2,439 research outputs found
Nucleosynthesis in Type I X-ray Bursts
Type I X-ray bursts are thermonuclear explosions that occur in the envelopes
of accreting neutron stars. Detailed observations of these phenomena have
prompted numerous studies in theoretical astrophysics and experimental nuclear
physics since their discovery over 35 years ago. In this review, we begin by
discussing key observational features of these phenomena that may be sensitive
to the particular patterns of nucleosynthesis from the associated thermonuclear
burning. We then summarize efforts to model type I X-ray bursts, with emphasis
on determining the nuclear physics processes involved throughout these bursts.
We discuss and evaluate limitations in the models, particularly with regard to
key uncertainties in the nuclear physics input. Finally, we examine recent,
relevant experimental measurements and outline future prospects to improve our
understanding of these unique environments from observational, theoretical and
experimental perspectives.Comment: Accepted by Prog. Part. Nucl. Phys., 45 pages, 14 figure
STARLIB: A Next-Generation Reaction-Rate Library for Nuclear Astrophysics
STARLIB is a next-generation, all-purpose nuclear reaction-rate library. For
the first time, this library provides the rate probability density at all
temperature grid points for convenient implementation in models of stellar
phenomena. The recommended rate and its associated uncertainties are also
included. Currently, uncertainties are absent from all other rate libraries,
and, although estimates have been attempted in previous evaluations and
compilations, these are generally not based on rigorous statistical
definitions. A common standard for deriving uncertainties is clearly warranted.
STARLIB represents a first step in addressing this deficiency by providing a
tabular, up-to-date database that supplies not only the rate and its
uncertainty but also its distribution. Because a majority of rates are
lognormally distributed, this allows the construction of rate probability
densities from the columns of STARLIB. This structure is based on a recently
suggested Monte Carlo method to calculate reaction rates, where uncertainties
are rigorously defined. In STARLIB, experimental rates are supplemented with:
(i) theoretical TALYS rates for reactions for which no experimental input is
available, and (ii) laboratory and theoretical weak rates. STARLIB includes all
types of reactions of astrophysical interest to Z = 83, such as (p,g), (p,a),
(a,n), and corresponding reverse rates. Strong rates account for thermal target
excitations. Here, we summarize our Monte Carlo formalism, introduce the
library, compare methods of correcting rates for stellar environments, and
discuss how to implement our library in Monte Carlo nucleosynthesis studies. We
also present a method for accessing STARLIB on the Internet and outline updated
Monte Carlo-based rates.Comment: Accepted for publication in the Astrophysical Journal Supplement
Series; 96 pages, 22 figure
Reaction rate uncertainties and the operation of the NeNa and MgAl chains during HBB in intermediate-mass AGB stars
We test the effect of proton-capture reaction rate uncertainties on the
abundances of the Ne, Na, Mg and Al isotopes processed by the NeNa and MgAl
chains during hot bottom burning (HBB) in asymptotic giant branch (AGB) stars
of intermediate mass between 4 and 6 solar masses and metallicities between
Z=0.0001 and 0.02. We provide uncertainty ranges for the AGB stellar yields,
for inclusion in galactic chemical evolution models, and indicate which
reaction rates are most important and should be better determined. We use a
fast synthetic algorithm based on detailed AGB models. We run a large number of
stellar models, varying one reaction per time for a very fine grid of values,
as well as all reactions simultaneously. We show that there are uncertainties
in the yields of all the Ne, Na, Mg and Al isotopes due to uncertain
proton-capture reaction rates. The most uncertain yields are those of 26Al and
23Na (variations of two orders of magnitude), 24Mg and 27Al (variations of more
than one order of magnitude), 20Ne and 22Ne (variations between factors 2 and
7). In order to obtain more reliable Ne, Na, Mg and Al yields from IM-AGB stars
the rates that require more accurate determination are: 22Ne(p,g)23Na,
23Na(p,g)24Mg, 25Mg(p,g)26Al, 26Mg(p,g)27Al and 26Al(p,g)27Si. Detailed
galactic chemical evolution models should be constructed to address the impact
of our uncertainty ranges on the observational constraints related to HBB
nucleosynthesis, such as globular cluster chemical anomalies.Comment: accepted for publication on Astronomy & Astrophysic
Charged-Particle Thermonuclear Reaction Rates: III. Nuclear Physics Input
The nuclear physics input used to compute the Monte Carlo reaction rates and
probability density functions that are tabulated in the second paper of this
series (Paper II) is presented. Specifically, we publish the input files to the
Monte Carlo reaction rate code RatesMC, which is based on the formalism
presented in the first paper of this series (Paper I). This data base contains
overwhelmingly experimental nuclear physics information. The survey of
literature for this review was concluded in November 2009.Comment: 132 page
Stent Placement for Coronary Stenosis in Kawasaki Disease: Case Report and Literature Review
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73875/1/j.1540-8183.2002.tb01030.x.pd
Scanning Electron Microscopy - Electron Beam Induced Current and Deep Level Transient Spectroscopy Studies of GaAs(In) Layers grown by Molecular Beam Epitaxy
Electrically active defects in indium-doped (0.6%) GaAs layers grown by Molecular Beam Epitaxy (MBE) on Si-doped (â1x1018 cm-3) GaAs substrates have been studied by the combination of two techniques: Scanning Electron Microscope - Electron Beam Induced Current (SEM-EBIC) technique, and Deep Level Transient Spectroscopy (DLTS). The epilayers studied were three microns thick. No electrically active defects were revealed by the EBIC micrographs in the top one micron of the epilayers, whereas a large number of non-propagating misfit dislocations were observed at the epilayer/substrate interface. DLTS measurements made in the dislocation free top region of the epilayer showed the presence of three well known traps, which had previously been observed to also exist near the interface. It is concluded that these traps are not related to misfit dislocations
Direct measurement of the 14N(p,g)15O S-factor
We have measured the 14N(p,g)15O excitation function for energies in the
range E_p = 155--524 keV. Fits of these data using R-matrix theory yield a
value for the S-factor at zero energy of 1.64(17) keV b, which is significantly
smaller than the result of a previous direct measurement. The corresponding
reduction in the stellar reaction rate for 14N(p,g)15O has a number of
interesting consequences, including an impact on estimates for the age of the
Galaxy derived from globular clusters.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Let
Experimental evidence of a natural parity state in Mg and its impact to the production of neutrons for the s process
We have studied natural parity states in Mg via the
Ne(Li,d)Mg reaction. Our method significantly improves the
energy resolution of previous experiments and, as a result, we report the
observation of a natural parity state in Mg. Possible spin-parity
assignments are suggested on the basis of published -ray decay
experiments. The stellar rate of the Ne(,)Mg
reaction is reduced and may give rise to an increase in the production of
s-process neutrons via the Ne(,n)Mg reaction.Comment: Published in PR
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Synthesis of tetrahydroquinoline enediyne core analogs of dynemicin
A process is described for the preparation of the core azobicyclo[7.3.1]tridecaenediyne moiety of the antitumor antibiotic dynemicin. The synthesis allows efficient production of the enediyne as a stable, compound in good yield from the adamantyl N-protected azabicyclo[7.3.1]tridecadiyne. The adamantyl protecting group is employed in the starting material, N-adamantyl dihydroquinoline or N-adamantyl 6-methoxy quinoline. Also disclosed are process for the synthesis of 3-hydroxy-6-methoxyquinoline and several N-substituted derivatives of azobicyclo[7.3.1]tridecaenediyne. Solid tumor and leukemia assays were performed on the analogs of dynemicin. The results suggest a method that these compounds will useful in treating certain types of leukemias and solid tumors. The disclosed synthesis provides a route to new dynemicin intermediates and analogs which will allow development of second and third generation dynemicins.Board of Regents, University of Texas Syste
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