AAS 207th Meeting, 8-12 January 2006
Session 124 Cool Stars
Poster, Wednesday, 9:20am-6:30pm, January 11, 2006, Exhibit Hall

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[124.04] Abundance Clues to the Natures of the Two {\em r}-Processes

J.J. Cowan (U. of Oklahoma), K.-L. Kratz, B. Pfeiffer, K. Farouqi (U. Mainz), C. Sneden (U. Texas), J. W. Truran (U. Chicago)

Abundances of heavier elements (barium and beyond) in many neutron-capture-rich halo stars accurately replicate the Solar System {\em r}-process pattern. However, abundances of lighter neutron-capture elements in these stars and those of short lived nuclear radioactivities in the early Solar System appear to require contributions from two distinct types of {\em r}-process synthesis events. We examine {\em r}-process theoretical predictions to further explore the implications of the solar and stellar observations. We find that the isotopic fractions of barium, and the elemental Ba/Eu abundance ratios in {\em r}-process rich low metallicity stars can only be matched by computations in which the neutron densities are in the range 23~\lesssim~log~nn~\lesssim~28. Further, our {\em r}-process calculations indicate that it is difficult to decouple production of the {2\rm nd}\ {\em r}-process abundance peak (A~\approx~130, including 129I) from the production of heavier elements in the barium region (A~\approx~140). For {\em r}-process conditions that successfully generate the heavy element pattern extending down to A=135, the relative abundance of 129I produced in this mass region appears to be at least ~90% of the observed solar value. Finally, in the neutron number density ranges required for production of the observed solar/stellar {3\rm rd} {\em r}-process peak (A~\approx~200), the predicted abundances of inter-peak element hafnuim (Z~=~72, A~\approx~160) follow closely those of {3\rm rd}-peak elements (osmium through lead). This suggests that abundance comparisons of hafnium to both rare-earth and {3\rm rd}-peak elements can shed further light on claims of invariance in the entire heavy end of the {\em r}-process abundance pattern.

This work has been supported in part by the National Science Foundation under grants AST 03-07279 (J.J.C.), AST 03-07495 (C.S.), and the Physics Frontier Center (JINA) PHY 02-16783 (J.W.T.), and the DOE under contract B341495 (J.W.T.). Support was also provided by the Deutsche Forschungsgemeinschaft (DFG) under contract KR 806/13-1, and the Helmholtz Gemeinschaft under grant VH-VI-061.

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Bulletin of the American Astronomical Society, 37 #4
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