57 research outputs found
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.
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'
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'
Halo Star Abundances and r-Process Synthesis
We review recent observational studies of heavy element abundances in low
metallicity stars and explore some implications of these results for
nucleosynthesis and early Galactic chemical evolution.Comment: 11 pages, 6 figures. To appear in Proceedings of Nuclei in the Cosmos
2000, Nuclear Physics
A Simple Model for r-Process Scatter and Halo Evolution
Recent observations of heavy elements produced by rapid neutron capture
(r-process) in the halo have shown a striking and unexpected behavior: within a
single star, the relative abundances of r-process elements heavier than Eu are
the same as the same as those of solar system matter, while across stars with
similar metallicity Fe/H, the r/Fe ratio varies over two orders of magnitude.
In this paper we present a simple analytic model which describes a star's
abundances in terms of its ``ancestry,'' i.e., the number of nucleosynthesis
events (e.g., supernova explosions) which contributed to the star's
composition. This model leads to a very simple analytic expression for the
abundance scatter versus Fe/H, which is in good agreement with the data and
with more sophisticated numerical models. We investigate two classes of
scenarios for r-process nucleosynthesis, one in which r-process synthesis
events occur in only \sim 4% of supernovae but iron synthesis is ubiquitous,
and one in which iron nucleosynthesis occurs in only about 9% of supernovae.
(the Wasserburg- Qian model). We find that the predictions in these scenarios
are similar for [Fe/H] \ga -2.5, but that these models can be readily
distinguished observationally by measuring the dispersion in r/Fe at [Fe/H] \la
-3.Comment: AASTeX, 21 pages, includes 4 figure
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