signal-to-noise ratios of 20 Å^(−1), which is typical for existing MRS of the red giant stars in Leo I, one of the most distant Milky Way satellites (250 kpc).

Item Type:Article
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URLURL TypeDescription Paper
Kirby, Evan N.0000-0001-6196-5162
Guhathakurta, Puragra0000-0001-8867-4234
Sneden, Christopher0000-0002-3456-5929
Alternate Title:Metallicity and Alpha-Element Abundance Measurement in Red Giant Stars from Medium Resolution Spectra
Additional Information:© 2008. The American Astronomical Society. Received 2008 February 22; accepted 2008 April 21. The authors gratefully acknowledge P. B. Stetson for providing photometry of all of the spectroscopic targets in this article and J. Simon and M. Geha for providing the DEIMOS observations of M79 and NGC 2419. We thank R. Kraft, D. Lai, C. Rockosi, I. Ivans, and M. Shetrone for extremely useful discussions. We acknowledge National Science Foundation grants AST 06-07708, AST 03-07966, and AST 06-07852 and NASA/STScI grants GO-10265.02 and GO-10134.02. E. N. K. is supported by a NSF Graduate Research Fellowship. Data herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The analysis pipeline used to reduce the DEIMOS data was developed at UC Berkeley with support from NSF grant AST 00-71048. Facilities: Keck:II(DEIMOS)
Funding AgencyGrant Number
NSFAST 06-07708
NSFAST 03-07966
NSFAST 06-07852
NSF Graduate Research FellowshipUNSPECIFIED
W. M. Keck FoundationUNSPECIFIED
NSFAST 00-71048
Subject Keywords:globular clusters: individual (M13 M15 M71 M79 NGC 2419 NGC 7006 NGC 7492) ; stars: abundances
Issue or Number:2
Record Number:CaltechAUTHORS:20190510-120909003
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Official Citation:Evan N. Kirby et al 2008 ApJ 682 1217
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:95414
Deposited By: George Porter
Deposited On:10 May 2019 22:02
Last Modified:16 Nov 2021 17:12

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