NOAO >   Observing Info >   Approved Programs >   2014A-0278

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Proposal Information for 2014A-0278


PI: Barbara Anthony-Twarog, University of Kansas, bjat@ku.edu
Address: Physics Department, 1251 Wescoe Hall Dr., Lawrence, KS 66045, U.S.A.

CoI: Bruce Twarog, University of Kansas
CoI: Con Deliyannis, Indiana University

Title: Constraining the Evolution and Origin of Li Using Giants in M92

Abstract: Three and a half years ago we began a comprehensive program focused on the collection and analysis of high-dispersion spectra in the Li 6708 Aregion for 150-330 stars per cluster from the tip of the RGB to below the MS-turnoff in each of the open clusters NGC 2158, NGC 6819, and NGC 7789. We have combined intermediate-band photometry and spectroscopy of samples typically an order-of-magnitude larger than any previous open cluster study, to map the main sequence and post-main sequence evolution of Li with age and metallicity. Our success in open clusters led us to push the study limits to exceptionally low metallicity by probing the evolution of stars on the giant branch in M92 ([Fe/H]=-2.7). With ~ 150 stars down to 1 mag below the horizontal branch, we planned to (a) test the impact of intracluster population inhomogeneities on the dispersion in Li, (b) constrain the primordial Li abundance among the dwarfs at the cluster turnoff, (c) observe the impact of the evolving surface convection zone on the Li abundance as the stars evolve to cooler T_eff and higher L, and (d) supply a direct comparison with the evolutionary patterns for the only other globular cluster giant branch studied in Li with the same level of completeness and precision, M4 ([Fe/H]=-1.10). Our summer observing run for this project was only partially successful thanks to poor/mediocre observing conditions(clouds and cirrus), allowing collection of high S/N spectra of 60 bright giants total in two configurations but only one-fourth the integration time needed for one faint configuration.


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