2,168 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
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
70Ge(p,gamma)71As and 76Ge(p,n)76As cross sections for the astrophysical p process: sensitivity of the optical proton potential at low energies
The cross sections of the 70Ge(p,gamma)71As and 76Ge(p,n)76As reactions have
been measured with the activation method in the Gamow window for the
astrophysical p process. The experiments were carried out at the Van de Graaff
and cyclotron accelerators of ATOMKI. The cross sections have been derived by
measuring the decay gamma-radiation of the reaction products. The results are
compared to the predictions of Hauser-Feshbach statistical model calculations
using the code NON-SMOKER. Good agreement between theoretical and experimental
S factors is found. Based on the new data, modifications of the optical
potential used for low-energy protons are discussed.Comment: Accepted for publication in Phys. Rev.
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
The 25Mg(p,g)Al reaction at low astrophysical energies
In the present work we report on a new measurement of resonance strengths in
the reaction 25Mg(p,gamma)26Al at E_cm= 92 and 189 keV. This study was
performed at the LUNA facility in the Gran Sasso underground laboratory using a
4pi BGO summing crystal. For the first time the 92 keV resonance was directly
observed and a resonance strength omega-gamma=(2.9+/-0.6)x10E-10 eV was
determined. Additionally, the gamma-ray branchings and strength of the 189 keV
resonance were studied with a high resolution HPGe detector yielding an
omega-gamma value in agreement with the BGO measurement, but 20% larger
compared to previous works.Comment: 6 pages, 4 figures, accepted for publication in Physics Letters
Measurement of the 18Ne(a,p_0)21Na reaction cross section in the burning energy region for X-ray bursts
The 18Ne(a,p)21Na reaction provides one of the main HCNO-breakout routes into
the rp-process in X-ray bursts. The 18Ne(a,p_0)21Na reaction cross section has
been determined for the first time in the Gamow energy region for peak
temperatures T=2GK by measuring its time-reversal reaction 21Na(p,a)18Ne in
inverse kinematics. The astrophysical rate for ground-state to ground-state
transitions was found to be a factor of 2 lower than Hauser-Feshbach
theoretical predictions. Our reduced rate will affect the physical conditions
under which breakout from the HCNO cycles occurs via the 18Ne(a,p)21Na
reaction.Comment: 5 pages, 3 figures, accepted for publication on Physical Review
Letter
Theoretical Evaluation of the Reaction Rates for 26Al(n,p)26Mg and 26Al(n,a)23Na
The reactions that destroy 26Al in massive stars have significance in a
number of astrophysical contexts. We evaluate the reaction rates of
26Al(n,p)26Mg and 26Al(n,a)23Na using cross sections obtained from the codes
EMPIRE and TALYS. These have been compared to the published rates obtained from
the non-smoker code and to some experimental data. We show that the results
obtained from EMPIRE and TALYS are comparable to those from non-smoker. We also
show how the theoretical results vary with respect to changes in the input
parameters. Finally, we present recommended rates for these reactions using the
available experimental data and our new theoretical results
Production of 26Al in stellar hydrogen-burning environments: spectroscopic properties of states in 27Si
Model predictions of the amount of the radioisotope 26Al produced in
hydrogen-burning environments require reliable estimates of the thermonuclear
rates for the 26gAl(p,{\gamma})27Si and 26mAl(p,{\gamma})27Si reactions. These
rates depend upon the spectroscopic properties of states in 27Si within about 1
MeV of the 26gAl+p threshold (Sp = 7463 keV). We have studied the
28Si(3He,{\alpha})27Si reaction at 25 MeV using a high-resolution
quadrupole-dipole-dipole-dipole magnetic spectrograph. For the first time with
a transfer reaction, we have constrained J{\pi} values for states in 27Si over
Ex = 7.0 - 8.1 MeV through angular distribution measurements. Aside from a few
important cases, we generally confirm the energies and spin-parity assignments
reported in a recent {\gamma}-ray spectroscopy study. The magnitudes of neutron
spectroscopic factors determined from shell-model calculations are in
reasonable agreement with our experimental values extracted using this
reaction.Comment: accepted for publication in Phys. Rev.
Primordial Nucleosynthesis
Primordial nucleosynthesis, or Big-Bang Nucleosynthesis (BBN), is one of the
three evidences for the Big-Bang model, together with the expansion of the
Universe and the Cosmic Microwave Background. There is a good global agreement
over a range of nine orders of magnitude between abundances of 4He, D, 3He and
7Li deduced from observations, and calculated in primordial nucleosynthesis.
This comparison was used to determine the baryonic density of the Universe. For
this purpose, it is now superseded by the analysis of the Cosmic Microwave
Background (CMB) radiation anisotropies. However, there remain, a yet
unexplained, discrepancy of a factor 3-5, between the calculated and observed
lithium primordial abundances, that has not been reduced, neither by recent
nuclear physics experiments, nor by new observations. We review here the
nuclear physics aspects of BBN for the production of 4He, D, 3He and 7Li, but
also 6Li, 9Be, 11B and up to CNO isotopes. These are, for instance, important
for the initial composition of the matter at the origin of the first stars.
Big-Bang nucleosynthesis, that has been used, to first constrain the baryonic
density, and the number of neutrino families, remains, a valuable tool to probe
the physics of the early Universe, like variation of "constants" or alternative
theories of gravity.Comment: Invited Plenary Talk given at the 11th International Conference on
Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA, May 27-June 1,
2012. To appear in the NN2012 Proceedings in Journal of Physics: Conference
Series (JPCS
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