32 research outputs found
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.
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.
Nuclear structure of the exotic mass region along the rp process path
Isomeric states in the nuclei along the rapid proton capture process path are
studied by the projected shell model. Emphasis is given to two waiting point
nuclei 68Se and 72Kr that are characterized by shape coexistence. Energy
surface calculations indicate that the ground state of these nuclei corresponds
to an oblate-deformed minimum, while the lowest state at the prolate-deformed
minimum can be considered as a shape isomer. The impact of these isomer states
on isotopic abundance in x-ray bursts is studied in a multi-mass-zone x-ray
burst model by assuming an upper-lower limit approach.Comment: Proceedings of Nuclei in Cosmos VIII, to appear in Nucl. Phys.
On the origin of the lightest Molybdenum isotopes
We discuss implications of recent precision measurements for the Rh93 proton
separation energy for the production of the lightest molybdenum isotopes in
proton-rich type II supernova ejecta. It has recently been shown that a novel
neutrino-induced process makes these ejecta a promising site for the production
of the light molybdenum isotopes and other "p-nuclei" with atomic mass near
100. The origin of these isotopes has long been uncertain. A distinguishing
feature of nucleosynthesis in neutrino-irradiated outflows is that the relative
production of Mo92 and Mo94 is set by a competition governed by the proton
separation energy of Rh93. We use detailed nuclear network calculations and the
recent experimental results for this proton separation energy to place
constraints on the outflow characteristics that produce the lightest molybdenum
isotopes in their solar proportions. It is found that for the conditions
calculated in recent two-dimensional supernova simulations, and also for a
large range of outflow characteristics around these conditions, the solar ratio
of Mo92 to Mo94 cannot be achieved. This suggests that either proton-rich winds
from type II supernova do not exclusively produce both isotopes, or that these
winds are qualitatively different than calculated in today's supernova models.Comment: 12 pages, 7 figures (3 color
The accretion and spreading of matter on white dwarfs
For a slowly rotating non-magnetized white dwarf the accretion disk extends
all the way to the star. Here the matter impacts and spreads towards the poles
as new matter continuously piles up behind it. We have solved the 3d
compressible Navier-Stokes equations on an axisymmetric grid to determine the
structure of this boundary layer for different viscosities corresponding to
different accretion rates. The high viscosity cases show a spreading BL which
sets off a gravity wave in the surface matter. The accretion flow moves
supersonically over the cusp making it susceptible to the rapid development of
gravity wave and/or Kelvin-Helmholtz instabilities. This BL is optically thick
and extends more than 30 degrees to either side of the disk plane after 3/4 of
a Keplerian rotation period (t=19s). The low viscosity cases also show a
spreading BL, but here the accretion flow does not set off gravity waves and it
is optically thin.Comment: 6 pages, 5 figures, requires autart.cl
The nuclear reaction waiting points, Mg22, Si26, S30, and Ar34, and bolometrically double peaked type I X-ray bursts
Type I X-ray bursts with a double peak in the bolometric luminosity have been
observed from several sources. The separation between the two peaks are on the
order of a few seconds. We propose a nuclear waiting point impedance in the
thermonuclear reaction flow to explain these observations. Nuclear structure
information suggests the potential waiting points: Mg22, Si26, S30 and Ar34,
which arise in conditions, where a further reaction flow has to await a
beta-decay, because the (alpha,p)-reaction is too weak to overcome the target
Coulomb-barrier and the (p,gamma)-reaction is quenched by photo-disintegration
at the burst temperature. The conclusion is that the effects of the
experimentally unknown S30(alpha,p)Cl33 and Ar34(alpha,p)K37 might be directly
visible in the observation of X-ray burst light curves.Comment: 5 pages, 3 figures, submitted to Astrophys. J. Let
Simulations of the Boundary Layer Between a White Dwarf and its Accretion Disk
Using a 2.5D time-dependent axisymmetric numerical code we recently
developed, we solve the full compressible Navier-Stokes equations (including an
alpha-viscosity prescription) to determine the structure of the boundary layer
between the white dwarf and the accretion disk in non-magnetic cataclysmic
varia ble systems. In this preliminary work, our numerical approach does not
include radiation. In the energy equation, we either take the dissipation
function (Phi) into account or we assumed that the energy is instantly radiated
away (Phi). For a slowly rotating non magnetized accreting white dwarf, the
accretion disk e xtends all the way to the stellar surface. There, the matter
impacts and spread s towards the poles as new matter continuously piles up
behind it. We carried out numerical simulations for different values of the
alpha viscosity parameter (alpha), corresponding to different mass accretion
rates. In the high viscosity cases (alpha=0.1), the spreading boundary layer
sets off a gravity wave in the s urface matter. The accretion flow moves
supersonically over the cusp making it s usceptible to the rapid development of
gravity wave and/or Kelvin-Helmholtz shea ring instabilities. This BL is
optically thick and extends more than 30 degrees to either side of the disk
plane after only 3/4 of a Keplerian rotation period (19s). In the low viscosity
cases (alpha=0.001), the spreading boundary layer does not set off gravity
waves and it is optically thin.Comment: final version, ApJ, in pres
Binary systems and their nuclear explosions
Peer ReviewedPreprin