AN EMPIRICAL-ANALYSIS OF ABSORPTION FEATURES IN SYNTHETIC SPECTRA

We have computed synthetic spectra in a grid of temperature, surface g ravity, [Fe/H], and [alpha/Fe] in order to investigate the sensitivity of the MgH and Mgb feature and of five Fe features in the region lamb dalambda 4900-5500 to changes in model parameters. After convolving th e spectra with a Gaussian in order to simulate the blend of features o bserved with medium resolution spectroscopy, we obtained an overall me tallicity from each feature with the empirical calibrations of late-ty pe stars by Friel & Janes [A&A, 267, 75 (1993)]. We find that, on the whole, output metallicities accurately reflect differences in model [F e/H], demonstrating that the calibration lines of Friel and Janes have accurately modeled index strength as a function of (B-V) and [Fe/H]. The Mg feature and, to a lesser extent, the Fe features at 5335, 501 1 , and 5270 angstrom are sensitive to surface gravity. The Mg feature i s also quite sensitive to [alpha/Fe]. These results, combined with sys tematic effects recently noted among observations of old open cluster giants, support the likelihood of varying [Mg/Fe] in the open cluster system.