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The Metallicity Distribution Function of Red Giants in the Large Magellanic Cloud

Citation

Cole, AA and Smecker-Hane, TA and Gallagher, JS, The Metallicity Distribution Function of Red Giants in the Large Magellanic Cloud, The Astronomical Journal, 120, (4) pp. 1808-1829. ISSN 0004-6256 (2000) [Refereed Article]

DOI: doi:10.1086/301564

Abstract

We report new metallicity determinations for 39 red giants in a 220 arcmin 2 region, 1°.8 southwest of the bar of the Large Magellanic Cloud. These abundance measurements are based on spectroscopy of the Ca II infrared triplet. We have carefully considered the effects of abundance ratios, the physics of Ca II line formation, the variation of red clump magnitude, and the contamination by foreground stars in our abundance analyses. The metallicity distribution function (MDF) shows a strong peak at [Fe/H] = -0.57 ± 0.04; a tail to abundances at least as low as [Fe/H] ≈ -1.6 brings the average abundance down to [Fe/H] = -0.64 ± 0.02. Half the red giants in our field fall within the range -0.83 ≤ [Fe/ H] ≤ -0.41. The MDF appears to be truncated at [Fe/H] ≈ -0.25; the exact value of the maximum abundance is subject to ∼0.1 dex uncertainty in the calibration of the Ca II IR triplet for young, metal-rich stars. We find a striking contrast in the shape of the MDF below [Fe/H] ≤ -1 between our inner disk field and the distant outer field studied by Olszewski: red giants deficient by more than a factor of 10 in heavy elements relative to the Sun are extremely scarce in the inner disk of the LMC. Our field star sample does not reproduce the full MDF of the LMC star clusters but seems similar to that of the intermediate-age (1-3 Gyr) clusters. We have also obtained abundance estimates using Strömgren photometry for ≈10 3 red giants in the same field. Photometry is the only practical way to measure abundances for the large numbers of stars necessary to lift age-metallicity degeneracy from our high-precision color-magnitude diagrams. The Strömgren measurements, which are sensitive to a combination of cyanogen and iron lines, correlate well with the Ca II measurements, but a metallicity-dependent offset is found. The offset may be due either to variations in the elemental abundance ratios due to galactic chemical evolution or to a metal-dependent mixing mechanism in RGB stars. An empirical relation between photometric and spectroscopic abundance estimates is derived. This will allow photometric abundance measurements to be placed on a consistent metallicity scale with spectroscopic metallicities, for very large numbers of stars.

Item Details

Item Type:Refereed Article
Research Division:Physical Sciences
Research Group:Astronomical and Space Sciences
Research Field:Galactic Astronomy
Objective Division:Expanding Knowledge
Objective Group:Expanding Knowledge
Objective Field:Expanding Knowledge in the Physical Sciences
Author:Cole, AA (Associate Professor Andrew Cole)
ID Code:45303
Year Published:2000
Web of Science® Times Cited:69
Deposited By:Physics
Deposited On:2007-07-10
Last Modified:2007-07-10
Downloads:0

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