Studies of Very Metal Poor Stars

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PI CohenMore than 20 years ago it was discovered that the tip of the giant branch in some galactic globular clusters is populated by stars which show wide variations in C, N, and O abundances. Subsequently less strong, but still real, variations in Na, Al and Mg were detected among the same stars. The goal of this project is to determine the variations of abundances of elements in metal poor stars, concentrating first on globular clusters, as a function of luminosity and evolutionary state. Our ability to explain and reproduce such variations is a stringent test of our understanding of many aspects of stellar evolution as well as of the early chemical history of the Galaxy. It is a test we sometimes fail. In previous work anticorrelated variations of O and Na were found in the globular cluster M71. Furthermore large anticorrelated variations in CH and CN also were found contrary to predictions by stellar evolution theory. The present project will continue and expand this work to more metal poor globular clusters, where the distinction between star-to-star abundance variations arising from dredge up versus primordial variations should be clearer. In addition, the study will extend to more metal poor clusters the work on the CH and CN bands to as low a luminosity as possible. In a related major program the abundance distributions and scatter among the most metal poor stars in the Galaxy, those with [Fe/H] =.3.0 dex, are being studied.***