John J. Cowan
David Ross Boyd Professor
B.A. 1970 George Washington
Ph.D. 1976 Maryland

STELLAR ABUNDANCES, NUCLEOSYNTHESIS, AND AGES

We have been making a large number of observational and theoretical studies of the heavy element abundances in Galactic halo stars. The observations have been made from the ground (Keck, McDonald, Kitt Peak) and from space (HST). Using the HST we have identified gold for the first time and uranium for only the second time in any halo star (BD+17 3248). Our abundance observations indicate the presence of rapid-neutron capture (i.e., r-process) elements in old Galactic halo and globular cluster stars. These observations demonstrate that the earliest generations of stars in the Galaxy, responsible for neutron-capture synthesis and the progenitors of the halo stars, were rapidly evolving. Abundance comparisons among large numbers of stars provide clues about the nature of neutron-capture element synthesis both during the earliest times and throughout the history of the Galaxy. In particular, these comparisons suggest differences in the way the heavier (including Ba and above) and lighter neutron capture elements are synthesized in nature. Understanding these differences will help to identify the astrophysical site (or sites) of and conditions in the r-process. The abundance comparisons also demonstrate a large star-to-star scatter in the neutron-capture/iron ratios at low metallicities- which disappears with increasing [Fe/H]- and suggests an early, chemically unmixed and inhomogeneous Galaxy. Our very recent neutron-capture element observations indicate that the early phases of Galactic nucleosynthesis, and the associated chemical evolution, are quite complex, with the yields from different (progenitor) mass-range stars contributing to different chemical mixes. Stellar abundance comparisons indicate a change from the r-process to the slow neutron capture (i.e., s-) process at higher metallicities (and later times) in the Galaxy. We are also using the observed abundances of the radioactive elements thorium and uranium in halo and globular cluster stars to determine the radioactive ages of the oldest stars in the Galaxy. These age estimates, clustering around 14 +/- 4 Gyr, provide lower limits on the age of the Galaxy and provide constraints on cosmological age determinations. During the last year we have focused on making more accurate abundance determinations in Galactic halo stars by utilizing new laboratory atomic data (in collaboration with J. E. Lawler at U. of Wisconsin and C. Sneden at U. of Texas) for several elements including Ho and Pt. This work has led to a new comprehensive abundance analysis of the well-known Galactic halo star CS 22892-052. We have also been examining abundance trends of certain neutron-capture elements, including La and Eu, in the Galaxy as a function of metallicity. Employing new recent HST and Keck observations of a number of metal-poor Galactic halo stars we are extending our studies to the lighter neutron-capture elements Ge and Ga. New chemical evolution studies, specifically for the elements Sr, Y and Zr, have recently been completed in collaboration with C. Travaglio (MPI and Torino), R. Gallino (Torino) and C. Sneden. Two undergraduate students, Jason Collier and Faith Jordan, were involved in these recent research projects.

UV, RADIO AND X-RAY OBSERVATIONS OF EXTRAGALACTIC SUPERNOVAE

RECENT PUBLICATIONS:

J. J. Cowan and F.-K. Thielemann, The r-Process in Supernovae,'' Physics Today, 57, 47 (2004).

C. Travaglio, R. Gallino, E. Arnone, J. J. Cowan, F. Jordan and C. Sneden, Galactic Evolution of Sr, Y, Zr: a multiplicity of stellar neutron capture components,'' astro-ph/0310189, Astrophys. J., 496, 864 (2004).

K.-L. Kratz, B. Pfeiffer, J. J. Cowan and C. Sneden, R-Process Chronometers,'' New Astronomy Reviews 48, 105 (2004).

J. E. Lawler, C. Sneden and J. J. Cowan, Improved Atomic Data for Ho II and New Holmium Abundances for the Sun and Three Metal-Poor Stars,'' Astrophys. J., 608, 850 (2004).

Y.-H. Chu, R. A. Gruendl, C. J. Stockdale, M. P. Rupen, J. J. Cowan and S. W. Teare, The Nature of SN 1961V,'' astro-ph/0402473, Astron. J., 127 , 2850 (2004)

J. Simmerer, C. Sneden, {\bf J. J. Cowan}, J. Collier, V. Woolf and J. Lawler, The Rise of the s-Process in the Galaxy,'' astro-ph/0410396, Astrophys. J., in press (2004)

E. A. Den Hartog, T. M. Herd, J. E. Lawler, C. Sneden, J. J. Cowan and T. C. Beers, Improved Laboratory Transition Probabilities for Pt I and Application to the Platinun Abundances of BD+17 3249 and the Sun,'' Astrophys. J., in press (2004).