835 research outputs found
Low heat conduction in white dwarf boundary layers?
X-ray spectra of dwarf novae in quiescence observed by Chandra and XMM-Newton
provide new information on the boundary layers of their accreting white dwarfs.
Comparison of observations and models allows us to extract estimates for the
thermal conductivity in the accretion layer and reach conclusions on the
relevant physical processes. We calculate the structure of the dense thermal
boundary layer that forms under gravity and cooling at the white dwarf surface
on accretion of gas from a hot tenuous ADAF-type coronal inflow. The
distribution of density and temperature obtained allows us to calculate the
strength and spectrum of the emitted X-ray radiation. They depend strongly on
the values of thermal conductivity and mass accretion rate. We apply our model
to the dwarf nova system VW Hyi and compare the spectra predicted for different
values of the thermal conductivity with the observed spectrum. We find a
significant deviation for all values of thermal conductivity that are a sizable
fraction of the Spitzer conductivity. A good fit arises however for a
conductivity of about 1% of the Spitzer value. This also seems to hold for
other dwarf nova systems in quiescence. We compare this result with thermal
conduction in other astrophysical situations. The highly reduced thermal
conductivity in the boundary layer requires magnetic fields perpendicular to
the temperature gradient. Locating their origin in the accretion of magnetic
fields from the hot ADAF-type coronal flow we find that dynamical effects of
these fields will lead to a spatially intermittent, localized accretion
geometry at the white dwarf surface.Comment: 8 pages, 5 figs, to appear in Astronomy & Astrophysic
Weak Disorder in Fibonacci Sequences
We study how weak disorder affects the growth of the Fibonacci series. We
introduce a family of stochastic sequences that grow by the normal Fibonacci
recursion with probability 1-epsilon, but follow a different recursion rule
with a small probability epsilon. We focus on the weak disorder limit and
obtain the Lyapunov exponent, that characterizes the typical growth of the
sequence elements, using perturbation theory. The limiting distribution for the
ratio of consecutive sequence elements is obtained as well. A number of
variations to the basic Fibonacci recursion including shift, doubling, and
copying are considered.Comment: 4 pages, 2 figure
Geoantineutrino Spectrum, 3He/4He-ratio Distribution in the Earth's Interior and Slow Nuclear Burning on the Boundary of the Liquid and Solid Phases of the Earth's Core
The description problem of geoantineutrino spectrum and reactor antineutrino
experimental spectrum in KamLAND, which takes place for antineutrino energy
\~2.8 MeV, and also the experimental results of the interaction of uranium
dioxide and carbide with iron-nickel and silicaalumina melts at high pressure
(5-10 GP?) and temperature (1600-2200C) have motivated us to consider the
possible consequences of the assumption made by V.Anisichkin and coauthors that
there is an actinid shell on boundary of liquid and solid phases of the Earth's
core. We have shown that the activation of a natural nuclear reactor operating
as the solitary waves of nuclear burning in 238U- and/or 232Th-medium (in
particular, the neutron- fission progressive wave of Feoktistov and/or
Teller-Ishikawa-Wood) can be such a physical consequence. The simplified model
of the kinetics of accumulation and burnup in U-Pu fuel cycle of Feoktistov is
developed. The results of the numerical simulation of neutron-fission wave in
two-phase UO2/Fe medium on a surface of the Earth's solid core are presented.
The georeactor model of 3He origin and the 3He/4He-ratio distribution in the
Earth's interior is offered. It is shown that the 3He/4He ratio distribution
can be the natural quantitative criterion of georeactor thermal power. On the
basis of O'Nions-Evensen-Hamilton geochemical model of mantle differentiation
and the crust growth supplied by actinid shell on the boundary of liquid and
solid phases of the Earth's core as a nuclear energy source (georeactor with
power of 30 TW), the tentative estimation of geoantineutrino intensity and
geoantineutrino spectrum on the Earth surface are given.Comment: 28 pages, 12 figures. Added text, formulas, figures and references.
Corrected equations. Changed content of some section
Heat flow of the Earth and resonant capture of solar 57-Fe axions
In a very conservative approach, supposing that total heat flow of the Earth
is exclusively due to resonant capture inside the Earth of axions, emitted by
57-Fe nuclei on Sun, we obtain limit on mass of hadronic axion: m_a<1.8 keV.
Taking into account release of heat from decays of 40-K, 232-Th, 238-U inside
the Earth, this estimation could be improved to the value: m_a<1.6 keV. Both
the values are less restrictive than limits set in devoted experiments to
search for 57-Fe axions (m_a<216-745 eV), but are much better than limits
obtained in experiments with 83-Kr (m_a<5.5 keV) and 7-Li (m_a<13.9-32 keV).Comment: 8 page
Optical Monitoring of 3C 390.3 from 1995 to 2004 and Possible Periodicities in the Historical Light Curve
We report V, R, and I band CCD photometry of the radio galaxy 3C 390.3
obtained with the 1.56-m telescope of the Shanghai Astronomical Observatory
from March 1995 to August 2004. Combining these data with data from the
literature, we have constructed a historical light curve from 1894 to 2004 and
searched for periodicities using the CLEANest program. We find possible periods
of 8.30+-1.17, 5.37+-0.49, 3.51+-0.21, and 2.13+-0.08 years.Comment: Accepted by AJ, 34 pages, 11 figure
Survival outcomes of patients with primary plasma cell leukemia (pPCL) treated with novel agents
Where Are All The Fallback Disks? Constraints on Propeller Systems
Fallback disks are expected to form around new-born neutron stars following a
supernova explosion. In almost all cases, the disk will pass through a
propeller stage. If the neutron star is spinning rapidly (initial period ms) and has an ordinary magnetic moment ( G cm), the
rotational power transferred to the disk by the magnetic field of the neutron
star will exceed the Eddington limit by many orders of magnitude, and the disk
will be rapidly disrupted. Fallback disks can thus survive only around
slow-born neutron stars and around black holes, assuming the latter do not
torque their surrounding disks as strongly as do neutron stars. This might
explain the apparent rarity of fallback disks around young compact objects.Comment: Submitted to Astrophysical Journal Letter
Using small molecules to facilitate exchange of bicarbonate and chloride anions across liposomal membranes
Bicarbonate is involved in a wide range of biological processes, which include respiration, regulation of intracellular pH and fertilization. In this study we use a combination of NMR spectroscopy and ion-selective electrode techniques to show that the natural product prodigiosin, a tripyrrolic molecule produced by microorganisms such as Streptomyces and Serratia, facilitates chloride/bicarbonate exchange (antiport) across liposomal membranes. Higher concentrations of simple synthetic molecules based on a 4,6-dihydroxyisophthalamide core are also shown to facilitate this antiport process. Although it is well known that proteins regulate Cl-/HCO3- exchange in cells, these results suggest that small molecules may also be able to regulate the concentration of these anions in biological systems
Enhancement of the electron electric dipole moment in gadolinium garnets
Effects caused by the electron electric dipole moment (EDM) in gadolinium
garnets are considered. Experimental studies of these effects could improve
current upper limit on the electron EDM by several orders of magnitude. We
suggest a consistent theoretical model and perform calculations of observable
effects in gadolinium gallium garnet and gadolinium iron garnet. Our
calculation accounts for both direct and exchange diagrams.Comment: 9 page
- …