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T.L. Bosler, T.A. Smecker-Hane (University of California, Irvine), A. McWilliam (Carnegie Institute of Washington)
We have calibrated the strength of the calcium II near-infrared triplet lines with calcium abundance, [Ca/H], derived from model atmosphere abundance analysis of neutral calcium lines for 141 red giant stars in 16 star clusters. The results can be applied to studies of nearby galaxies to yield more accurate metallicities than the current calcium triplet calibration. The strong absorption lines from singly-ionized calcium (Ca II) at 8498Å, 8542Åand 8662Åare excellent observational tools for inferring metallicity because they are strong enough to observe in distant red giant stars and their equivalent widths are linearly related to iron abundance, [Fe/H]. The dependence of Ca II on [Fe/H] has been well-calibrated empirically for globular cluster stars, and some recent work has extended the calibration to younger, more metal-rich stars. However, there is a fundamental problem with inferring [Fe/H] from Ca II because the calibration depends on the Galactic calcium to iron ratio, [Ca/Fe], but the dependence of [Ca/Fe] on [Fe/H] is determined by the details of a galaxy's evolution. We have derived consistent measure of the relationship of Ca II to [Ca/H] independent of galactic evolution.
The Ca II to [Ca/H] calibration was made using data from telescopes at UCO-Lick and Keck Observatories combined with Ca II measurements from Rutledge, et al. 1997, PASP, 109, 907. We used low-dispersion spectra to measure the equivalent widths of the Ca II lines, and we determined [Ca/H] from model atmosphere abundance analysis of numerous Ca I and Fe I, II lines in high-dispersion spectra. While most previous work on the CaII calibration used globular clusters with -1.9 < [Fe/H] < -0.5, we observed both open and globular clusters with ages 2 to 14 Gyrs and with -2.3 < [Fe/H] < +0.4 to get a widely-applicable calibration.
Financial support for this project was provided by National Science Foundation grant AST-0070985 to T. Smecker-Hane. T. Bosler also thanks the ACS Foundation for fellowship support.
The author(s) of this abstract have provided an email address for comments about the abstract: tbosler@uci.edu
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.