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John J. Cowan Ast-0707447 Annual Project Report-year Three

We had a very productive third year with many significant new results and discoveries. The P.I. was assisted in these studies by two of his graduate students, Jason Collier and Julie Ruppert Dalhed, who were partially supported by the grant and worked on grant-related studies. Reference numbers refer to the " Publications Resulting from the NSF Award. " • We are using both ground-based (Keck, HET) and space-based (Hubble Space Telescope, HST) telescopes to make extensive studies of Galactic halo stars. These stars contain the nucleosynthesis products (from the rapid neutron capture process, i.e., the r-process) from the earliest generations of stars – the progenitors of the halo stars. The observed stellar abundance distributions – from the lightest neutron-capture elements, such as Ge, along with some of the heaviest, including Pt – are providing new clues about the earliest Galactic r-process nucleosynthesis. These in turn will help to identify the characteristics and nature of the first stars in the Galaxy and the Universe. For example, our observational studies of the ultra-metal-poor star CS 22892–052 have demonstrated that the abundances of the heaviest (Z ≥ 56) n-capture elements are consistent with a scaled solar r (but not s-)-process pattern (see Figure 1, and #1, #5, & #25). We have strengthened and extended those findings. Our comprehensive stellar abundance comparisons show this same scaled solar system r-process elemental distribution for the heavy neutron-capture elements in a group of several (r-process rich) halo stars, not just in CS 22892–052 (see Figure 2, and refs. # 1, 5, 8, & 25). This agreement strongly suggests that there is one r-process site, at least for elements Z ≥ 56, and these relative elemental (and perhaps isotopic-see # 3, 5, 15, 17, & 25) r-process abundances have not changed over the history of the Galaxy. • To extend and amplify our studies of individual or small numbers of stars, the P.I. has become part of a collaboration (principally with astronomers at the University of Texas) to study large numbers of metal-poor halo stars. This project, known as CASH (Chemical Abundances of Stars in the Halo), is a comprehensive, multi-year, high-resolution spectroscopic survey of more than 1,000 very metal-poor stars ([Fe/H] < −2.0) discovered during the course of previous medium-resolution spectroscopic searches, but not yet observed at high spectral resolution. This survey will represent, by over an order of magnitude, the largest such high-resolution effort of metal-poor …