114 research outputs found
An Approximation for the rp-Process
Hot (explosive) hydrogen burning or the Rapid Proton Capture Process
(rp-process) occurs in a number of astrophysical environments. Novae and X-ray
bursts are the most prominent ones, but accretion disks around black holes and
other sites are candidates as well. The expensive and often multidimensional
hydro calculations for such events require an accurate prediction of the
thermonuclear energy generation, while avoiding full nucleosynthesis network
calculations. In the present investigation we present an approximation scheme
applicable in a temperature range which covers the whole range of all presently
known astrophysical sites. It is based on the concept of slowly varying
hydrogen and helium abundances and assumes a kind of local steady flow by
requiring that all reactions entering and leaving a nucleus add up to a zero
flux. This scheme can adapt itself automatically and covers situations at low
temperatures, characterized by a steady flow of reactions, as well as high
temperature regimes where a -equilibrium is established.
In addition to a gain of a factor of 15 in computational speed over a full
network calculation, and an energy generation accurate to more than 15 %, this
scheme also allows to predict correctly individual isotopic abundances. Thus,
it delivers all features of a full network at a highly reduced cost and can
easily be implemented in hydro calculations.Comment: 18 pages, LaTeX using astrobib and aas2pp4, includes PostScript
figures; Astrophysical Journal, in press. PostScript source also available at
http://quasar.physik.unibas.ch/preps.htm
Stringency of the 2-His–1-Asp Active-Site Motif in Prolyl 4-Hydroxylase
The non-heme iron(II) dioxygenase family of enzymes contain a common 2-His–1-carboxylate iron-binding motif. These enzymes catalyze a wide variety of oxidative reactions, such as the hydroxylation of aliphatic C–H bonds. Prolyl 4-hydroxylase (P4H) is an α-ketoglutarate-dependent iron(II) dioxygenase that catalyzes the post-translational hydroxylation of proline residues in protocollagen strands, stabilizing the ensuing triple helix. Human P4H residues His412, Asp414, and His483 have been identified as an iron-coordinating 2-His–1-carboxylate motif. Enzymes that catalyze oxidative halogenation do so by a mechanism similar to that of P4H. These halogenases retain the active-site histidine residues, but the carboxylate ligand is replaced with a halide ion. We replaced Asp414 of P4H with alanine (to mimic the active site of a halogenase) and with glycine. These substitutions do not, however, convert P4H into a halogenase. Moreover, the hydroxylase activity of D414A P4H cannot be rescued with small molecules. In addition, rearranging the two His and one Asp residues in the active site eliminates hydroxylase activity. Our results demonstrate a high stringency for the iron-binding residues in the P4H active site. We conclude that P4H, which catalyzes an especially demanding chemical transformation, is recalcitrant to change
Sensitivity of p-Process Nucleosynthesis to Nuclear Reaction Rates in a 25 Solar Mass Supernova Model
The astrophysical p process, which is responsible for the origin of the
proton rich stable nuclei heavier than iron, was investigated using a full
nuclear reaction network for a type II supernova explosion when the shock front
passes through the O/Ne layer. Calculations were performed with a multi-layer
model adopting the seed of a pre-explosion evolution of a 25 solar mass star.
The reaction flux was calculated to determine the main reaction path and
branching points responsible for synthesizing the proton rich nuclei. In order
to investigate the impact of nuclear reaction rates on the predicted p-process
abundances, extensive simulations with different sets of collectively and
individually modified neutron-, proton-, alpha-capture and photodisintegration
rates have been performed. These results are not only relevant to explore the
nuclear physics related uncertainties in p-process calculations but are also
important for identifying the strategy and planning of future experiments.Comment: 27 pages, 24 figures, accepted for publication in ApJ
Nuclear uncertainties in the NeNa-MgAl cycles and production of 22Na and 26Al during nova outbursts
Classical novae eject significant amounts of nuclear processed material into
the interstellar medium. Among the isotopes synthesized during such explosions,
two radioactive nuclei deserve a particular attention: 22Na and 26Al. In this
paper, we investigate the nuclear paths leading to 22Na and 26Al production
during nova outbursts by means of an implicit, hydrodynamic code that follows
the course of the thermonuclear runaway from the onset of accretion up to the
ejection stage. New evolutionary sequences of ONe novae have been computed,
using updated nuclear reaction rates relevant to 22Na and 26Al production.
Special attention is focused on the role played by nuclear uncertainties within
the NeNa and MgAl cycles in the synthesis of such radioactive species. From the
series of hydrodynamic models, which assume upper, recommended or lower
estimates of the reaction rates, we derive limits on the production of both
22Na and 26Al. We outline a list of nuclear reactions which deserve new
experimental investigations in order to reduce the wide dispersion introduced
by nuclear uncertainties in the 22Na and 26Al yields.Comment: 46 pages, 4 figures. Accepted for publication in The Astrophysical
Journa
Research on Fundamental Breach of Contract:from the perspective of CISG
根本违约是从英国法中产生的一种违约形态,被《联合国国际货物销售合同公约》采纳后成为一个国际性的条款。根本违约制度在《公约》中具有极为重要的地位,扮演了中心的角色。一般认为,根本违约是合同当事人违反了合同中最重要、根本性的条款而构成的违约,另一方当事人不仅有权要求损害赔偿,更重要的是有权宣告合同无效(解除合同)的制度。我国《合同法》第94条也规定了根本违约制度。本文对《公约》的根本违约制度进行了较为全面、深入的分析探讨,指出了我国《合同法》根本违约制度的缺陷并提出了改进的建议。全文除前言和结语外,正文共分为四章:第一章:根本违约制度之国际概览。本章以宏观的角度介绍了根本违约制度在英美法和大陆法两...The mechanism of fundamental breach of contract was derived from British law and adopted by CISG, and had then been developed into an international term. This mechanism plays an extremely important role in CISG. Generally, the mechanism is invoked where one party violates the most important and fundamental terms of the contract and the other party thus has the right to claim damages and to declare...学位:法学硕士院系专业:法学院法律系_国际法学(含国际公法、国际私法、国际经济法)学号:20040810
Investigation of the 19Na via resonance elastic scattering
The structure of the unbound proton-rich isotope 19Na was studied in
resonance elastic scattering of a radioactive 18Ne beam on a proton target
using the thick-target inverse-kinematics method. The experiment covered
excitation energy range from 0.5 to 2.7 MeV in c.m.s. Only one state of 19Na
(the second excited state) was observed. A combined R-matrix and
potential-model analysis was performed. The spin and parity assignment of this
second excited state was confirmed to be 1/2+. We showthat the position of the
1/2+ state significantly affects the reaction rate through that state but the
total reaction rate remains unchanged since the 18Ne(2p,gamma) proceeds mostly
via the ground and first excited states in 19Na at stellar temperatures.Comment: 13 pages, 5 figure
Reaction Rates Uncertainties and the Production of F19 in AGB Stars
We present nucleosynthesis calculations and the resulting 19F stellar yields
for a large set of models with different masses and metallicity. We find that
the production of fluorine depends on the temperature of the convective pulses,
the amount of primary 12C mixed into the envelope by third dredge up and the
extent of the partial mixing zone. Then we perform a detailed analysis of the
reaction rates involved in the production of 19F and the effects of their
uncertainties. We find that the major uncertainties are associated with the
14C(alpha,gamma)18O and the 19F(alpha,p)22Ne reaction rates. For these two
reactions we present new estimates of the rates and their uncertainties. The
importance of the partial mixing zone is reduced when using our estimate for
the 14C(alpha,gamma)18O rate. Taking into account both the uncertainties
related to the partial mixing zone and those related to nuclear reactions, the
highest values of 19F enhancements observed in AGB stars are not matched by the
models. This is a problem that will have to be revised by providing a better
understanding of the formation and nucleosynthesis in the partial mixing zone,
also in relation to reducing the uncertainties of the 14C(alpha,gamma)18O
reaction rate. At the same time the possible effect of Cool Bottom Processing
at the base of the convective envelope should be included in the computation of
AGB nucleosynthesis. This process could in principle help matching the highest
19F abundances observed by decreasing the C/O ratio at the surface of the star,
while leaving the 19F abundance unchanged.Comment: 40 pages, 8 figures, accepted for publication on the Astrophysical
Journa
Catching Element Formation In The Act
Gamma-ray astronomy explores the most energetic photons in nature to address
some of the most pressing puzzles in contemporary astrophysics. It encompasses
a wide range of objects and phenomena: stars, supernovae, novae, neutron stars,
stellar-mass black holes, nucleosynthesis, the interstellar medium, cosmic rays
and relativistic-particle acceleration, and the evolution of galaxies. MeV
gamma-rays provide a unique probe of nuclear processes in astronomy, directly
measuring radioactive decay, nuclear de-excitation, and positron annihilation.
The substantial information carried by gamma-ray photons allows us to see
deeper into these objects, the bulk of the power is often emitted at gamma-ray
energies, and radioactivity provides a natural physical clock that adds unique
information. New science will be driven by time-domain population studies at
gamma-ray energies. This science is enabled by next-generation gamma-ray
instruments with one to two orders of magnitude better sensitivity, larger sky
coverage, and faster cadence than all previous gamma-ray instruments. This
transformative capability permits: (a) the accurate identification of the
gamma-ray emitting objects and correlations with observations taken at other
wavelengths and with other messengers; (b) construction of new gamma-ray maps
of the Milky Way and other nearby galaxies where extended regions are
distinguished from point sources; and (c) considerable serendipitous science of
scarce events -- nearby neutron star mergers, for example. Advances in
technology push the performance of new gamma-ray instruments to address a wide
set of astrophysical questions.Comment: 14 pages including 3 figure
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