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H. R. Jacobson (NOAO/UT Austin), E. D. Friel (NSF), C. A. Pilachowski (IU Bloomington), S. Balachandran (UMD), E. Barrett (UHI), L. Fullton (STScI)
Old open clusters yield information on the chemical properties and chemical evolution of the Galactic disk. Despite an increasing number of modern photometric studies of open clusters, few detailed abundance analyses have been done, particularly of the oldest ones. We present here the first in a number of spectroscopic studies of the old cluster population, with results for Collinder 261, estimated to be 7--10 Gyr old (Janes & Phelps 1994, Mazur et al.\ 1995, Gozzoli et al.\ 1996). High resolution echelle spectra have been obtained of four K giants in the cluster using the CTIO 4m telescope. Abundances were determined differentially relative to Arcturus from over 100 atomic absorption lines. Starting with photometrically-determined atmospheric parameters, we generated Fe abundances as functions of excitation potential, line strength, and wavelength using the LTE code MOOG (Sneden, 1973). Teff, log(g), and vt were varied until Fe abundances showed no dependence upon excitation potential, equivalent width, and microturbulent velocity. Using the final atmospheric parameters thus obtained, we were able to better constrain the reddening of the cluster (E(B-V)\approx0.33) as well as calculate chemical abundances. We found an average [Fe/H] = (-0.21±0.08) relative to the Sun, which we present along with measurements of O, Mg, Ca, Ti, Al, Si, Ni, and Cr. To check the validity of our methods, we obtained high-resolution spectra of two K giants in the well studied M 67 (Balachandran 1995). Using the same line list and model atmospheres, atmospheric parameters obtained from Balachandran (1995), and the equivalent widths of 21 spectral lines, we calculated an average [Fe/H] = (0.00±0.05) relative to the Sun. This result, also presented here, validates the method of our study.
H. R. Jacobson's research was supported by the NOAO/KPNO REU Program, funded by the National Science Foundation.
The author(s) of this abstract have provided an email address for comments about the abstract: hrj@astro.as.utexas.edu