616 research outputs found
Neutrino Capture and r-Process Nucleosynthesis
We explore neutrino capture during r-process nucleosynthesis in
neutrino-driven ejecta from nascent neutron stars. We focus on the interplay
between charged-current weak interactions and element synthesis, and we
delineate the important role of equilibrium nuclear dynamics. During the period
of coexistence of free nucleons and light and/or heavy nuclei, electron
neutrino capture inhibits the r-process. At all stages, capture on free
neutrons has a larger impact than capture on nuclei. However, neutrino capture
on heavey nuclei by itself, if it is very strong, is also detrimental to the
r-process until large nuclear equilibrium clusters break down and the classical
neutron-capture phase of the r-process begins. The sensitivity of the r-process
to neutrino irradiation means that neutrino-capture effects can strongly
constrain the r-process site, neutrino physics, or both. These results apply
also to r-process scenarios other than neutrino-heated winds.Comment: 20 pages, 17 figures, Submitted to Physical Review
Neutrino-Accelerated Hot Hydrogen Burning
We examine the effects of significant electron anti-neutrino fluxes on
hydrogen burning. Specifically, we find that the bottleneck weak nuclear
reactions in the traditional pp-chain and the hot CNO cycle can be accelerated
by anti-neutrino capture, increasing the energy generation rate. We also
discuss how anti-neutrino capture reactions can alter the conditions for break
out into the rp-process. We speculate on the impact of these considerations for
the evolution and dynamics of collapsing very- and super- massive compact
objects.Comment: 14 pages, 6 figures, submitted to ApJ; minor content chang
Evidence for an Intense Neutrino Flux during -Process Nucleosynthesis?
We investigate the possibility that neutrino capture on heavy nuclei competes
with beta decay in the environment where the -Process elements are
synthesized. We find that such neutrino capture is not excluded by existing
abundance determinations. We show that inclusion of significant neutrino
capture on the (neutron number) N=82 waiting point nuclei can allow the
inferred abundances of these species to provide a good fit to steady weak (beta
decay plus neutrino capture) flow equilibrium. In fact, for particular choices
of neutrino flux conditions, this fit is improved over the case where nuclei
change their charge by beta decay alone. However, this improved fit can be
realized only if neutrino capture plays a negligible role in nuclear decay back
toward stability. We discuss the implications of these considerations for
current proposed sites and models for -Process nucleosynthesis.Comment: 10 pages, plain tex, submitted to ApJ
Weak Charge-Changing Flow in Expanding r-Process Environments
We assess the prospects for attaining steady nuclear flow equilibrium in
expanding r-process environments where beta decay and/or neutrino capture
determine the nuclear charge-changing rates. For very rapid expansions, we find
that weak steady flow equilibrium normally cannot be attained. However, even
when neutron capture processes freeze out in such nonequilibrium conditions,
abundance ratios of nuclear species in the r-process peaks might still mimic
those attained in weak steady flow. This result suggests that the r-process
yield in a regime of rapid expansion can be calculated reliably only when all
neutron capture, photodisintegration, and weak interaction processes are fully
coupled in a dynamical calculation. We discuss the implications of these
results for models of the r-process sited in rapidly expanding neutrino-heated
ejecta.Comment: 21 pages, AAS LaTex, 2 postscript figure
Neutrino-Neutrino Scattering and Matter-Enhanced Neutrino Flavor Transformation in Supernovae
We examine matter-enhanced neutrino flavor transformation
() in the region above the neutrino
sphere in Type II supernovae. Our treatment explicitly includes contributions
to the neutrino-propagation Hamiltonian from neutrino-neutrino forward
scattering. A proper inclusion of these contributions shows that they have a
completely negligible effect on the range of - vacuum
mass-squared difference, , and vacuum mixing angle, , or
equivalently , required for enhanced supernova shock re-heating.
When neutrino background effects are included, we find that -process
nucleosynthesis from neutrino-heated supernova ejecta remains a sensitive probe
of the mixing between a light and a with a
cosmologically significant mass. Neutrino-neutrino scattering contributions are
found to have a generally small effect on the
parameter region probed by -process nucleosynthesis. We point out that the
nonlinear effects of the neutrino background extend the range of sensitivity of
-process nucleosynthesis to smaller values of .Comment: 38 pages, tex, DOE/ER/40561-150-INT94-00-6
Evidence of Multiple r-Process Sites in the Early Galaxy: New Observations of CS 22892-052
First results are reported of a new abundance study of neutron-capture
elements in the ultra-metal-poor (UMP; [Fe/H] = -3.1) halo field giant star CS
22892-052. Using new high resolution, high signal-to-noise spectra, abundances
of more than 30 neutron-capture elements (Z>30) have been determined. Six
elements in the 40<Z<56 domain (Nb, Ru, Rh, Pd, Ag and Cd) have been detected
for the first time in a UMP star. Abundances are also derived for three of the
heaviest stable elements (Os, Ir, and Pb). A second transition of thorium,
Th{4086}, confirms the abundance deduced from the standard Th{4019} line, and
an upper limit to the abundance of uranium is established from the absence of
the U{3859} line. As found in previous studies, the abundances of the heavier
(Z>=56) stable neutron-capture elements in CS 22892-052 match well the scaled
solar system r-process abundance distribution. From the observed Th abundance,
an average age of ~= 16 +/- 4 Gyr is derived for cs22892-052, consistent with
the lower age limit of ~= 11 Gyr derived from the upper limit on the U
abundance. The concordance of scaled solar r-process and CS 22892-052
abundances breaks down for the lighter neutron-capture elements, supporting
previous suggestions that different r-process production sites are responsible
for lighter and heavier neutron-capture elements.Comment: To be published in the Astrophysical Journal Letter
Nuclear Physics Neutrino PreTown Meeting: Summary and Recommendations
In preparation for the nuclear physics Long Range Plan exercise, a group of
104 neutrino physicists met in Seattle September 21-23 to discuss both the
present state of the field and the new opportunities of the next decade. This
report summarizes the conclusions of that meeting and presents its
recommendations. Further information is available at the workshop's web site.
This report will be further reviewed at the upcoming Oakland Town Meeting.Comment: Latex, 31 pages. This version has been updated to include final
Comments from the working group
Can a Large Neutron Excess Help Solve the Baryon Loading Problem in Gamma-Ray Burst Fireballs?
We point out that the baryon-loading problem in Gamma-Ray Burst (GRB) models
can be amelioriated if a significant fraction of the baryons which inertially
confine the fireball are converted to neutrons. A high neutron fraction in some
circumstances can result in a reduced transfer of energy from relativistic
light particles in the fireball to baryons. The energy needed to produce the
required relativistic flow in the GRB is consequently reduced, in some cases by
orders of magnitude. This could be relevant to GRB models because a high
neutron-to-proton ratio has been calculated in neutron star-merger fireball
environments. Significant neutron excess also could occur near compact objects
with high neutrino fluxes.Comment: 5 pages, 2 figures, to appear in Phys. Rev. Let
Sterile neutrinos and supernova nucleosynthesis
A light sterile neutrino species has been introduced to explain
simultaneously the solar and atmospheric neutrino puzzles and the results of
the LSND experiment, while providing for a hot component of dark matter.
Employing this scheme of neutrino masses and mixings, we show how
matter-enhanced active-sterile neutrino transformation followed by
active-active neutrino transformation can solve robustly the neutron deficit
problem encountered by models of r-process nucleosynthesis associated with
neutrino-heated supernova ejecta.Comment: 29 pages, 3 postscript figures, submitted to Phys. Rev.
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