659 research outputs found
On the very long term evolutionary behavior of hydrogen-accreting Low-Mass CO white dwarfs
Hydrogen-rich matter has been added to a CO white dwarf of initial mass 0.516
\msun at the rates and \msun \yrm1, and results are
compared with those for a white dwarf of the same initial mass which accretes
pure helium at the same rates. For the chosen accretion rates, hydrogen burns
in a series of recurrent mild flashes and the ashes of hydrogen burning build
up a helium layer at the base of which a He flash eventually occurs. In
previous studies involving accretion at higher rates and including initially
more massive WDs, the diffusion of energy inward from the H shell-flashing
region contributes to the increase in the temperature at the base of the helium
layer, and the mass of the helium layer when the He flash begins is
significantly smaller than in a comparison model accreting pure helium; the He
shell flash is not strong enough to develop into a supernova explosion. In
contrast, for the conditions adopted here, the temperature at the base of the
He layer becomes gradually independent of the deposition of energy by H shell
flashes, and the mass of the He layer when the He flash occurs is a function
only of the accretion rate, independent of the hydrogen content of the accreted
matter. When the He flash takes place, due to the high degeneracy at the base
of the He layer, temperatures in the flashing zone will rise without a
corresponding increase in pressure, nuclear burning will continue until nuclear
statistical equilibrium is achieved; the model will become a supernova, but not
of the classical type Ia variety.Comment: 14 pages and 3 Postscript figures, Accepted for publication on ApJ
Letter
A Sea Level Equation for seismic perturbations
Large earthquakes are a potentially important source of relative sea level variations, since they can drive global deformation and simultaneously perturb the gravity field of the Earth. For the first time, we formalize a gravitationally self-consistent, integral sea level equation suitable for earthquakes, in which we account both for direct effects by the seismic dislocation and for the feedback from water loading associated with sea level changes. Our approach builds upon the well-established theory first proposed in the realm of glacio-isostatic adjustment modelling. The seismic sea level equation is numerically implemented to model sea level signals following the 2004 Sumatra–Andaman earthquake, showing that surface loading from ocean water redistribution (so far ignored in post-seismic deformation modelling) may account for a significant fraction of the total computed post-seismic sea level variatio
Long term continuous radon monitoring in a seismically active area
We present the results of a long term, continuous radon monitoring experiment started in April 2010 in a seismically active area, affected during the 2010-2013 data acquisition time window by an intense micro seismic activity and by several small seismic events. We employed both correlation and cross-correlation analyses in order to investigate possible relationship existing between the collected radon data, seismic events and meteorological parameters. Our results do not support the feasibility of a robust one-to-one association between the small magnitude earthquakes characterizing the local seismic activity and single radon measurement anomalies, but evidence significant correlation patterns between the spatio-temporal variations of seismic moment release and soil radon emanations, the latter being anyway dominantly modulated by meteorological parameters variations
Outer core density heterogeneity and the discrepancy between PKP and PcP travel time observations
We derive 3-D maps of the Earth’s mantle, CMB and outer
core by means of least squares tomographic inversions. The data set includes
compressional wave travel time measurements associated with the phases P,
PcP, PKPbc, PKPdf, all based on the bulletins of the International Seismological
Centre (1964-1995), after source relocation by Antolik et al. [2001].
Maps of the CMB derived independently from only core-reflected (PcP) or
only core-refracted (PKP) phases are not well correlated. We study the radial
coherence of whole-Earth tomographic images, to investigate potential
trade-offs between CMB undulations and velocity anomalies in the mantle
and/or outer core. We find that imaged lateral heterogeneities in the outer
core are correlated with the topography of the CMB. This, together with the
studies of Wahr and De Vries [1989] and Piersanti et al. [2001], suggests that
the core anomalies might not be entirely fictitious
Prion expression is activated by Adenovirus 5 infection and affects the adenoviral cycle in human cells
The prion protein is a cell surface glycoprotein whose physiological role remains elusive, while its implication
in transmissible spongiform encephalopathies (TSEs) has been demonstrated. Multiple interactions between
the prion protein and viruses have been described: viruses can act as co-factors in TSEs and life cycles of
different viruses have been found to be controlled by prion modulation.
We present data showing that human Adenovirus 5 induces prion expression. Inactivated Adenovirus did
not alter prion transcription, while variants encoding for early products did, suggesting that the prion is
stimulated by an early adenoviral function. Down-regulation of the prion through RNA interference showed
that the prion controls adenovirus replication and expression.
These data suggest that the prion protein could play a role in the defense strategy mounted by the host
during viral infection, in a cell autonomous manner. These results have implications for the study of the prion
protein and of associated TSEs
Outer core density heterogeneity and the discrepancy between PKP and PcP travel time observations
We derive 3-D maps of the Earth’s mantle, CMB and outer
core by means of least squares tomographic inversions. The data set includes
compressional wave travel time measurements associated with the phases P,
PcP, PKPbc, PKPdf, all based on the bulletins of the International Seismological
Centre (1964-1995), after source relocation by Antolik et al. [2001].
Maps of the CMB derived independently from only core-reflected (PcP) or
only core-refracted (PKP) phases are not well correlated. We study the radial
coherence of whole-Earth tomographic images, to investigate potential
trade-offs between CMB undulations and velocity anomalies in the mantle
and/or outer core. We find that imaged lateral heterogeneities in the outer
core are correlated with the topography of the CMB. This, together with the
studies of Wahr and De Vries [1989] and Piersanti et al. [2001], suggests that
the core anomalies might not be entirely fictitious
Global seismic tomography and modern parallel computers
A fast technological progress is providing seismic tomographers with computers
of rapidly increasing speed and RAM, that are not always properly taken
advantage of. Large computers with both shared-memory and distributedmemory
architectures have made it possible to approach the tomographic
inverse problem more accurately. For example, resolution can be quantified
from the resolution matrix rather than checkerboard tests; the covariance
matrix can be calculated to evaluate the propagation of errors from data to
model parameters; the L-curve method can be applied to determine a range
of acceptable regularization schemes. We show how these exercises can be
implemented efficiently on different hardware architectures
Long term continuous radon monitoring in a seismically active area
We present the results of a long term, continuous radon monitoring experiment started in April 2010 in a seismically active area, affected during the 2010-2013 data acquisition time window by an intense micro seismic activity and by several small seismic events. We employed both correlation and cross-correlation analyses in order to investigate possible relationship existing between the collected radon data, seismic events and meteorological parameters. Our results do not support the feasibility of a robust one-to-one association between the small magnitude earthquakes characterizing the local seismic activity and single radon measurement anomalies, but evidence significant correlation patterns between the spatio-temporal variations of seismic moment release and soil radon emanations, the latter being anyway dominantly modulated by meteorological parameters variations
Application of the Post-Widder Laplace inversion algorithm to postseismic rebound models
The postseismic response of a viscoelastic Earth can be computed analytically with a normal-mode approach, based on the application of propagator methods. This framework suffers from many limitations, mostly connected with the
solution of the secular equation, whose degree scales with the number of viscoelastic layers so that only low-resolution models can be practically solved. Recently, a
viable alternative to the normal-mode approach has been proposed, based on the Post-Widder inversion formula. This method allows to overcome some of the intrinsic limitations of the normal-mode approach, so that Earth models with arbitrary radial resolution can be employed and general linear non-Maxwell rheologies can be implemented. In this work, we test the robustness of the method against a standard normal-mode approach in order to optimize computation performance while ensuring the solution stability. As an application, we address the issue of finding the minimum number of layers with distinct elastic properties needed to accurately describe the postseismic relaxation of a realistic Earth model
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