263 research outputs found
Ground and excited states Gamow-Teller strength distributions of iron isotopes and associated capture rates for core-collapse simulations
This paper reports on the microscopic calculation of ground and excited
states Gamow-Teller (GT) strength distributions, both in the electron capture
and electron decay direction, for Fe. The associated electron and
positron capture rates for these isotopes of iron are also calculated in
stellar matter. These calculations were recently introduced and this paper is a
follow-up which discusses in detail the GT strength distributions and stellar
capture rates of key iron isotopes. The calculations are performed within the
framework of the proton-neutron quasiparticle random phase approximation
(pn-QRPA) theory. The pn-QRPA theory allows a microscopic
\textit{state-by-state} calculation of GT strength functions and stellar
capture rates which greatly increases the reliability of the results. For the
first time experimental deformation of nuclei are taken into account. In the
core of massive stars isotopes of iron, Fe, are considered to be
key players in decreasing the electron-to-baryon ratio () mainly via
electron capture on these nuclide. The structure of the presupernova star is
altered both by the changes in and the entropy of the core material.
Results are encouraging and are compared against measurements (where possible)
and other calculations. The calculated electron capture rates are in overall
good agreement with the shell model results. During the presupernova evolution
of massive stars, from oxygen shell burning stages till around end of
convective core silicon burning, the calculated electron capture rates on
Fe are around three times bigger than the corresponding shell model
rates. The calculated positron capture rates, however, are suppressed by two to
five orders of magnitude.Comment: 18 pages, 12 figures, 10 table
rp-Process weak-interaction mediated rates of waiting-point nuclei
Electron capture and positron decay rates are calculated for
neutron-deficient Kr and Sr waiting point nuclei in stellar matter. The
calculation is performed within the framework of pn-QRPA model for rp-process
conditions. Fine tuning of particle-particle, particle-hole interaction
parameters and a proper choice of the deformation parameter resulted in an
accurate reproduction of the measured half-lives. The same model parameters
were used to calculate stellar rates. Inclusion of measured Gamow-Teller
strength distributions finally led to a reliable calculation of weak rates that
reproduced the measured half-lives well under limiting conditions. For the
rp-process conditions, electron capture and positron decay rates on Kr
and Sr are of comparable magnitude whereas electron capture rates on
Sr and Kr are 1--2 orders of magnitude bigger than the
corresponding positron decay rates. The pn-QRPA calculated electron capture
rates on Kr are bigger than previously calculated. The present
calculation strongly suggests that, under rp-process conditions, electron
capture rates form an integral part of weak-interaction mediated rates and
should not be neglected in nuclear reaction network calculations as done
previously.Comment: 13 pages, 4 figures, 4 tables; Astrophysics and Space Science (2012
Stellar neutrino energy loss rates due to Mg suitable for O+Ne+Mg core simulations
Neutrino losses from proto-neutron stars play a pivotal role to decide if
these stars would be crushed into black holes or explode as supernovae. Recent
observations of subluminous Type II-P supernovae (e.g., 2005cs, 2003gd, 1999br,
1997D) were able to rejuvenate the interest in 8-10 M stars which
develop O+Ne+Mg cores. Simulation results of O+Ne+Mg cores show varying results
in converting the collapse into an explosion. The neutrino energy loss rates
are important input parameters in core collapse simulations. Proton-neutron
quasi-particle random phase approximation (pn-QRPA) theory has been used for
calculation of neutrino energy loss rates due to Mg in stellar matter.
The rates are presented on a detailed density-temperature grid suitable for
simulation purposes. The calculated neutrino energy loss rates are enhanced up
to more than one order of magnitude compared to the shell model calculations
and favor a lower entropy for the core of these massive stars.Comment: 20 pages, 4 figures, 2 table
-decay of key titanium isotopes in stellar environment
Amongst iron regime nuclei, -decay rates on titanium isotopes are
considered to be important during the late phases of evolution of massive
stars. The key -decay isotopes during presupernova evolution were
searched from available literature and a microscopic calculation of the decay
rates were performed using the proton-neutron quasiparticle random phase
approximation (pn-QRPA) theory. As per earlier simulation results electron
capture and -decay on certain isotopes of titanium are considered to be
important for the presupernova evolution of massive stars. Earlier the stellar
electron capture rates and neutrino energy loss rates due to relevant titanium
isotopes were presented. In this paper we finally present the -decay
rates of key titanium isotopes in stellar environment. The results are also
compared against previous calculations. The pn-QRPA -decay rates are
bigger at high stellar temperatures and smaller at high stellar densities
compared to the large scale shell model results. This study can prove useful
for the core-collapse simulators.Comment: 14 pages, 5 figure
Microscopic Calculations of Weak Interaction Rates of Nuclei in Stellar Environment for A = 18 to 100
We report here the microscopic calculation of weak interaction rates in
stellar matter for 709 nuclei with A = 18 to 100 using a generalized form of
proton-neutron quasiparticle RPA model with separable Gamow-Teller forces. This
is the first ever extensive microscopic calculation of weak rates calculated
over a wide temperature-density grid which includes 10^7 \leq T(K) \leq 30
\times 10^9 and 10 \leq \rho Y_{e} (gcm^{-3}) \leq 10^{11}, and over a larger
mass range. Particle emission processes from excited states, previously
ignored, are taken into account, and are found to significantly affect some
decay rates. The calculated capture and decay rates take into
consideration the latest experimental energy levels and value
compilations. Our calculation of electron capture and -decay rates, in
the -shell, show considerable differences with a recently reported shell
model diagonalization approach calculation.Comment: 9 page
Neutrino energy loss rates and positron capture rates on Co for presupernova and supernova physics
Proton-neutron quasi-particle random phase approximation (pn-QRPA) theory has
recently being used for calculation of stellar weak interaction rates of
-shell nuclide with success. Neutrino losses from proto-neutron stars play
a pivotal role to decide if these stars would be crushed into black holes or
explode as supernovae. The product of abundance and positron capture rates on
Co is substantial and as such can play a role in fine tuning of input
parameters of simulation codes specially in the presupernova evolution.
Recently we introduced our calculation of capture rates on Co, in a
luxurious model space of , employing the pn-QRPA theory with a
separable interaction. Simulators, however, may require these rates on a fine
scale. Here we present for the first time an expanded calculation of the
neutrino energy loss rates and positron capture rates on Co on an
extensive temperature-density scale. These type of scale is appropriate for
interpolation purposes and of greater utility for simulation codes. The pn-QRPA
calculated neutrino energy loss rates are enhanced roughly up to two orders of
magnitude compared with the large-scale shell model calculations and favor a
lower entropy for the core of massive stars.Comment: 27 pages, 6 figures, 5 table
Comparative study of Gamow-Teller strength distributions in the odd-odd nucleus 50V and its impact on electron capture rates in astrophysical environments
Gamow-Teller (GT) strength transitions are an ideal probe for testing nuclear
structure models. In addition to nuclear structure, GT transitions in nuclei
directly affect the early phases of Type Ia and Type-II supernovae core
collapse since the electron capture rates are partly determined by these GT
transitions. In astrophysics, GT transitions provide an important input for
model calculations and element formation during the explosive phase of a
massive star at the end of its life-time. Recent nucleosynthesis calculations
show that odd-odd and odd-A nuclei cause the largest contribution in the rate
of change of lepton-to-baryon ratio. In the present manuscript, we have
calculated the GT strength distributions and electron capture rates for odd-odd
nucleus 50V by using the pn-QRPA theory. At present 50V is the first
experimentally available odd-odd nucleus in fp-shell nuclei. We also compare
our GT strength distribution with the recently measured results of a
50V(d,2He)50Ti experiment, with the earlier work of Fuller, Fowler, and Newman
(referred to as FFN) and subsequently with the large-scale shell model
calculations. One curious finding of the paper is that the Brink's hypothesis,
usually employed in large-scale shell model calculations, is not a good
approximation to use at least in the case of 50V. SNe Ia model calculations
performed using FFN rates result in overproduction of 50Ti, and were brought to
a much acceptable value by employing shell model results. It might be
interesting to study how the composition of the ejecta using presently reported
QRPA rates compare with the observed abundances.Comment: 16 pages, 5 figure
Fine-Grid Calculations for Stellar Electron and Positron Capture Rates on Fe-Isotopes
The acquisition of precise and reliable nuclear data is a prerequisite to
success for stellar evolution and nucleosynthesis studies. Core-collapse
simulators find it challenging to generate an explosion from the collapse of
the core of massive stars. It is believed that a better understanding of the
microphysics of core-collapse can lead to successful results. The weak
interaction processes are able to trigger the collapse and control the
lepton-to-baryon ratio () of the core material. It is suggested that the
temporal variation of within the core of a massive star has a pivotal
role to play in the stellar evolution and a fine-tuning of this parameter at
various stages of presupernova evolution is the key to generate an explosion.
During the presupernova evolution of massive stars, isotopes of iron, mainly
Fe, are considered to be key players in controlling ratio
via electron capture on these nuclide. Recently an improved microscopic
calculation of weak interaction mediated rates for iron isotopes was introduced
using the proton-neutron quasiparticle random phase approximation (pn-QRPA)
theory. The pn-QRPA theory allows a microscopic \textit{state-by-state}
calculation of stellar capture rates which greatly increases the reliability of
calculated rates. The results were suggestive of some fine-tuning of the
ratio during various phases of stellar evolution. Here we present for
the first time the fine-grid calculation of the electron and positron capture
rates on Fe. Core-collapse simulators may find this calculation
suitable for interpolation purposes and for necessary incorporation in the
stellar evolution codes.Comment: 21 pages, 6 ps figures and 2 table
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