Página 1 dos resultados de 50 itens digitais encontrados em 0.195 segundos

## Indirect study of (11)B(p,alpha(0))(8)Be and (10)B(p,alpha)(7)Be reactions at astrophysical energies by means of the Trojan Horse Method: recent results

Fonte: ELSEVIER SCIENCE BV
Publicador: ELSEVIER SCIENCE BV

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

45.68%

Nuclear (p,alpha) reactions destroying the so-called ""light-elements"" lithium, beryllium and boron have been largely studied in the past mainly because their role in understanding some astrophysical phenomena, i.e. mixing-phenomena occurring in young F-G stars [1]. Such mechanisms transport the surface material down to the region close to the nuclear destruction zone, where typical temperatures of the order of similar to 10(6) K are reached. The corresponding Gamow energy E(0)=1.22 (Z(x)(2)Z(X)(2)T(6)(2))(1/3) [2] is about similar to 10 keV if one considers the ""boron-case"" and replaces in the previous formula Z(x) = 1, Z(X) = 5 and T(6) = 5. Direct measurements of the two (11)B(p,alpha(0))(8)Be and (10)B(p,alpha)(7)Be reactions in correspondence of this energy region are difficult to perform mainly because the combined effects of Coulomb barrier penetrability and electron screening [3]. The indirect method of the Trojan Horse (THM) [4-6] allows one to extract the two-body reaction cross section of interest for astrophysics without the extrapolation-procedures. Due to the THM formalism, the extracted indirect data have to be normalized to the available direct ones at higher energies thus implying that the method is a complementary tool in solving some still open questions for both nuclear and astrophysical issues [7-12].

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## Modeling of laser-generated radiative blast waves, with applications to late-term supernova remnants

Fonte: Universidade Rice
Publicador: Universidade Rice

ENG

Relevância na Pesquisa

35.74%

The goal of laser astrophysics is to provide a means by which aspects of specific astrophysical phenomena can be reproduced in the laboratory. Although the hydrodynamic instabilities of early supernova remnants have already been studied using this method, the role of significant radiative losses in shock propagation (for example, in late-term remnants) has only been imperfectly modeled. This thesis introduces an improved self-similar analytic approach to radiative blast-wave evolution where the total amount of energy loss remains constant in proportion to the energy flux entering the shock front. The approximation is solved for the cases in which both energy loss from the shock front and heating of the shock (due to the presence of ionization precursors) are significant. Because this solution is independent of the exact method of cooling, it is appropriate for both the laboratory and astrophysical regimes. In addition, this thesis applies the analytic approximation to laboratory-produced radiative blast waves as well as to numerical models of these experimental blast waves. These results will allow for better design of laser-based experiments with further applications to astrophysical phenomena, as well as for an increase in the understanding of the challenges involved in scaling radiative phenomena between laboratory experiments and astrophysical theory.

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## Conservative special relativistic radiative transfer for multidimensional astrophysical simulations: motivation and elaboration

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 24/10/2005

Relevância na Pesquisa

45.57%

Many astrophysical phenomena exhibit relativistic radiative flows. While
velocities in excess of $v \sim 0.1c$ can occur in these systems, it has been
common practice to approximate radiative transfer to $\cO(v/c)$. In the case of
neutrino transport in core-collapse supernovae, this approximation gives rise
to an inconsistency between the lepton number transfer and lab frame energy
transfer, which have different $\cO(v/c)$ limits. A solution used in
spherically symmetric $\cO(v/c)$ simulations has been to retain, for energy
accounting purposes, the $\cO(v^2/c^2)$ terms in the lab frame energy transfer
equation that arise from the $\cO(v/c)$ neutrino number transport equation.
Avoiding the proliferation of such ``extra'' $\cO(v^2/c^2)$ terms in the
absence of spherical symmetry motivates a special relativistic formalism, which
we exhibit in coordinates sufficiently general to encompass Cartesian,
spherical, and cylindrical coordinate systems.; Comment: 9 pages

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## Ongoing Space Physics - Astrophysics Connections

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 16/03/2005

Relevância na Pesquisa

35.56%

I review several ongoing connections between space physics and astrophysics:
a) Measurements of energetic particle spectra have confirmed theoretical
prediction of the highest energy to which shocks can accelerate particles, and
this has direct bearing on the origin of the highest energy cosmic rays. b)
Mass ejection in solar flares may help us understand photon ejection in the
giant flares of magnetar outbursts. c) Measurements of electron heat fluxes in
the solar wind can help us understand whether heat flux in tenuous
astrophysical plasma is in accordance with the classical Spitzer-Harm formula
or whether it is reduced well below this value by plasma instabilities.; Comment: To appear in Proceedings of "Critical Phenomena in the Solar System",
Ein-Boqeq, March, 2004

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## Large collection of astrophysical S-factors and its compact representation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 14/04/2012

Relevância na Pesquisa

35.57%

Numerous nuclear reactions in the crust of accreting neutron stars are
strongly affected by dense plasma environment. Simulations of superbursts, deep
crustal heating and other nuclear burning phenomena in neutron stars require
astrophysical S-factors for these reactions (as a function of center-of-mass
energy E of colliding nuclei). A large database of S-factors is created for
about 5000 non-resonant fusion reactions involving stable and unstable isotopes
of Be, B, C, N, O, F, Ne, Na, Mg, and Si. It extends the previous database of
about 1000 reactions involving isotopes of C, O, Ne, and Mg. The calculations
are performed using the Sao Paulo potential and the barrier penetration
formalism. All calculated S-data are parameterized by an analytic model for
S(E) proposed before [Phys. Rev. C 82, 044609 (2010)] and further elaborated
here. For a given reaction, the present S(E)-model contains three parameters.
These parameters are easily interpolated along reactions involving isotopes of
the same elements with only seven input parameters, giving an ultracompact,
accurate, simple, and uniform database. The S(E) approximation can also be used
to estimate theoretical uncertainties of S(E) and nuclear reaction rates in
dense matter, as illustrated for the case of the 34Ne+34Ne reaction in the
inner crust of an accreting neutron star.; Comment: 13 pages...

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## Dark Energy Stars

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

45.47%

Event horizons and closed time-like curves cannot exist in the real world for
the simple reason that they are inconsistent with quantum mechanics. Following
ideas originated by Robert Laughlin, Pawel Mazur, Emil Mottola, David Santiago,
and the speaker it is now possible to describe in some detail what happens
physically when one approaches and crosses a region of space-time where
classical general relativity predicts there should be an infinite red shift
surface. This quantum critical physics provides a new perspective on a variety
of enigmatic astrophysical phenomena, including supernovae explosions, gamma
ray bursts, positron emission, and dark matter.; Comment: 4 pages, 2 figures, corrected typos

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## Astrophysical Observations: Lensing and Eclipsing Einstein's Theories

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/03/2005

Relevância na Pesquisa

45.58%

Albert Einstein postulated the equivalence of energy and mass, developed the
theory of special relativity, explained the photoelectric effect, and described
Brownian motion in five papers, all published in 1905, 100 years ago. With
these papers, Einstein provided the framework for understanding modern
astrophysical phenomena. Conversely, astrophysical observations provide one of
the most effective means for testing Einstein's theories. Here, I review
astrophysical advances precipitated by Einstein's insights, including
gravitational redshifts, gravitational lensing, gravitational waves, the
Lense-Thirring effect, and modern cosmology. A complete understanding of
cosmology, from the earliest moments to the ultimate fate of the universe, will
require developments in physics beyond Einstein, to a unified theory of gravity
and quantum physics.; Comment: 6 pages, 4 figures (resol. Reduced to 85 dpi)

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## Radiation hydrodynamics integrated in the code PLUTO

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 20/09/2013

Relevância na Pesquisa

35.54%

The transport of energy through radiation is very important in many
astrophysical phenomena. In dynamical problems the time-dependent equations of
radiation hydrodynamics have to be solved. We present a newly developed
radiation-hydrodynamics module specifically designed for the versatile MHD code
PLUTO. The solver is based on the flux-limited diffusion approximation in the
two-temperature approach. All equations are solved in the co-moving frame in
the frequency independent (grey) approximation. The hydrodynamics is solved by
the different Godunov schemes implemented in PLUTO, and for the radiation
transport we use a fully implicit scheme. The resulting system of linear
equations is solved either using the successive over-relaxation (SOR) method
(for testing purposes), or matrix solvers that are available in the PETSc
library. We state in detail the methodology and describe several test cases in
order to verify the correctness of our implementation. The solver works in
standard coordinate systems, such as Cartesian, cylindrical and spherical, and
also for non-equidistant grids. We have presented a new radiation-hydrodynamics
solver coupled to the MHD-code \PLUTO that is a modern, versatile and efficient
new module for treating complex radiation hydrodynamical problems in
astrophysics. As test cases...

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## Early attempts at atmospheric simulations for the Cherenkov Telescope Array

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

35.51%

The Cherenkov Telescope Array (CTA) will be the world's first observatory for
detecting gamma-rays from astrophysical phenomena and is now in its prototyping
phase with construction expected to begin in 2015/16. In this work we present
the results from early attempts at detailed simulation studies performed to
assess the need for atmospheric monitoring. This will include discussion of
some lidar analysis methods with a view to determining a range resolved
atmospheric transmission profile. We find that under increased aerosol density
levels, simulated gamma-ray astronomy data is systematically shifted leading to
softer spectra. With lidar data we show that it is possible to fit atmospheric
transmission models needed for generating lookup tables, which are used to
infer the energy of a gamma-ray event, thus making it possible to correct
affected data that would otherwise be considered unusable.; Comment: 5 pages, 6 figures, updated author metadata

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## Effects of Environment and Energy Injection on Gamma-Ray Burst Afterglows

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 18/08/2000

Relevância na Pesquisa

35.57%

There is growing evidence that some long gamma-ray bursts (GRBs) arise from
the core collapse of massive stars, and thus it is inevitable that the
environments of these GRBs are preburst stellar winds or dense media. We
studied, for the first time, the wind model for afterglows based on the
Blandford-McKee self-similar solution of a relativistic shock, and suggested
that GRB 970616 is an interactor with a stellar wind. We also proposed a dense
medium model for some afterglows, e.g., the steepening in the light curve of
the R-band afterglow of GRB 990123 may be caused by the adiabatic shock which
has evolved from an ultrarelativistic phase to a nonrelativistic phase in a
dense medium. We further discussed the dense medium model in more details, and
investigated the effects of synchrotron self absorption and energy injection. A
shock in a dense medium becomes nonrelativistic rapidly after a short
relativistic phase. The afterglow from the shock at the nonrelativistic stage
decays more rapidly than at the relativistic stage. Since some models for GRB
energy sources predicted that a strongly magnetic millisecond pulsar may be
born during GRB formation, we discussed the effect of such a pulsar on the
evolution of the nonrelativistic shock through magnetic dipole radiation. We
found that in the pulsar energy injection case...

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## Magnetic Extraction of Spin Energy from a Black Hole

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 23/08/2000

Relevância na Pesquisa

35.56%

Numerous variations have been proposed on the original suggestion by
Blandford and Znajek that magnetic fields could be used to extract rotational
energy from black holes. A new categorization of these variations is proposed
so that they may be considered in a systematic way. "Black hole spindown" is
defined more precisely, distinguishing decrease in the spin parameter a/M from
decreases in angular momentum a and rotational kinetic energy, M - M_i. Several
key physical questions are raised: Can the "stretched horizon" of a black hole
communicate with the outside world? Do accretion disks bring any net magnetic
flux to the black holes at their centers? Is the magnetic field adjacent to a
black hole force-free everywhere?; Comment: to appear in "Explosive Phenomena in Astrophysical Compact Objects,
Proceedings of the 1st KIAS Astrophysics Workshop"

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## Nuclear Astrophysics with Radioactive Beams

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

45.55%

#Nuclear Theory#Astrophysics - Instrumentation and Methods for Astrophysics#Nuclear Experiment#Physics - Instrumentation and Detectors

The quest to comprehend how nuclear processes influence astrophysical
phenomena is driving experimental and theoretical research programs worldwide.
One of the main goals in nuclear astrophysics is to understand how energy is
generated in stars, how elements are synthesized in stellar events and what the
nature of neutron stars is. New experimental capabilities, the availability of
radioactive beams and increased computational power paired with new
astronomical observations have advanced the present knowledge. This review
summarizes the progress in the field of nuclear astrophysics with a focus on
the role of indirect methods and reactions involving beams of rare isotopes.; Comment: 121 pages, 27 figures, 510 references, to appear in Physics Reports.
Minor typos and references fixed

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## Three-Dimensional Magnetic Reconnection

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 02/03/2009

Relevância na Pesquisa

35.59%

The importance of magnetic reconnection as an energy release mechanism in
many solar, stellar, magnetospheric and astrophysical phenomena has long been
recognised. Reconnection is the only mechanism by which magnetic fields can
globally restructure, enabling them to access a lower energy state. Over the
past decade, there have been some major advances in our understanding of
three-dimensional reconnection. In particular, the key characteristics of 3D
magnetohydrodynamic (MHD) reconnection have been determined. For instance, 3D
reconnection (i) occurs with or without nulls, (ii) occurs continuously and
continually throughout a diffusion region and (iii) is driven by counter
rotating flows.
Furthermore, analysis of resistive 3D MHD magnetic experiments have revealed
some intriguing effects relating to where and how reconnection occurs. To
illustrate these new features, a series of constant-resistivity experiments,
involving the interaction of two opposite-polarity magnetic sources in an
overlying field, are considered. Such a simple interaction represents a typical
building block of the Sun's magnetic atmosphere. By following the evolution of
the magnetic topology, we are able to explain where, how and at what rate the
reconnection occurs. Remarkably there can be up to five energy release sites at
anyone time (compared to one in the potential case) and the duration of the
interaction increases (more than doubles) as the resistivity decreases (by a
factor of 16). The decreased resistivity also leads to a higher peak ohmic
dissipation and more energy being released in total...

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## Periodic Box FHNC calculations of neutron star crustal matter. (I)

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/06/2011

Relevância na Pesquisa

35.51%

Neutron star crustal matter, whose properties are relevant in many models
aimed at explaining observed astrophysical phenomena, has so far always been
studied using a mean field approach. In order to check the results obtained in
this way, a sensible next step is to make use of a realistic nuclear potential.
The present paper extends the periodic-box Fermi HyperNetted Chain method to
include longitudinal-isospin dependence of the correlations, making feasible a
study of asymmetric crustal matter. Results are presented for the symmetry
energy, the low-density neutron star equation of state and the single particle
neutron and proton energies.; Comment: 27 pages, submitted to PRC

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## Rydberg atoms in astrophysics

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 13/08/2012

Relevância na Pesquisa

45.53%

Elementary processes in astrophysical phenomena traditionally attract
researchers attention. At first this can be attributed to a group of
hemi-ionization processes in Rydberg atom collisions with ground state parent
atoms. This processes might be studied as a prototype of the elementary process
of the radiation energy transformation into electrical one. The studies of
nonlinear mechanics have shown that so called regime of dynamic chaos should be
considered as typical, rather than exceptional situation in Rydberg atoms
collision. From comparison of theory with experimental results it follows that
a such kind of stochastic dynamic processes, occurred during the single
collision, may be observed.

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## A Macroscopic Description of a Generalized Self-Organized Criticality System: Astrophysical Applications

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

45.6%

We suggest a generalized definition of self-organized criticality (SOC)
systems: SOC is a critical state of a nonlinear energy dissipation system that
is slowly and continuously driven towards a critical value of a system-wide
instability threshold, producing scale-free, fractal-diffusive, and
intermittent avalanches with powerlaw-like size distributions. We develop here
a macroscopic description of SOC systems that provides an equivalent
description of the complex microscopic fine structure, in terms of
fractal-diffusive transport (FD-SOC). Quantitative values for the size
distributions of SOC parameters (length scales $L$, time scales $T$, waiting
times $\Delta t$, fluxes $F$, and energies $E$) are derived from first
principles, using the scale-free probability conjecture, $N(L) dL \propto
L^{-d}$, for Euclidean space dimension $d$. We apply this model to
astrophysical SOC systems, such as lunar craters, the asteroid belt, Saturn
ring particles, magnetospheric substorms, radiation belt electrons, solar
flares, stellar flares, pulsar glitches, soft gamma-ray repeaters, black-hole
objects, blazars, and cosmic rays. The FD-SOC model predicts correctly the size
distributions of 8 out of these 12 astrophysical phenomena, and indicates
non-standard scaling laws and measurement biases for the others.

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## Magnetic Energy and Helicity Budgets in the Active-Region Solar Corona. I. Linear Force-Free Approximation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 27/06/2007

Relevância na Pesquisa

35.54%

We self-consistently derive the magnetic energy and relative magnetic
helicity budgets of a three-dimensional linear force-free magnetic structure
rooted in a lower boundary plane. For the potential magnetic energy we derive a
general expression that gives results practically equivalent to those of the
magnetic Virial theorem. All magnetic energy and helicity budgets are
formulated in terms of surface integrals applied to the lower boundary, thus
avoiding computationally intensive three-dimensional magnetic field
extrapolations. We analytically and numerically connect our derivations with
classical expressions for the magnetic energy and helicity, thus presenting a
so-far lacking unified treatment of the energy/helicity budgets in the
constant-alpha approximation. Applying our derivations to photospheric vector
magnetograms of an eruptive and a noneruptive solar active regions, we find
that the most profound quantitative difference between these regions lies in
the estimated free magnetic energy and relative magnetic helicity budgets. If
this result is verified with a large number of active regions, it will advance
our understanding of solar eruptive phenomena. We also find that the
constant-alpha approximation gives rise to large uncertainties in the
calculation of the free magnetic energy and the relative magnetic helicity.
Therefore...

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## Evolution of Solar Magnetic Field and Associated Multi-wavelength Phenomena: Flare events on 20 November 2003

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 07/12/2009

Relevância na Pesquisa

35.56%

We analyze H-alpha images, soft X-ray profiles, magnetograms, extreme
ultra-violet images and radio observations of two homologous flare events
(M1.4/1N and M9.6/2B) on 20 November 2003 in the active region NOAA 10501 and
study properties of reconnection between twisted filament systems, energy
release and associated launch of coronal mass ejections (CMEs). During both
events twisted filaments observed in H-alpha approached each other and
initiated the flare processes. However, the second event showed the formation
of cusp as the filaments interacted. The rotation of sunspots of opposite
polarities, inferred from magnetograms likely powered the twisted filaments and
injection of helicity. Along the current sheet between these two opposite
polarity sunspots, the shear was maximum, which could have caused the twist in
the filament. At the time of interaction between filaments, the reconnection
took place and flare emission in thermal and non-thermal energy ranges attained
the maximum. The radio signatures revealed the opening of field lines resulting
from reconnection. The H-alpha images and radio data provide the inflow speed
leading to reconnection and the scale size of particle acceleration region. The
first event produced a narrow and slow CME...

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## A localised subgrid scale model for fluid dynamical simulations in astrophysics I: Theory and numerical tests

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 23/01/2006

Relevância na Pesquisa

45.54%

We present a one-equation subgrid scale model that evolves the turbulence
energy corresponding to unresolved velocity fluctuations in large eddy
simulations. The model is derived in the context of the Germano consistent
decomposition of the hydrodynamical equations. The eddy-viscosity closure for
the rate of energy transfer from resolved toward subgrid scales is localised by
means of a dynamical procedure for the computation of the closure parameter.
Therefore, the subgrid scale model applies to arbitrary flow geometry and
evolution. For the treatment of microscopic viscous dissipation a
semi-statistical approach is used, and the gradient-diffusion hypothesis is
adopted for turbulent transport. A priori tests of the localised eddy-viscosity
closure and the gradient-diffusion closure are made by analysing data from
direct numerical simulations. As an a posteriori testing case, the large eddy
simulation of thermonuclear combustion in forced isotropic turbulence is
discussed. We intend the formulation of the subgrid scale model in this paper
as a basis for more advanced applications in numerical simulations of complex
astrophysical phenomena involving turbulence.; Comment: 18 pages, 10 figures, accepted for publication in Astron. Astrophys.;
3D visualisations not included; complete PDF version can be downloaded from
http://www.astro.uni-wuerzburg.de/%7Eschmidt/Paper/SGSModel_I_AA.pdf

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## Investigation of thermonuclear $^{18}$Ne($\alpha$,$p$)$^{21}$Na rate via resonant elastic scattering of $^{21}$Na+$p$

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 18/03/2014

Relevância na Pesquisa

35.57%

The $^{18}$Ne($\alpha$,$p$)$^{21}$Na reaction is thought to be one of the key
breakout reactions from the hot CNO cycles to the rp-process in type I x-ray
bursts. In this work, the resonant properties of the compound nucleus $^{22}$Mg
have been investigated by measuring the resonant elastic scattering of
$^{21}$Na+$p$. An 89 MeV $^{21}$Na radioactive beam delivered from the CNS
Radioactive Ion Beam Separator bombarded an 8.8 mg/cm$^2$ thick polyethylene
(CH$_{2}$)$_{n}$ target. The $^{21}$Na beam intensity was about
2$\times$10$^{5}$ pps, with a purity of about 70% on target. The recoiled
protons were measured at the center-of-mass scattering angles of
$\theta_{c.m.}$$\approx$175.2${^\circ}$, 152.2${^\circ}$, and 150.5${^\circ}$
by three sets of $\Delta E$-$E$ telescopes, respectively. The excitation
function was obtained with the thick-target method over energies
$E_x$($^{22}$Mg)=5.5--9.2 MeV. In total, 23 states above the proton-threshold
in $^{22}$Mg were observed, and their resonant parameters were determined via
an $R$-matrix analysis of the excitation functions. We have made several new
$J^{\pi}$ assignments and confirmed some tentative assignments made in previous
work. The thermonuclear $^{18}$Ne($\alpha$,$p$)$^{21}$Na rate has been
recalculated based on our recommended spin-parity assignments. The
astrophysical impact of our new rate has been investigated through one-zone
postprocessing x-ray burst calculations. We find that the
$^{18}$Ne($\alpha$...

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