AAS 195th Meeting, January 2000
Session 44. Late Stages of Stellar Evolution
Display, Thursday, January 13, 2000, 9:20am-6:30pm, Grand Hall

## [44.04] Neutron-Capture Elements in Low Metallicity Halo Giants

C. Sneden, R.S. French (Univ. of Texas at Austin), J.J. Cowan (Univ. of Oklahoma), J.E. Lawler (Univ. of Wisconsin), F. Primas (ESO), T.C. Beers (Michigan State Univ.), J.W. Truran (Univ. of Chicago)

We are conducting a high resolution (R~~q 30,000) ultraviolet spectroscopic survey of 10 very metal-poor halo giants using the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope. The general goal is to determine abundances of several key neutron-capture elements (Z~>~30) that have no transitions accessible to ground-based spectroscopy. The stars chosen for STIS observations have -3.0 \leq~[Fe/H]~\leq --1.4, but they all have large relative overabundances of neutron-capture elements relative to iron. For example, all target stars have [Eu/Fe]~> +0.5. We also have obtained STIS high resolution spectra of the well-studied halo giant HD 122563, whose substantial neutron-capture-element deficiency renders all transitions of these elements undetectably weak amid the forest of Fe-peak and OH lines in its UV spectrum. We develop synthetic spectrum line lists through iterative attempts to match the HD~122563 spectrum, and then use these in performing line-by-line differential abundance analyses of the neutron-capture-rich program stars.

Detections of all possible neutron-capture elements are important, but for now we focus on Os, Ir, Pt, and Au because these are the heaviest of the stable elements (the so-called 3^{rd} neutron-capture peak elements). With only half of the scheduled observations in hand, we have already detected at least six lines of \ion{Pt}{1} in most targets, as well as lines of \ion{Os}{1}, \ion{Ge}{1}, \ion{Zr}{2}, \ion{Pb}{1}, and \ion{Ba}{2}. Additionally, we may have struck gold'' with the probable detection of the \ion{Au}{1} resonance lines at 2428, 2676~Å.

The STIS spectra, derived with full model atmosphere, synthetic spectrum analyses, yield abundances that will shed light on star-to-star abundance variations of 3rd neutron-capture peak elements. Additionally, these abundances will be employed along with ground-based abundances of thorium to provide new estimates of the Galactic age.

This research is supported by NASA STScI grant GO-08342 and NSF grants AST-9618364 to C.S. and AST-9618332 to J.J.C.