1,462 research outputs found
Efficient Searches for r-Process-Enhanced, Metal-Poor Stars
Neutron-capture-enhanced, metal-poor stars are of central importance to
developing an understanding of the operation of the r-process in the early
Galaxy, thought to be responsible for the formation of roughly half of all
elements beyond the iron peak. A handful of neutron-capture-rich, metal-poor
stars with [Fe/H] < -2.0 have already been identified, including the well known
r-process-enhanced stars CS 22892-052 and CS 31082-001. However, many questions
of fundamental interest can only be addressed with the assemblage of a much
larger sample of such stars, so that general properties can be distinguished.
We describe a new effort, HERES: The Hamburg/ESO R-Process-Enhanced Star
survey, nearing completion, which will identify on the order of 5-10 additional
highly r-process-enhanced, metal-poor stars, and in all likelihood, a similar
or greater number of mildly r-process-enhanced, metal-poor stars in the halo of
the Galaxy. HERES is based on rapid "snapshot" spectra of over 350 candidate
halo giants with [Fe/H] < -2.0, obtained at moderately high resolution, and
with moderate signal-to-noise ratios, using the UVES spectrograph on the
European VLT 8m telescope.Comment: Contributed paper to The Eighth Nuclei in the Cosmos conference, to
appear (in refereed form) in Nuclear Physics
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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
A holistic abundance analysis of r-rich stars
The chemical abundances of metal-poor stars are an excellent test bed by
which to set new constraints on models of neutron-capture processes at low
metallicity. Some r-process-rich (hereafter r-rich) metal-poor stars, such as
HD221170, show an overabundance of the heavier neutron-capture elements and
excesses of lighter neutron-capture elements. The study of these r-rich stars
could give us a better understanding of weak and main r-process nucleosynthesis
at low metallicity. Based on conclusions from the observation of metal-poor
stars and neutron-capture element nucleosynthesis theory, we set up a model to
determine the relative contributions from weak and main r-processes to the
heavy-element abundances in metal-poor stars. Using this model, we find that
the abundance patterns of light elements for most sample stars are close to the
pattern of weak r-process stars, and those of heavier neutron-capture elements
very similar to the pattern of main r-process stars, while the lighter
neutron-capture elements can be fitted by the mixing of weak and main r-process
material. The production of weak r-process elements appears to be associated
with the light elements, while the production of main r-process elements is
almost decoupled from that of the light elements. We compare our results with
the observed data at low metallicities, showing that the predicted trends are
in good agreement with the observed trends, at least for the metallicity range
[Fe/H] < -2.1. For most sample stars, the abundance patterns of both
neutron-capture elements and light elements could be best explained by a star
formed in a molecular cloud that has been polluted by both weak and main
r-process material.Comment: 9 pages, 5 figure
Study of isotopic fractions and abundances of the neutron-capture elements in HD 175305
The chemical abundances of metal-poor stars are excellent sources of
information for setting new constraints on models of Galactic chemical
evolution at low metallicities. In this paper we present an attempt to fit the
elemental abundances observed in the bright, metal-poor giant HD 175305, and
derive isotopic fractions using a parametric model. The observed abundances can
be wellmatched by the combined contributions froms- and r-processmaterial. The
component coefficients of the r- and s-processes are C1 = 3.220 and C3 = 1.134,
respectively. The Smisotopic fraction in this star where the observed
neutron-capture elements are produced is predicted to be f 152+154 =0.582,which
suggests that, even though the r-process is predominantly responsible for the
synthesis of the neutron-capture elements in the early Galaxy, the onset of the
s-process had already occurred at this metallicity of [Fe/H] = -1.6
Heavy Element Dispersion in the Metal-Poor Globular Cluster M92
Dispersion among the light elements is common in globular clusters (GCs),
while dispersion among heavier elements is less common. We present detection of
r-process dispersion relative to Fe in 19 red giants of the metal-poor GC M92.
Using spectra obtained with the Hydra multi-object spectrograph on the WIYN
Telescope at Kitt Peak National Observatory, we derive differential abundances
for 21 species of 19 elements. The Fe-group elements, plus Y and Zr, are
homogeneous at a level of 0.07-0.16 dex. The heavy elements La, Eu, and Ho
exhibit clear star-to-star dispersion spanning 0.5-0.8 dex. The abundances of
these elements are correlated with one another, and we demonstrate that they
were produced by r-process nucleosynthesis. This r-process dispersion is not
correlated with the dispersion in C, N, or Na in M92, indicating that r-process
inhomogeneities were present in the gas throughout star formation. The
r-process dispersion is similar to that previously observed in the metal-poor
GC M15, but its origin in M15 or M92 is unknown at present.Comment: Accepted for publication in the Astronomical Journal (22 pages, 12
figures). v2: references update
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.
Automated Stellar Spectral Classification and Parameterization for the Masses
Stellar spectroscopic classification has been successfully automated by a
number of groups. Automated classification and parameterization work best when
applied to a homogeneous data set, and thus these techniques primarily have
been developed for and applied to large surveys. While most ongoing large
spectroscopic surveys target extragalactic objects, many stellar spectra have
been and will be obtained. We briefly summarize past work on automated
classification and parameterization, with emphasis on the work done in our
group. Accurate automated classification in the spectral type domain and
parameterization in the temperature domain have been relatively easy. Automated
parameterization in the metallicity domain, formally outside the MK system, has
also been effective. Due to the subtle effects on the spectrum, automated
classification in the luminosity domain has been somewhat more difficult, but
still successful. In order to extend the use of automated techniques beyond a
few surveys, we present our current efforts at building a web-based automated
stellar spectroscopic classification and parameterization machine. Our proposed
machinery would provide users with MK classifications as well as the
astrophysical parameters of effective temperature, surface gravity, mean
abundance, abundance anomalies, and microturbulence.Comment: 5 pages; to appear in The Garrison Festschrift conference proceeding
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