1,236 research outputs found
The reactions and ashes of thermonuclear explosions on neutron stars
This paper reports on the detailed rp-process reaction flow on an accreting
neutron star and the resulting ashes of a type I X-ray burst. It is obtained by
coupling a 298 isotope reaction network to a self-consistent one-dimensional
model calculation with a constant accretion rate of dM/dt=1.0e17g/s (0.09
Eddington).Comment: 4 pages, 2 figures, submitted to the INPC2004 proceeding
Hydrodynamic Models of Type I X-Ray Bursts: Metallicity Effects
Type I X-ray bursts are thermonuclear stellar explosions driven by
charged-particle reactions. In the regime for combined H/He-ignition, the main
nuclear flow is dominated by the rp-process (rapid proton-captures and beta+
decays), the 3 alpha-reaction, and the alpha-p-process (a suite of (alpha,p)
and (p,gamma) reactions). The main flow is expected to proceed away from the
valley of stability, eventually reaching the proton drip-line beyond A = 38.
Detailed analysis of the relevant reactions along the main path has only been
scarcely addressed, mainly in the context of parameterized one-zone models. In
this paper, we present a detailed study of the nucleosynthesis and nuclear
processes powering type I X-ray bursts. The reported 11 bursts have been
computed by means of a spherically symmetric (1D), Lagrangian, hydrodynamic
code, linked to a nuclear reaction network that contains 325 isotopes (from 1H
to 107Te), and 1392 nuclear processes. These evolutionary sequences, followed
from the onset of accretion up to the explosion and expansion stages, have been
performed for 2 different metallicities to explore the dependence between the
extension of the main nuclear flow and the initial metal content. We carefully
analyze the dominant reactions and the products of nucleosynthesis, together
with the the physical parameters that determine the light curve (including
recurrence times, ratios between persistent and burst luminosities, or the
extent of the envelope expansion). Results are in qualitative agreement with
the observed properties of some well-studied bursting sources. Leakage from the
predicted SbSnTe-cycle cannot be discarded in some of our models. Production of
12C (and implications for the mechanism that powers superbursts), light
p-nuclei, and the amount of H left over after the bursting episodes will also
be discussed.Comment: 78 pages (pdf), including 34 figures. Accepted for publication in The
Astrophysical Journal Suppl. Serie
Explosive hydrogen burning during type I X-ray bursts
Explosive hydrogen burning in type I X-ray bursts (XRBs) comprise charged
particle reactions creating isotopes with masses up to A~100. Since charged
particle reactions in a stellar environment are very temperature sensitive, we
use a realistic time-dependent general relativistic and self-consistent model
of type I x-ray bursts to provide accurate values of the burst temperatures and
densities. This allows a detailed and accurate time-dependent identification of
the reaction flow from the surface layers through the convective region and the
ignition region to the neutron star ocean. Using this, we determine the
relative importance of specific nuclear reactions in the X-ray burst.Comment: 53 pages, 24 figures, submitted to Astrophys.
Halflife of 56Ni in cosmic rays
A measurement of the 56Ni cosmic ray abundance has been discussed as a
possible tool to determine the acceleration time scale of relativistic
particles in cosmic rays. This conjecture will depend on the halflife of
totally ionized 56Ni which can only decay by higher-order forbidden
transitions. We have calculated this halflife within large-scale shell model
calculations and find t_{1/2} \approx 4 \times 10^4 years, only slightly larger
than the currently available experimental lower limit, but too short for 56Ni
to serve as a cosmic ray chronometer.Comment: 3 pages, 1 figur
Extracting the rp-process from X-ray burst light curves
The light curves of type I X-ray bursts (XRBs) result from energy released
from the atmosphere of a neutron star when accreted hydrogen and helium ignite
and burn explosively via the rp-process. Since charged particle reaction rates
are both density and very temperature dependent, a simulation model must
provide accurate values of these variables to predict the reaction flow. This
paper uses a self-consistent one-dimensional model calculation with a constant
accretion rate of dM/dt=5e16g/s (0.045 Eddington) and reports on the detailed
rp-process reaction flow of a given burst.Comment: 4 pages, submitted to Nucl. Phys. A as part of the Nuclei in Cosmos 8
proceeding
The importance of 15O(a,g)19Ne to X-ray bursts and superbursts
One of the two breakout reactions from the hot CNOcycle is 15O(a,g)19Ne,
which at low temperatures depends strongly on the resonance strength of the
4.033 MeV state in 19Ne. An experimental upper limit has been placed on its
strength, but the lower limit on the resonance strength and thereby the
astrophysical reaction rate is unconstrained experimentally. However, this
breakout reaction is crucial to the thermonuclear runaway which causes type I
X-ray bursts on accreting neutron stars. In this paper we exploit astronomical
observations in an attempt to constrain the relevant nuclear physics and deduce
a lower limit on the reaction rate. Our sensitivity study implies that if the
rate were sufficiently small, accreting material would burn stably without
bursts. The existence of type I X-ray bursts and superbursts consequently
suggests a lower limit on the 15O(a,g)19Ne reaction rate at low temperatures.Comment: 10 pages, 4 figures, uses apj.sty, accepted for publ. in Astrophys.
B\'ezier curves that are close to elastica
We study the problem of identifying those cubic B\'ezier curves that are
close in the L2 norm to planar elastic curves. The problem arises in design
situations where the manufacturing process produces elastic curves; these are
difficult to work with in a digital environment. We seek a sub-class of special
B\'ezier curves as a proxy. We identify an easily computable quantity, which we
call the lambda-residual, that accurately predicts a small L2 distance. We then
identify geometric criteria on the control polygon that guarantee that a
B\'ezier curve has lambda-residual below 0.4, which effectively implies that
the curve is within 1 percent of its arc-length to an elastic curve in the L2
norm. Finally we give two projection algorithms that take an input B\'ezier
curve and adjust its length and shape, whilst keeping the end-points and
end-tangent angles fixed, until it is close to an elastic curve.Comment: 13 pages, 15 figure
- …