1,373 research outputs found
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
A Two-Zone Model for Type I X-ray Bursts on Accreting Neutron Stars
We construct a two-zone model to describe H and He burning on the surface of
an accreting neutron star and use it to study the triggering of type I X-ray
bursts. Although highly simplified, the model reproduces all of the bursting
regimes seen in the more complete global linear stability analysis of Narayan &
Heyl (2003), including the regime of delayed mixed bursts. The results are also
consistent with observations of type I X-ray bursts. At accretion rates Mdot <
0.1 Mdot_Edd, thermonuclear He burning via the well-known thin-shell thermal
instability triggers bursts. As Mdot increases, however, the trigger mechanism
evolves from the fast thermal instability to a slowly growing overstability
involving both H and He burning. The competition between nuclear heating via
the beta-limited CNO cycle and the triple-alpha process on the one hand, and
radiative cooling via photon diffusion and emission on the other hand, drives
oscillations with a period approximately equal to the H-burning timescale. If
these oscillations grow, the gradually rising temperature at the base of the
helium layer eventually provokes a thin-shell thermal instability and hence a
delayed mixed burst. For Mdot > 0.25 Mdot_Edd, there is no instability or
overstability, and there are no bursts. Nearly all other theoretical models
predict that bursts should occur for all Mdot < Mdot_Edd, in conflict with both
our results and observations. We suggest that this discrepancy arises from the
assumed strength of the hot CNO cycle breakout reaction 15O(alpha,gamma)19Ne in
these other models. That observations agree much better with the results of
Narayan & Heyl and our two-zone model, both of which neglect breakout
reactions, may imply that the true 15O(alpha,gamma)19Ne cross section is much
smaller than assumed in previous investigations.Comment: 13 pages, 8 figures, accepted by Ap
Convergence to a self-similar solution in general relativistic gravitational collapse
We study the spherical collapse of a perfect fluid with an equation of state
by full general relativistic numerical simulations. For 0, it has been known that there exists a general relativistic counterpart
of the Larson-Penston self-similar Newtonian solution. The numerical
simulations strongly suggest that, in the neighborhood of the center, generic
collapse converges to this solution in an approach to a singularity and that
self-similar solutions other than this solution, including a ``critical
solution'' in the black hole critical behavior, are relevant only when the
parameters which parametrize initial data are fine-tuned. This result is
supported by a mode analysis on the pertinent self-similar solutions. Since a
naked singularity forms in the general relativistic Larson-Penston solution for
0, this will be the most serious known counterexample against
cosmic censorship. It also provides strong evidence for the self-similarity
hypothesis in general relativistic gravitational collapse. The direct
consequence is that critical phenomena will be observed in the collapse of
isothermal gas in Newton gravity, and the critical exponent will be
given by , though the order parameter cannot be the black
hole mass.Comment: 22 pages, 15 figures, accepted for publication in Physical Review D,
reference added, typos correcte
Models for Type I X-Ray Bursts with Improved Nuclear Physics
Multi-zone models of Type I X-ray bursts are presented that use an adaptive
nuclear reaction network of unprecedented size, up to 1300 isotopes. Sequences
of up to 15 bursts are followed for two choices of accretion rate and
metallicity. At 0.1 Eddington (and 0.02 Eddington for low metallicity),
combined hydrogen-helium flashes occur. The rise times, shapes, and tails of
these light curves are sensitive to the efficiency of nuclear burning at
various waiting points along the rp-process path and these sensitivities are
explored. The bursts show "compositional inertia", in that their properties
depend on the fact that accretion occurs onto the ashes of previous bursts
which contain left-over hydrogen, helium and CNO nuclei. This acts to reduce
the sensitivity of burst properties to metallicity. For the accretion rates
studied, only the first anomalous burst in one model produces nuclei as heavy
as A=100, other bursts make chiefly nuclei with A~64. The amount of carbon
remaining after hydrogen-helium bursts is typically <1% by mass, and decreases
further as the ashes are periodically heated by subsequent bursts. At the lower
accretion rate of 0.02 Eddington and solar metallicity, the bursts ignite in a
hydrogen-free helium layer. At the base of this layer, up to 90% of the helium
has already burned to carbon prior to the unstable ignition. These
helium-ignited bursts have briefer, brighter light curves with shorter tails,
very rapid rise times (<0.1 s), and ashes lighter than the iron group.Comment: Submitted to the Astrophysical Journal (42 pages; 27 figures
Magnetic and superconducting properties of Cd2Re2O7: Cd NMR and Re NQR
We report Cd NMR and Re NQR studies on Cd2Re2O7, the first superconductor
among pyrochlore oxides Tc=1 K. Re NQR spectrum at zero magnetic field below
100 K rules out any magnetic or charge order. The spin-lattice relaxation rate
below Tc exhibits a pronounced coherence peak and behaves within the
weak-coupling BCS theory with nearly isotropic energy gap. Cd NMR results point
to moderate ferromagnetic enhancement at high temperatures followed by rapid
decrease of the density of states below the structural transition temperature
of 200 K.Comment: 4 pages, 4 figure
The cooling rate of neutron stars after thermonuclear shell flashes
Thermonuclear shell flashes on neutron stars are detected as bright X-ray
bursts. Traditionally, their decay is modeled with an exponential function.
However, this is not what theory predicts. The expected functional form for
luminosities below the Eddington limit, at times when there is no significant
nuclear burning, is a power law. We tested the exponential and power-law
functional forms against the best data available: bursts measured with the
high-throughput Proportional Counter Array (PCA) on board the Rossi X-ray
Timing Explorer. We selected a sample of 35 'clean' and ordinary (i.e., shorter
than a few minutes) bursts from 14 different neutron stars that 1) show a large
dynamic range in luminosity, 2) are the least affected by disturbances by the
accretion disk and 3) lack prolonged nuclear burning through the rp-process. We
find indeed that for every burst a power law is a better description than an
exponential function. We also find that the decay index is steep, 1.8 on
average, and different for every burst. This may be explained by contributions
from degenerate electrons and photons to the specific heat capacity of the
ignited layer and by deviations from the Stefan-Boltzmann law due to changes in
the opacity with density and temperature. Detailed verification of this
explanation yields inconclusive results. While the values for the decay index
are consistent, changes of it with the burst time scale, as a proxy of ignition
depth, and with time are not supported by model calculations.Comment: 10 pages, 7 figures, recommended for publication in A&
Formation of Primordial Stars in a LCDM Universe
We study the formation of the first generation of stars in the standard cold
dark matter model, using a very high-resolution hydordynamic simulations. Our
simulation achieves a dynamic range of 10^{10} in length scale. With accurate
treatment of atomic and molecular physics, it allows us to study the
chemo-thermal evolution of primordial gas clouds to densities up to n =
10^{16}/cc without assuming any a priori equation of state; a six orders of
magnitudes improvement over previous three-dimensional calculations. All the
relevant atomic and molecular cooling and heating processes, including cooling
by collision-induced continuum emission, are implemented. For calculating
optically thick H2 cooling at high densities, we use the Sobolev method. To
examine possible gas fragmentation owing to thermal instability, we compute
explicitly the growth rate of isobaric perturbations. We show that the cloud
core does not fragment in either the low-density or high-density regimes. We
also show that the core remains stable against gravitational deformation and
fragmentation. We obtain an accurate gas mass accretion rate within a 10 Msun
innermost region around the protostar. The protostar is accreting the
surrounding hot gas at a rate of 0.001-0.01 Msun/yr. From these findings we
conclude that primordial stars formed in early minihalos are massive. We carry
out proto-stellar evolution calculations using the obtained accretion rate. The
resulting mass of the first star is M_ZAMS = 60-100 Msun, with the exact mass
dependent on the actual accretion rate.Comment: 27 pages, 13 embedded figures. Revised versio
Anomalous electric conductions in KSbO3-type metallic rhenium oxides
Single crystals of KSbO3-type rhenium oxides, La4Re6Orho(T)=\rho_{0}+AT^{n}(n \approx 1.6)$ in
a wide temperature range between 5 K and 300 K, which is extraordinary for
three-dimensional metals without strong electron correlations. The resistivity
of Bi3Re3O11 shows an anomaly around at 50 K, where the magnetic susceptibility
also detects a deviation from ordinary Pauli paramagnetism.Comment: 13 pages, 7 figures. J. Phys. Soc. Japan, in pres
The biocompatibility of titanium in a buffer solution: compared effects of a thin film of TiO2 deposited by MOCVD and of collagen deposited from a gel
This study aims at evaluating the biocompatibility of titanium surfaces modified according two different ways: (i) deposition of a bio-inert, thin film of rutile TiO2 by chemical vapour deposition (MOCVD), and (ii) biochemical treatment with collagen gel, in order to obtain a bio-interactive coating. Behind the comparison is the idea that either the bio-inert or the bio-active coating has specific advantages when applied to implant treatment, such as the low price of the collagen treatment for instance. The stability in buffer solution was evaluated by open circuit potential (OCP) for medium time and cyclic voltametry. The OCP stabilized after 5104 min for all the specimens except the collagen treated sample which presented a stable OCP from the first minutes. MOCVD treated samples stabilized to more electropositive values. Numeric results were statistically analysed to obtain the regression equations for long time predictable evolution. The corrosion parameters determined from cyclic curves revealed that the MOCVD treatment is an efficient way to improve corrosion resistance. Human dermal fibroblasts were selected for cell culture tests, taking into account that these cells are present in all bio-interfaces, being the main cellular type of connective tissue. The cells grew on either type of surface without phenotype modification. From the reduction of yellow, water-soluble 3-(4,5-dimethyldiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT cytotoxicity test), MOCVD treated samples offer better viability than mechanically polished Ti and collagen treated samples as well. Cell spreading, as evaluated from microscope images processed by the program Sigma Scan, showed also enhancement upon surface modification. Depending on the experimental conditions, MOCVD deposited TiO2 exhibits different nanostructures that may influence biological behaviour. The results demonstrate the capacity of integration in simulated physiologic liquids for an implant pretreated by either method
- …