20 research outputs found
Estimates of Stellar Weak Interaction Rates for Nuclei in the Mass Range A=65-80
We estimate lepton capture and emission rates, as well as neutrino energy
loss rates, for nuclei in the mass range A=65-80. These rates are calculated on
a temperature/density grid appropriate for a wide range of astrophysical
applications including simulations of late time stellar evolution and x-ray
bursts. The basic inputs in our single particle and empirically inspired model
are i) experimentally measured level and weak decay information, ii) estimates
of matrix elements for allowed experimentally-unmeasured transitions based on
the systematics of experimentally observed allowed transitions, and iii)
estimates of the centroids of the GT resonances motivated by shell model
calculations in the fp shell as well as by (n,p) and (p,n) experiments.
Transitions involving Fermi resonances (isobaric analog states) are also
included and dominate the rates for many interesting proton rich nuclei for
which an experimentally-determined ground state lifetime is unavailable. To
compare our results with more detailed shell model based calculations we also
calculate weak rates for nuclei in the mass range A=60-65 for which Langanke
and Martinez-Pinedo have provided rates. The typical deviation in the electron
capture and B- decay rates for these ~30 nuclei is less than a factor of two or
three for a wide range of temperature and density appropriate for pre-supernova
stellar evolution. We also discuss some subtleties associated with the
partition functions used in calculations of stellar weak rates and show that
the proper treatment of the partition functions is essential for estimating
high temperature beta decay rates. Partition functions based on un-converged
Lanczos calculations can result in estimates of high temperature beta decay
rates that are systematically low.Comment: Tables of rates for nuclei in the mass range A=66-110 are available
from J. Prue
On the Contribution of Gamma Ray Bursts to the Galactic Inventory of Some Intermediate Mass Nuclei
Light curves from a growing number of Gamma Ray Bursts (GRBs) indicate that
GRBs copiously produce radioactive Ni moving outward at fractions of the speed
of light. We calculate nuclear abundances of elements accompanying the
outflowing Ni under the assumption that this Ni originates from a wind blown
off of a viscous accretion disk. We also show that GRB's likely contribute
appreciably to the galactic inventory of 42Ca, 45Sc, 46Ti, 49Ti, 63Cu, and may
be a principal site for the production of 64Zn.Comment: 11 pages, 1 figur
Neutrinos from the Propagation of a Relativistic Jet Through a Star
We discuss the neutrino signature of a relativistic jet propagating through a
stellar envelope, a scenario realized in the collapsar model for Gamma Ray
Bursts (GRBs). It is shown that the dramatic slowing of the jet deep within the
star is accompanied by inelastic neutron-nucleon collisions and the conversion
of a substantial fraction of the jet kinetic energy to neutrinos. These
neutrinos have observed energies in the range two to tens of GeV and an
estimated detection rate comparable to or larger than the detection rate of GeV
neutrinos from other GRB-related processes. The time delay between the arrival
of these neutrinos and the GRB photons is tens of seconds. An observation of
this delay would provide an indication that the GRB jet originated in a massive
star.Comment: To appear in Ap
Light Element Synthesis in High Entropy Relativistic Flows Associated with Gamma Ray Bursts
We calculate and discuss the light element freeze-out nucleosynthesis in high
entropy winds and fireballs for broad ranges of entropy-per-baryon, dynamic
timescales characterizing relativistic expansion, and neutron-to-proton ratios.
With conditions characteristic of Gamma Ray Bursts (GRBs) we find that
deuterium production can be prodigious, with final abundance values 2H/H
approximately 2%, depending on the fireball isospin, late time dynamics, and
the effects of neutron decoupling- induced high energy non-thermal nuclear
reactions. This implies that there potentially could be detectable local
enhancements in the deuterium abundance associated with GRB events.Comment: 14 pages 3 figure
On the origin of the lightest Molybdenum isotopes
We discuss implications of recent precision measurements for the Rh93 proton
separation energy for the production of the lightest molybdenum isotopes in
proton-rich type II supernova ejecta. It has recently been shown that a novel
neutrino-induced process makes these ejecta a promising site for the production
of the light molybdenum isotopes and other "p-nuclei" with atomic mass near
100. The origin of these isotopes has long been uncertain. A distinguishing
feature of nucleosynthesis in neutrino-irradiated outflows is that the relative
production of Mo92 and Mo94 is set by a competition governed by the proton
separation energy of Rh93. We use detailed nuclear network calculations and the
recent experimental results for this proton separation energy to place
constraints on the outflow characteristics that produce the lightest molybdenum
isotopes in their solar proportions. It is found that for the conditions
calculated in recent two-dimensional supernova simulations, and also for a
large range of outflow characteristics around these conditions, the solar ratio
of Mo92 to Mo94 cannot be achieved. This suggests that either proton-rich winds
from type II supernova do not exclusively produce both isotopes, or that these
winds are qualitatively different than calculated in today's supernova models.Comment: 12 pages, 7 figures (3 color
The difficulty in using orphan afterglows to measure gamma-ray burst beaming
If gamma-ray burst (GRB) emission is strongly collimated then GRBs occur
throughout the Universe at a rate much higher than is detected. Since the
emission from the optical afterglow is thought to be more isotropic than the
gamma-ray emission, it has been hypothesized that a search for orphan
afterglows (those without the triggering GRB) would allow strong constraints to
be placed on the degree of GRB collimation. We show here that, within the
context of leading models of GRB jet evolution, measurement of the GRB beaming
angle using optical orphan searches is extremely difficult, perhaps impossible
in practice. This is because in the leading model of GRB jets, the effective
afterglow beaming angle scales with the jet angle for small angles, and so the
ratio of detected orphan afterglows to GRBs is independent of the jet opening
angle. Thus, the number of expected afterglow detections is the same for
moderate jet angles (e.g. 20 deg) as for arbitrarily small jet angles (<< 0.1
deg). For nearly isotropic GRB geometry, or for radio afterglow searches in
which the jet has become non-relativistic, the ratio of afterglows to GRBs may
give information on collimation. However, using a simple model we estimate the
expected number of orphan detections in current supernova surveys, and find
this number to be less than one, for all jet opening angles. Even for future
supernova surveys, the small detection rate and lack of dependence on
collimation angle appear to ruin the prospects of determining GRB beaming by
this method. Radio searches may provide the best hope to find the missing
orphans.Comment: 12 pages, 3 figures, submitted to Ap
Nucleosynthesis in Outflows from the Inner Regions of Collapsars
We consider nucleosynthesis in outflows originating from the inner regions of
viscous accretion disks formed after the collapse of a rotating massive star.
We show that wind-like outflows driven by viscous and neutrino heating can
efficiently synthesize Fe-group elements moving at near-relativistic
velocities. The mass of 56Ni synthesized and the asymptotic velocities attained
in our calculations are in accord with those inferred from observations of
SN1998bw and SN2003dh. These steady wind-like outflows are generally proton
rich, characterized by only modest entropies, and consequently synthesize
essentially nothing heavier than the Fe-group elements. We also discuss
bubble-like outflows resulting from rapid energy deposition in localized
regions near or in the accretion disk. These intermittent ejecta emerge with
low electron fraction and are a promising site for the synthesis of the A=130
r-process peak elements.Comment: 35 pages, 6 figures, added discussion of the influence of nuclear
recombination on wind dynamics, to appear in Ap
On Steady State Neutrino Heated Ultra-Relativistic Winds from Compact Objects
We study steady state winds from compact objects in the regime where the wind
velocity at infinity is ultra-relativistic. This may have relevance to some
models of Gamma-Ray-Bursts (GRB's). Particular attention is paid to the case
where neutrinos provide the heating. Unless the neutrino luminosity is very
large, L>10^{54} erg/s, the only allowed steady state solutions are those where
energy deposition is dominated by neutrino-antineutrino annihilation at the
sonic point. In this case, the matter temperature near the neutron star surface
is low, less than 1MeV for typical neutrino luminosities. This is in contrast
to the case for sub-relativistic winds discussed in the context of supernovae
where the matter temperature near the neutron star approximates the temperature
characterizing the neutrinos. We also investigate the setting of the neutron to
proton ratio (n/p) in these winds and find that only for large (>10 MeV)
electron neutrino or electron anti-neutrino temperatures is n/p entirely
determined by neutrino capture. Otherwise, n/p retains an imprint of conditions
in the neutron star