Stellar Nucleosynthesis in the Hyades Open Cluster

We report a comprehensive light element (Li, C, N, O, Na, Mg, and Al) abundance analysis of three solar-type main sequence (MS) dwarfs and three red giant branch (RGB) clump stars in the Hyades open cluster using highresolution and high signal-to-noise spectroscopy. The abundances have been derived in a self-consistent fashion, and for each group (MS or RGB), the CNO abundances are found to be in excellent star-to-star agreement. Using the dwarfs to infer the initial composition of the giants, the combined abundance patterns confirm that the giants have undergone the first dredge-up and that material processed by the CN cycle has been mixed to the surface layers. The observed abundances are compared to predictions of a standard stellar model based on the Clemson-American University of Beirut (CAUB) stellar evolution code. The model reproduces the observed evolution of the N and O abundances, as well as the previously derived C/C ratio, but it fails to predict by a factor of 1.5 the observed level of C depletion. A similar discord appears to exist in previously reported observed and modeled C abundances of giants in the Galactic disk. Random uncertainties in the mean abundances and uncertainties related to possible systematic errors in the Hyades dwarf and giant parameter scales cannot account for the discrepancy in the observed and modeled abundances. Li abundances are derived to determine if non-canonical extra mixing, like that seen in low-mass metal-poor giants, has occurred in the Hyades giants. The Based on data taken with the Harlan J. Smith 2.7-m telescope at The McDonald Observatory of the University of Texas at Austin. National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ, 85719; [email protected] Department of Physics and Astronomy, Clemson University, 118 Kinard Laboratory, Clemson, SC, 29634; [email protected], [email protected]