88 research outputs found
Propagation of the First Flames in Type Ia Supernovae
We consider the competition of the different physical processes that can
affect the evolution of a flame bubble in a Type Ia supernovae -- burning,
turbulence and buoyancy. Even in the vigorously turbulent conditions of a
convecting white dwarf, thermonuclear burning that begins at a point near the
center (within 100 km) of the star is dominated by the spherical laminar
expansion of the flame, until the burning region reaches kilometers in size.
Consequently flames that ignite in the inner ~20 km promptly burn through the
center, and flame bubbles anywhere must grow quite large--indeed, resolvable by
large-scale simulations of the global system--for significant motion or
deformation occur. As a result, any hot-spot that successfully ignites into a
flame can burn a significant amount of white dwarf material. This potentially
increases the stochastic nature of the explosion compared to a scenario where a
simmering progenitor can have small early hot-spots float harmlessly away.
Further, the size where the laminar flame speed dominates other relevant
velocities sets a characteristic scale for fragmentation of larger flame
structures, as nothing--by definition--can easily break the burning region into
smaller volumes. This makes possible the development of semi-analytic
descriptions of the earliest phase of the propagation of burning in a Type Ia
supernovae, which we present here. Our analysis is supported by fully resolved
numerical simulations of flame bubbles.Comment: 33 pages, 14 figures, accepted for publication in Ap
Analysis of the gastrin-releasing peptide receptor gene in Italian patients with autism spectrum disorders
The gastrin-releasing peptide receptor (GRPR) was implicated for the first time in the pathogenesis of Autism spectrum disorders (ASD) by Ishikawa-Brush et al. [Ishikawa-Brush et al. (1997): Hum Mol Genet 6: 1241-1250]. Since this original observation, only one association study [Marui et al. (2004): Brain Dev 26: 5-7] has further investigated, though unsuccessfully, the involvement of the GRPR gene in ASD. With the aim of contributing further information to this topic we have sequenced the entire coding region and the intron/exon junctions of the GRPR gene in 149 Italian autistic patients. The results of this study led to the identification of four novel point mutations, two of which, that is, C6S and L181F, involve amino acid changes identified in two patients with ASD and Rett syndrome, respectively. Both the leucine at position 181 and the cysteine at position 6 are strongly conserved in vertebrates. C6S and L181F mutant proteins were expressed in COS-7 and BALB/3T3 cells, but they did not affect either GRP's binding affinity or its potency for stimulating phospholipase C-mediated production of inositol 1,4,5-trisphosphate. In summary, our results do not provide support for a major role of the GRPR gene in ASD in the population of patients we have studied. However, there is a potential role of C6S and L181F mutations on GRPR function, and possibly in the pathogenesis of the autistic disorders in the two patient
The LOFT perspective on neutron star thermonuclear bursts
This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of thermonuclear X-ray bursts on
accreting neutron stars. For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large
Observatory for X-ray Timin
On Validating an Astrophysical Simulation Code
We present a case study of validating an astrophysical simulation code. Our
study focuses on validating FLASH, a parallel, adaptive-mesh hydrodynamics code
for studying the compressible, reactive flows found in many astrophysical
environments. We describe the astrophysics problems of interest and the
challenges associated with simulating these problems. We describe methodology
and discuss solutions to difficulties encountered in verification and
validation. We describe verification tests regularly administered to the code,
present the results of new verification tests, and outline a method for testing
general equations of state. We present the results of two validation tests in
which we compared simulations to experimental data. The first is of a
laser-driven shock propagating through a multi-layer target, a configuration
subject to both Rayleigh-Taylor and Richtmyer-Meshkov instabilities. The second
test is a classic Rayleigh-Taylor instability, where a heavy fluid is supported
against the force of gravity by a light fluid. Our simulations of the
multi-layer target experiments showed good agreement with the experimental
results, but our simulations of the Rayleigh-Taylor instability did not agree
well with the experimental results. We discuss our findings and present results
of additional simulations undertaken to further investigate the Rayleigh-Taylor
instability.Comment: 76 pages, 26 figures (3 color), Accepted for publication in the ApJ
The Response of Model and Astrophysical Thermonuclear Flames to Curvature and Stretch
Critically understanding the `standard candle'-like behavior of Type Ia
supernovae requires understanding their explosion mechanism. One family of
models for Type Ia Supernovae begins with a deflagration in a Carbon-Oxygen
white dwarf which greatly accelerates through wrinkling and flame
instabilities. While the planar speed and behavior of astrophysically-relevant
flames is increasingly well understood, more complex behavior, such as the
flame's response to stretch and curvature, has not been extensively explored in
the astrophysical literature; this behavior can greatly enhance or suppress
instabilities and local flame-wrinkling, which in turn can increase or decrease
the bulk burning rate. In this paper, we explore the effects of curvature on
both nuclear flames and simpler model flames to understand the effect of
curvature on the flame structure and speed.Comment: 25 pages; accepted to ApJ; fixed author field
On Simulating Type Ia Supernovae
Type Ia supernovae are bright stellar explosions distinguished by
standardizable light curves that allow for their use as distance indicators for
cosmological studies. Despite their highly successful use in this capacity, the
progenitors of these events are incompletely understood. We describe simulating
type Ia supernovae in the paradigm of a thermonuclear runaway occurring in a
massive white dwarf star. We describe the multi-scale physical processes that
realistic models must incorporate and the numerical models for these that we
employ. In particular, we describe a flame-capturing scheme that addresses the
problem of turbulent thermonuclear combustion on unresolved scales. We present
the results of our study of the systematics of type Ia supernovae including
trends in brightness following from properties of the host galaxy that agree
with observations. We also present performance results from simulations on
leadership-class architectures.Comment: 13 pages, 3 figures, accepted to proceedings of the Conference on
Computational Physics, Oct. 30 - Nov. 3, 201
Binary systems and their nuclear explosions
Peer ReviewedPreprin
Urticaria and infections
Urticaria is a group of diseases that share a distinct skin reaction pattern. Triggering of urticaria by infections has been discussed for many years but the exact role and pathogenesis of mast cell activation by infectious processes is unclear. In spontaneous acute urticaria there is no doubt for a causal relationship to infections and all chronic urticaria must have started as acute. Whereas in physical or distinct urticaria subtypes the evidence for infections is sparse, remission of annoying spontaneous chronic urticaria has been reported after successful treatment of persistent infections. Current summarizing available studies that evaluated the course of the chronic urticaria after proven Helicobacter eradication demonstrate a statistically significant benefit compared to untreated patients or Helicobacter-negative controls without urticaria (p < 0.001). Since infections can be easily treated some diagnostic procedures should be included in the routine work-up, especially the search for Helicobacter pylori. This review will update the reader regarding the role of infections in different urticaria subtypes
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