AAS 195th Meeting, January 2000
Session 3. Globular Clusters
Display, Wednesday, January 12, 2000, 9:20am-6:30pm, Grand Hall

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[3.14] A Globular Cluster Abundance Scale Based on 2.2\micron\ Spectra of Giant Stars

J. A. Frogel, A. W. Stephens, S. Ramirez, D.L. DePoy (OSU)

We have established an [Fe/H] scale for globular clusters based on medium resolution (1500-3000) infrared K band spectra. Our technique uses the Na I (2.2\mum) and Ca I (2.26\mum) atomic line blends and the first band head of CO (2.29\mum). Calibration comes from spectra of more than 100 giant stars in 15 Galactic globular clusters which have good optical abundance determinations. The technique is valid for globular clusters with -1.8<[Fe/H]<-0.1, and is accurate to ±0.1 dex with observations of only a few stars per cluster.

Strong molecular absorption throughout the optical region of the spectrum reduces the observed light from the giants in a cluster by up to several magnitudes, making them appear significantly fainter than stars which are much further down the giant branch. On the other hand, our technique capitalizes on the fact that the stars on the upper part of the giant branch are the brightest and reddest stars in the infrared. This makes them easy to identify and separate from foreground stars even on "snapshot", non-photometric images, minimizes effects of interstellar extinction, and requires only short exposure spectroscopic observations.

Our technique allows for accurate metallicity determinations of globular clusters which are difficult to study optically, either because of high extinction or crowding, such as toward the Galactic center. As an example, we have determined new abundances for NGC 6528 (-1.28), NGC 6539 (-0.74), HP 1 (-0.44), Liller 1 (-0.10), Terzan 2 (-0.69), and Terzan 4 (-1.56). The new values for NGC 6528 and HP 1 differ significantly from previous determinations based primarily on optical photometry; the old values were -0.7 and -1.5 respectively. This system can also be extended to extra-galactic clusters via observations with NGST.

The early phases of this work was partially supported by National Science Foundation grant AST-9218281.

The author(s) of this abstract have provided an email address for comments about the abstract: frogel@astronomy.ohio-state.edu

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