87 research outputs found
The 13N(d,n)14O Reaction and the Astrophysical 13N(p,g)14O Reaction Rate
N()O is one of the key reactions in the hot CNO cycle
which occurs at stellar temperatures around 0.1. Up to now, some
uncertainties still exist for the direct capture component in this reaction,
thus an independent measurement is of importance. In present work, the angular
distribution of the N()O reaction at = 8.9
MeV has been measured in inverse kinematics, for the first time. Based on the
distorted wave Born approximation (DWBA) analysis, the nuclear asymptotic
normalization coefficient (ANC), , for the ground state of
O N + is derived to be fm. The
N()O reaction is analyzed with the R-matrix approach,
its astrophysical S-factors and reaction rates at energies of astrophysical
relevance are then determined with the ANC. The implications of the present
reaction rates on the evolution of novae are then discussed with the reaction
network calculations.Comment: 17 pages and 8 figure
Radially extended kinematics and stellar populations of the massive ellipticals NGC1600, NGC4125 and NGC7619. Constraints on the outer dark halo density profile
We present high quality long slit spectra along the major and minor axes out
to 1.5-2 Re (14-22 kpc) of three bright elliptical galaxies (NGC1600, NGC4125,
NGC7619) obtained at the Hobby-Eberly Telescope (HET). We derive stellar
kinematic profiles and Lick/IDS indices (Hbeta, Mgb, Fe5015, Fe5270, Fe5335,
Fe5406). Moreover, for NGC4125 we derive gas kinematics and emission line
strengths. We model the absorption line strengths using Simple Stellar
Populations models that take into account the variation of [\alpha/Fe] and
derive ages, total metallicity and element abundances. Overall, we find that
the three galaxies have old and [\alpha/Fe] overabundant stellar populations
with no significant gradients. The metallicity is supersolar at the center with
a strong negative radial gradient. For NGC4125, several pieces of evidence
point to a recent dissipational merger event. We calculate the broad band color
profiles with the help of SSP models. All of the colors show sharp peaks at the
center of the galaxies, mainly caused by the metallicity gradients, and agree
well with the measured colors. Using the Schwarzschild's axisymmetric orbit
superposition technique, we model the stellar kinematics to constrain the dark
halos of the galaxies. We use the tight correlation between the Mgb strength
and local escape velocity to set limits on the extent of the halos by testing
different halo sizes. Logarithmic halos - cut at 60 kpc -minimize the overall
scatter of the Mgb-Vesc relation. Larger cutoff radii are found if the dark
matter density profile is decreasing more steeply at large radii.Comment: Accepted for publication in Astronomy and Astrophysic
New determination of the 13C(a, n)16O reaction rate and its influence on the s-process nucleosynthesis in AGB stars
We present a new measurement of the -spectroscopic factor
() and the asymptotic normalization coefficient (ANC) for the 6.356
MeV 1/2 subthreshold state of O through the C(B,
Li)O transfer reaction and we determine the -width of this
state. This is believed to have a strong effect on the rate of the
C(, )O reaction, the main neutron source for {\it
slow} neutron captures (the -process) in asymptotic giant branch (AGB)
stars. Based on the new width we derive the astrophysical S-factor and the
stellar rate of the C(, )O reaction. At a temperature
of 100 MK our rate is roughly two times larger than that by \citet{cau88} and
two times smaller than that recommended by the NACRE compilation. We use the
new rate and different rates available in the literature as input in
simulations of AGB stars to study their influence on the abundances of selected
-process elements and isotopic ratios. There are no changes in the final
results using the different rates for the C(, )O
reaction when the C burns completely in radiative conditions. When the
C burns in convective conditions, as in stars of initial mass lower than
2 M_\sun and in post-AGB stars, some changes are to be expected, e.g.,
of up to 25% for Pb in our models. These variations will have to be carefully
analyzed when more accurate stellar mixing models and more precise
observational constraints are available
Neutrino-driven wind simulations and nucleosynthesis of heavy elements
Neutrino-driven winds, which follow core-collapse supernova explosions,
present a fascinating nuclear astrophysics problem that requires understanding
advanced astrophysics simulations, the properties of matter and neutrino
interactions under extreme conditions, the structure and reactions of exotic
nuclei, and comparisons against forefront astronomical observations. The
neutrino-driven wind has attracted vast attention over the last 20 years as it
was suggested to be a candidate for the astrophysics site where half of the
heavy elements are produced via the r-process. In this review, we summarize our
present understanding of neutrino-driven winds from the dynamical and
nucleosynthesis perspectives. Rapid progress has been made during recent years
in understanding the wind with improved simulations and better micro physics.
The current status of the fields is that hydrodynamical simulations do not
reach the extreme conditions necessary for the r-process and the proton or
neutron richness of the wind remains to be investigated in more detail.
However, nucleosynthesis studies and observations point already to
neutrino-driven winds to explain the origin of lighter heavy elements, such as
Sr, Y, Zr.Comment: Submitted to: J. Phys. G: Nucl. Phy
Have Superheavy Elements been Produced in Nature?
We discuss the possibility whether superheavy elements can be produced in
Nature by the astrophysical rapid neutron capture process. To this end we have
performed fully dynamical network r-process calculations assuming an
environment with neutron-to-seed ratio large enough to produce superheavy
nuclei. Our calculations include two sets of nuclear masses and fission
barriers and include all possible fission channels and the associated fission
yield distributions. Our calculations produce superheavy nuclei with A ~ 300
that however decay on timescales of days.Comment: 12 pages, 11 figure
Nuclear astrophysics: the unfinished quest for the origin of the elements
Half a century has passed since the foundation of nuclear astrophysics. Since
then, this discipline has reached its maturity. Today, nuclear astrophysics
constitutes a multidisciplinary crucible of knowledge that combines the
achievements in theoretical astrophysics, observational astronomy,
cosmochemistry and nuclear physics. New tools and developments have
revolutionized our understanding of the origin of the elements: supercomputers
have provided astrophysicists with the required computational capabilities to
study the evolution of stars in a multidimensional framework; the emergence of
high-energy astrophysics with space-borne observatories has opened new windows
to observe the Universe, from a novel panchromatic perspective; cosmochemists
have isolated tiny pieces of stardust embedded in primitive meteorites, giving
clues on the processes operating in stars as well as on the way matter
condenses to form solids; and nuclear physicists have measured reactions near
stellar energies, through the combined efforts using stable and radioactive ion
beam facilities. This review provides comprehensive insight into the nuclear
history of the Universe and related topics: starting from the Big Bang, when
the ashes from the primordial explosion were transformed to hydrogen, helium,
and few trace elements, to the rich variety of nucleosynthesis mechanisms and
sites in the Universe. Particular attention is paid to the hydrostatic
processes governing the evolution of low-mass stars, red giants and asymptotic
giant-branch stars, as well as to the explosive nucleosynthesis occurring in
core-collapse and thermonuclear supernovae, gamma-ray bursts, classical novae,
X-ray bursts, superbursts, and stellar mergers.Comment: Invited Review. Accepted for publication in "Reports on Progress in
Physics" (version with low-resolution figures
Horizons: nuclear astrophysics in the 2020s and beyond
Nuclear astrophysics is a field at the intersection of nuclear physics and astrophysics, which seeks to understand the nuclear engines of astronomical objects and the origin of the chemical elements. This white paper summarizes progress and status of the field, the new open questions that have emerged, and the tremendous scientific opportunities that have opened up with major advances in capabilities across an ever growing number of disciplines and subfields that need to be integrated. We take a holistic view of the field discussing the unique challenges and opportunities in nuclear astrophysics in regards to science, diversity, education, and the interdisciplinarity and breadth of the field. Clearly nuclear astrophysics is a dynamic field with a bright future that is entering a new era of discovery opportunities
Horizons: Nuclear Astrophysics in the 2020s and Beyond
Nuclear Astrophysics is a field at the intersection of nuclear physics and
astrophysics, which seeks to understand the nuclear engines of astronomical
objects and the origin of the chemical elements. This white paper summarizes
progress and status of the field, the new open questions that have emerged, and
the tremendous scientific opportunities that have opened up with major advances
in capabilities across an ever growing number of disciplines and subfields that
need to be integrated. We take a holistic view of the field discussing the
unique challenges and opportunities in nuclear astrophysics in regards to
science, diversity, education, and the interdisciplinarity and breadth of the
field. Clearly nuclear astrophysics is a dynamic field with a bright future
that is entering a new era of discovery opportunities.Comment: 96 pages. Submitted to Journal of Physics
The LOFT mission concept: a status update
The Large Observatory For x-ray Timing (LOFT) is a mission concept which was proposed to ESA as M3 and M4 candidate in the framework of the Cosmic Vision 2015-2025 program. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument and the uniquely large field of view of its wide field monitor, LOFT will be able to study the behaviour of matter in extreme conditions such as the strong gravitational field in the innermost regions close to black holes and neutron stars and the supra-nuclear densities in the interiors of neutron stars. The science payload is based on a Large Area Detector (LAD, >8m2 effective area, 2-30 keV, 240 eV spectral resolution, 1 degree collimated field of view) and a Wide Field Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source location accuracy, 300 eV spectral resolution). The WFM is equipped with an on-board system for bright events (e.g., GRB) localization. The trigger time and position of these events are broadcast to the ground within 30 s from discovery. In this paper we present the current technical and programmatic status of the mission
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