1,873 research outputs found
The rp-Process in Neutrino-driven Winds
Recent hydrodynamic simulations of core-collapse supernovae with accurate
neutrino transport suggest that the bulk of the early neutrino-heated ejecta is
proton rich, in which the production of some interesting proton-rich nuclei is
expected. As suggested in recent nucleosynthesis studies, the rapid
proton-capture (rp) process takes place in such proton-rich environments by
bypassing the waiting point nuclei with the beta-lives of a few minutes via the
faster capture of neutrons continuously supplied from the neutrino absorption
by protons. In this study, the nucleosynthesis calculations are performed with
the wide ranges of the neutrino luminosities and the electron fractions (Ye),
using the semi-analytic models of proto-neutron star winds. The masses of
proto-neutron stars are taken to be 1.4 Msolar and 2.0 Msolar, where the latter
is regarded as the test for somewhat high entropy winds (about a factor of
two). For Ye > 0.52, the neutrino-induced rp-process takes place in many wind
trajectories, and the p-nuclei up to A ~ 130 are synthesized with interesting
amounts. However, 92Mo is somewhat underproduced compared to those with similar
mass numbers. For 0.46 < Ye < 0.49, on the other hand, 92Mo is significantly
enhanced by the nuclear flows in the vicinity of the abundant 90Zr that
originates from the alpha-process at higher temperature. The nucleosynthetic
yields are averaged over the ejected masses of winds, and further the Ye
distribution predicted by the recent hydrodynamic simulation of a core-collapse
supernova. Comparison of the mass-Ye-averaged yields to the solar compositions
implies that the neutrino-driven winds can be potentially the origin of light
p-nuclei up to A ~ 110, including 92,94Mo and 96,98Ru that cannot be explained
by other astrophysical sites.Comment: 29 pages, 18 figures, accepted for publication in Ap
Magnetic domain walls in constrained geometries
Magnetic domain walls have been studied in micrometer-sized Fe20Ni80 elements
containing geometrical constrictions by spin-polarized scanning electron
microscopy and numerical simulations. By controlling the constriction
dimensions, the wall width can be tailored and the wall type modified. In
particular, the width of a 180 degree Neel wall can be strongly reduced or
increased by the constriction geometry compared with the wall in unconstrained
systems.Comment: 4 pages, 6 figure
Entire solutions of fully nonlinear elliptic equations with a superlinear gradient term
International audienceIn this paper we consider second order fully nonlinear operators with an additive superlinear gradient term. Like in the pioneering paper of Brezis for the semilinear case, we obtain the existence of entire viscosity solutions, defined in all the space, without assuming global bounds. A uniqueness result is also obtained for special gradient terms, subject to a convexity/concavity type assumption where superlinearity is essential and has to be handled in a different way from the linear case
Unstable Nonradial Oscillations on Helium Burning Neutron Stars
Material accreted onto a neutron star can stably burn in steady state only
when the accretion rate is high (typically super-Eddington) or if a large flux
from the neutron star crust permeates the outer atmosphere. For such situations
we have analyzed the stability of nonradial oscillations, finding one unstable
mode for pure helium accretion. This is a shallow surface wave which resides in
the helium atmosphere above the heavier ashes of the ocean. It is excited by
the increase in the nuclear reaction rate during the oscillations, and it grows
on the timescale of a second. For a slowly rotating star, this mode has a
frequency of approximately 20-30 Hz (for l=1), and we calculate the full
spectrum that a rapidly rotating (>>30 Hz) neutron star would support. The
short period X-ray binary 4U 1820--30 is accreting helium rich material and is
the system most likely to show this unstable mode,especially when it is not
exhibiting X-ray bursts. Our discovery of an unstable mode in a thermally
stable atmosphere shows that nonradial perturbations have a different stability
criterion than the spherically symmetric thermal perturbations that generate
type I X-ray bursts.Comment: Accepted for publication in Astrophysical Journal, 22 pages, 14
figure
Hydrostatic Expansion and Spin Changes During Type I X-Ray Bursts
We present calculations of the spin-down of a neutron star atmosphere due to
hydrostatic expansion during a Type I X-ray burst. We show that (i) Cumming and
Bildsten overestimated the spin-down of rigidly-rotating atmospheres by a
factor of two, and (ii) general relativity has a small (5-10%) effect on the
angular momentum conservation law. We rescale our results to different neutron
star masses, rotation rates and equations of state, and present some detailed
rotational profiles. Comparing with recent observations of large frequency
shifts in MXB 1658-298 and 4U 1916-053, we find that the spin-down expected if
the atmosphere rotates rigidly is a factor of two to three less than the
observed values. If differential rotation is allowed to persist, we find that
the upper layers of the atmosphere spin down by an amount comparable to the
observed values; however, there is no compelling reason to expect the observed
spin frequency to be that of only the outermost layers. We conclude that
hydrostatic expansion and angular momentum conservation alone cannot account
for the largest frequency shifts observed during Type I bursts.Comment: Submitted to the Astrophysical Journal (13 pages, including 4
figures
P-Process Nucleosynthesis inside Supernova-Driven Supercritical Accretion Disks
We investigate p-process nucleosynthesis in a supercritical accretion disk
around a compact object of 1.4 M_solar, using the self-similar solution of an
optically thick advection dominated flow. Supercritical accretion is expected
to occur in a supernova with fallback material accreting onto a new-born
compact object. It is found that appreciable amounts of p-nuclei are
synthesized via the p-process in supernova-driven supercritical accretion disks
(SSADs) when the accretion rate m_dot = M_dot c^2/(16 L_Edd) >10^5, where L_Edd
is the Eddington luminosity. Abundance profiles of p-nuclei ejected from SSADs
have similar feature to those of the oxygen/neon layers in Type II supernovae
when the abundance of the fallback gas far from the compact object is that of
the oxygen/neon layers in the progenitor. The overall abundance profile is in
agreement with that of the solar system. Some p-nuclei, such as Mo, Ru, Sn, and
La, are underproduced in the SSADs as in Type II supernovae. If the fallback
gas is mixed with a small fraction of proton through Rayleigh-Taylor
instability during the explosion, significant amounts of Mo92 are produced
inside the SSADs. Ru96 and La138 are also produced when the fallback gas
contains abundant proton though the overall abundance profile of p-nuclei is
rather different from that of the solar system. The p-process nucleosynthesis
in SSADs contributes to chemical evolution of p-nuclei, in particular Mo92, if
several percents of fallback matter are ejected via jets and/or winds.Comment: 15 pages, 7 figures included, 3 tables, LaTeX emulateapj5.sty,
accepted for publication by the Astronomical Journal (March, 2003
The rp Process Ashes from Stable Nuclear Burning on an Accreting Neutron Star
We calculate the nucleosynthesis during stable nuclear burning on an
accreting neutron star. This is appropriate for weakly magnetic neutron stars
accreting at near-Eddington rates in low mass X-ray binaries, and for most
accreting X-ray pulsars. We show that the nuclear burning proceeds via the
rapid proton capture process (rp process), and makes nuclei far beyond the iron
group. The final mixture of nuclei consists of elements with a range of masses
between approximately A=60 and A=100. The average nuclear mass of the ashes is
set by the extent of helium burning via (alpha,p) reactions, and depends on the
local accretion rate.
Our results imply that the crust of these accreting neutron stars is made
from a complex mixture of heavy nuclei, with important implications for its
thermal, electrical and structural properties. A crustal lattice as impure as
our results suggest will have a conductivity set mostly by impurity scattering,
allowing more rapid Ohmic diffusion of magnetic fields than previously
estimated.Comment: To appear in the Astrophysical Journal (33 pages, LaTeX, including 11
postscript figures
Proton-Deuteron Elastic Scattering from 2.5 to 22.5 MeV
We present the results of a calculation of differential cross sections and
polarization observables for proton-deuteron elastic scattering, for proton
laboratory energies from 2.5 to 22.5 MeV. The Paris potential parametrisation
of the nuclear force is used. As solution method for the charged-composite
particle equations the 'screening and renormalisation approach' is adopted
which allows to correctly take into account the Coulomb repulsion between the
two protons. Comparison is made with the precise experimental data of Sagara et
al. [Phys. Rev. C 50, 576 (1994)] and of Sperison et al. [Nucl. Phys. A422, 81
(1984)].Comment: 24 pages, 8 eps figures, uses REVTe
Periodic Thermonuclear X-ray Bursts from GS 1826-24 and the Fuel Composition as a Function of Accretion Rate
We analyze 24 type I X-ray bursts from GS 1826-24 observed by the Rossi X-ray
Timing Explorer between 1997 November and 2002 July. The bursts observed
between 1997-98 were consistent with a stable recurrence time of 5.74 +/- 0.13
hr. The persistent intensity of GS 1826-24 increased by 36% between 1997-2000,
by which time the burst interval had decreased to 4.10 +/- 0.08 hr. In 2002
July the recurrence time was shorter again, at 3.56 +/- 0.03 hr. The bursts
within each epoch had remarkably identical lightcurves over the full approx.
150 s burst duration; both the initial decay timescale from the peak, and the
burst fluence, increased slightly with the rise in persistent flux. The
decrease in the burst recurrence time was proportional to Mdot^(-1.05+/-0.02)
(where Mdot is assumed to be linearly proportional to the X-ray flux), so that
the ratio alpha between the integrated persistent and burst fluxes was
inversely correlated with Mdot. The average value of alpha was 41.7 +/- 1.6.
Both the alpha value, and the long burst durations indicate that the hydrogen
is burning during the burst via the rapid-proton (rp) process. The variation in
alpha with Mdot implies that hydrogen is burning stably between bursts,
requiring solar metallicity (Z ~ 0.02) in the accreted layer. We show that
solar metallicity ignition models naturally reproduce the observed burst
energies, but do not match the observed variations in recurrence time and burst
fluence. Low metallicity models (Z ~ 0.001) reproduce the observed trends in
recurrence time and fluence, but are ruled out by the variation in alpha. We
discuss possible explanations, including extra heating between bursts, or that
the fraction of the neutron star covered by the accreted fuel increases with
Mdot.Comment: 9 pages, 6 figures, accepted by ApJ. Minor revisions following the
referee's repor
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