An Observational Perspective On Some Aspects Of Early Stellar Nucleosynthesis

Some basic abundance results for low metallicity stars that were formed in the early days of the Milky Way Galaxy are summarized. Discussion is centered on two nucleosynthetic groups: the light a elements (Mg, Si, and Ca), and the neutron-capture elements (those heavier than the Fe group, atomic numbers greater than 30). Emphasis is placed on the present state of stellar spectroscopic and atomic transition data.Astronom

The Early Formation, Evolution and Age of the Neutron-Capture Elements in the Early Galaxy

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 several halo stars show that the heaviest neutron-capture elements (including Ba and heavier) are consistent with a scaled solar system r-process abundance distribution, while the lighter such elements do not conform to the solar pattern. These comparisons suggest two r-process sites or at least two different sets of astrophysical conditions. The large star-to-star scatter observed in the neutron-capture/iron ratios at low metallicities -- which disappears with increasing [Fe/H] -- suggests an early, chemically unmixed and inhomogeneous Galaxy. The stellar abundances indicate a change from the r-process to the slow neutron capture (i.e., s-) process at higher metallicities in the Galaxy. The detection of thorium in halo and globular cluster stars offers a promising, independent age-dating technique that can put lower limits on the age of the Galaxy.Comment: 6 pages, 3 figures; To appear in the proceedings of the 20th Texas Symposium on Relativistic Astrophysics, J. C. Wheeler & H. Martel (eds.

Nuclear Chronometers

Observations of metal-poor Galactic halo stars indicate that the abundance pattern of the (heaviest) neutron-capture elements is consistent with the scaled solar system r-process abundances. Utilizing the radioactive (r-process) element thorium, age determinations have been made for several of these same stars, placing constraints on both Galactic and cosmological age estimates.Comment: 6 pages, 2 figures. To appear in the Proceedings of ``Cosmic Evolution'

Galactic Cosmochronometry from Radioactive Elements in the Spectra of Very Old Metal-Poor Stars

In a short review of neutron-capture elemental abundances in Galactic halo stars, emphasis is placed on the use of these elements to estimate the age of the Galactic halo. Two prominent characteristics of neutron-capture elements in halo stars are their large star-to-star scatter in the overall abundance level with respect to lighter elements, and the dominance of r-process abundance patterns at lowest stellar metallicities. The r-process abundance signature potentially allows the direct determination of the age of the earliest Galactic halo nucleosynthesis events, but further developments in r-process theory, high resolution spectroscopy of very metal-poor stars, and in basic atomic data are needed to narrow the uncertainties in age estimates. Attention is brought to the importance of accurate transition probabilities in neutron-capture element cosmochronometry. Recent progress in the transition probabilities of rare earth elements is discussed, along with suggestions for future work on other species.Comment: 19 pages, 5 figures; To appear in Physica Script

Neutron-Capture Element Trends in the Halo

In a brief review of abundances neutron-capture elements (Z > ~30) in metal-poor halo stars, attention is called to their star-to-star scatter, the dominance of r-process synthesis at lowest metallicities, the puzzle of the lighter members of this element group, and the possibility of a better r-/s-process discriminant.Comment: 6 pages, 2 figures. To appear in the Proceedings of ``Cosmic Evolution'