Isotopic abundance analysis of field and cluster stars

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2004-05

Authors

Yong, David C., 1974-

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Abstract

Our understanding of the evolution of the Galaxy is built in part on the interpretation of observed abundance ratios in stars of different metallicity. While elemental abundances may be easier to measure, the production of the individual isotopes can occur in different stellar sites via different nucleosynthetic processes. Therefore, isotope ratios provide a more detailed insight into the chemical evolution of the Galaxy. We conducted a search for cool metal-poor stars from which we measured Mg isotope ratios to study Galactic chemical evolution. Our data are in fair agreement with predictions that assume the isotopes are the product of massive stars. We find an intrinsic scatter at all metallicities which may be due to ejecta from AGB stars. Consideration of the elemental abundances in our sample shows that the mass of the AGB star providing the ejecta dictates whether overabundances of Mg isotopes, s-process elements, or both are observed. To gauge non-LTE effects and inadequacies in the model atmospheres, we measured Mg isotope ratios in the Hyades open cluster. While the model atmospheres did not reproduce ionization equilibrium for Fe, the Mg isotope ratios were not sensitive to these problems. We also used isotope ratios to study globular cluster chemical evolution. Every well studied Galactic globular cluster shows star-to-star differences in and correlations between the light element abundances. The mechanism responsible for these variations remains poorly understood. We measured Mg isotope ratios in the giant stars of the globular cluster NGC 6752. Varying degrees of pollution by AGB stars of the same generation as the observed stars may explain the abundance variations of light elements and isotope ratios. For all elements heavier than Al, there are no star-to-star abundance variations and the mean abundance ratios are in good agreement with other globular clusters and field stars at the same metallicity.

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