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

Using the Very Large Array (VLA) we have been following the long-term radio behavior of intermediate-age (i.e., 10--100 year old) extragalactic supernovae. We have found that these supernovae, such as SN 1970G in M101 and SN 1923A in M83, are still emitting in the radio decades after the supernova explosion. These observations are designed to understand how supernovae evolve into supernova remnants (SNRs), which typically take at least 100 years to become radio emitters. The observations also provide an indication of the circumstellar mass-loss rate, which affects the level and duration of the radio emission, from the supernova progenitor star. In collaboration with You-Hua Chu (U. of Illinois) we have also examined in some detail the radio and optical emission of one very unusual object, SN 1961V, in NGC 1058. Combining HST (STIS) and radio observations we are attempting to determine the exact nature of this object - whether it is in fact a supernova or a luminous blue variable star similar to Eta Carinae. Our recent observations indicate an SN occurred in the vicinity of SN 1961V a few decades ago. In addition we have been making coordinated multi-wavelength (Kitt Peak, VLA and Chandra) observations of point sources in nearby face-on galaxies. We are trying to identify previously undetected supernovae or SNRS. Our VLA observations are also being used to distinguish High \& Low Mass X-ray Binaries (HMXBs \& LMXBs) from supernova remnants (SNRs). We are also trying to identify the population of SNRs, HMXBs, and LMXBs in spiral galaxies and whether there are massive black holes (MBHs) in these spiral galaxies. To resolve the central region of one galaxy (M83), a new (radio) VLBI experiment has been approved to use the Long Baseline Array (LBA) of Australia Telescope. These observations will assist in the possible detection of coincident radio/x-ray sources in the complex nuclear region. Chris Stockdale (now graduated and a professor at Marquette University) and Larry Maddox worked with me on these projects. A new collaboration with Steven Tingay (Swinburne University of Technology, Australia) was also established for the LBA experiment.

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).