138 research outputs found
In-shock Cooling in Numerical Simulations
We model a one-dimensional shock-tube using smoothed particle hydrodynamics
and investigate the consequences of having finite shock-width in numerical
simulations. We investigate the cooling of gas during passage through the shock
for different cooling regimes. For a shock temperature of 10^5K, the maximum
temperature of the gas is much reduced and the cooling time was reduced by a
factor of 2. At lower temperatures, we are especially interested in the
production of molecular Hydrogen and so we follow the ionization level and H_2
abundance across the shock. This regime is particularly relevent to simulations
of primordial galaxy formation for halos in which the virial temperature of the
galaxy is sufficiently high to partially re-ionize the gas. The effect of
in-shock cooling is substantial: the maximum temperature the gas reaches
compared to the theoretical temperature was found to vary between 0.15 and 0.81
for the simulations performed. The downstream ionization level is reduced from
the theoretical level by a factor of between 2.4 and 12.5, and the resulting
H_2 abundance was found to be reduced to a fraction of 0.45 to 0.74 of its
theoretical value. At temperatures above 10^5K, radiative shocks are unstable
and will oscillate. We reproduce these oscillations and find good agreement
with the previous work of Chevalier and Imamura (1982), and Imamura, Wolff and
Durisen (1984). The effect of in-shock cooling in such shocks is difficult to
quantify, but is undoubtedly present.Comment: 8 pages, LaTeX, 7 figure
A Novel Longitudinal Mode in the Coupled Quantum Chain Compound KCuF3
Inelastic neutron scattering measurements are reported that show a new
longitudinal mode in the antiferromagnetically ordered phase of the spin-1/2
quasi-one-dimensional antiferromagnet KCuF3. This mode signals the cross-over
from one-dimensional to three-dimensional behavior and indicates a reduction in
the ordered spin moment of a spin-1/2 antiferromagnet. The measurements are
compared with recent quantum field theory results and are found to be in
excellent agreement. A feature of the data not predicted by theory is a damping
of the mode by decay processes to the transverse spin-wave branches.Comment: 9 pages of text plus 4 postscript figures (1 color
In The Neighbourhood of Tame Monsters: A study of galaxies near low-redshift quasars
The impact of quasars on their galaxy neighbours is an important factor in
the understanding of galaxy evolution models. The aim of this work is to
characterize the intermediate-scale environments of quasars at low redshift (z
0.2) with the most statistically complete sample to date using the seventh
data release of the Sloan Digital Sky Survey. We have used 305 quasar-galaxy
associations with spectroscopically measured redshifts within the projected
distance range of 350 kpc, to calculate how surface densities of galaxies,
colors, degree of ionization, dust extinction and star-formation rates change
as a function of the distance to our quasar sample. We also identify the
companion Active Galactic Nuclei from our main galaxy sample and calculate
surface density for different galaxy types. We have done this in three
different quasar-galaxy redshift difference ranges z 0.001, 0.006,
and 0.012. Our results suggest that there is a significant increase of the
surface density of blue neighbours around our low-redshift quasar sample that
is steeper than around non-active field galaxies of the same luminosity and
redshift range. This may indicate that quasar formation is accomplished via a
merging scenario. No significant changes in star formation rate, dust
extinction, degree of ionization or color as a function of distance from the
quasars was observed. We could not observe any effects from quasars on their
companion galaxies.Comment: Submitted to Astronomy & Astrophysics November 22 2011; Accepted into
Astronomy & Astrophysics on April 23 2012. Keywords: active, nuclei, star
formation, quasars: general, interactions, evolutio
Potential for anthropogenic disturbances to influence evolutionary change in the life history of a threatened salmonid
Although evolutionary change within most species is thought to occur slowly, recent studies have identified cases where evolutionary change has apparently occurred over a few generations. Anthropogenically altered environments appear particularly open to rapid evolutionary change over comparatively short time scales. Here, we consider a Pacific salmon population that may have experienced life-history evolution, in response to habitat alteration, within a few generations. Historically, juvenile fall Chinook salmon (Oncorhynchus tshawytscha) from the Snake River migrated as subyearlings to the ocean. With changed riverine conditions that resulted from hydropower dam construction, some juveniles now migrate as yearlings, but more interestingly, the yearling migration tactic has made a large contribution to adult returns over the last decade. Optimal life-history models suggest that yearling juvenile migrants currently have a higher fitness than subyearling migrants. Although phenotypic plasticity likely accounts for some of the change in migration tactics, we suggest that evolution also plays a significant role. Evolutionary change prompted by anthropogenic alterations to the environment has general implications for the recovery of endangered species. The case study we present herein illustrates the importance of integrating evolutionary considerations into conservation planning for species at risk
Small-scale biocomplexity in coastal Atlantic cod supporting a Darwinian perspective on fisheries management
Harvesting of marine resources raises concerns about how to identify and preserve biocomplexity, including the diversity of life histories found within and among wild populations of a species. In order to fully accomplish this, there is a need to elucidate the underlying causes of phenotypic variation, and how this variation responds to environmental changes. In general, both evolutionary (genetic) and nonevolutionary (plastic) responses may occur. Plastic responses to environmental change are expected to shift the phenotype along a reaction norm, while an evolutionary response is expected to shift the reaction norm itself. Here, we assess the maturation patterns of coastal Atlantic cod (Gadus morhua) in Skagerrak, where studies using neutral markers have revealed genetically differentiated populations of this harvested fish within tens of kilometres of coastline. Our results suggest that physiological state prior to the spawning season, as well as juvenile growth, both influence the probability of completing sexual maturation at a given age. Furthermore, our results point towards a spatial structuring of this plasticity (i.e. the maturation reaction norms) comparable with population connectivity inferred from neutral markers. We argue that such fine-scale biocomplexity calls for a Darwinian approach to fisheries management
A Good Long Look at the Black Hole Candidates LMC X-1 and LMC X-3
We present results from 170ksec long RXTE observations of LMC X-1 and LMC
X-3, taken in 1996 December, where their spectra can be described by a disc
black body plus an additional soft (Gamma~2.8) high-energy power-law (detected
up to 50keV in LMC X-3). These observations, as well as archival ASCA
observations, constrain any narrow Fe line present in the spectra to have an
equivalent width <90eV, broad lines (~150eV EW, \sigma ~ 1keV) are permitted.
We also study the variability of LMC X-1. Its X-ray power spectral density
(PSD) is approximately f^{-1} between 10^{-3} and 0.3Hz with a rms variability
of ~7%. Above 5keV the PSD shows evidence of a break at f > 0.2Hz, possibly
indicating an outer disc radius of ~1000GM/c^2 in this likely wind-fed system.
Furthermore, the coherence function between variability in the > 5keV band and
variablity in the lower energy bands is extremely low. We discuss the
implications of these observations for the mechanisms.Comment: MNRAS, in press, clearified discussion, esp. on Fe lin
Induction of antigen-specific tolerance through hematopoietic stem cell-mediated gene therapy: the future for therapy of autoimmune disease?
Based on the principle that immune ablation followed by HSC-mediated recovery purges disease-causing leukocytes to interrupt autoimmune disease progression, hematopoietic stem cell transplantation (HSCT) has been increasingly used as a treatment for severe autoimmune diseases. Despite clinically-relevant outcomes, HSCT is associated with serious iatrogenic risks and is suitable only for the most serious and intractable diseases. A further limitation of autologous HSCT is that relapse rates can be high, suggesting disease-causing leukocytes are incompletely purged or the environmental and genetic determinants that drive disease remain active. Incorporation of antigen-specific tolerance approaches that synergise with autologous HSCT could reduce or prevent relapse. Further, by reducing the requirement for highly toxic immune-ablation and instead relying on antigen-specific tolerance, the clinical utility of HSCT could be significantly diversified. Substantial progress has been made exploring HSCT-mediated induction of antigen-specific tolerance in animal models but studies have focussed on primarily on prevention of autoimmune diseases. However, as diagnosis of autoimmune disease is often not made until autoimmune disease is well developed and populations of autoantigen-specific pathogenic effector and memory T cells have become well established, immunotherapies must be developed to address effector and memory T-cell responses which have traditionally been considered the key impediment to immunotherapy. Here, focusing on T-cell mediated autoimmune diseases we review progress made in antigen-specific immunotherapy using HSCT-mediated approaches, induction of tolerance in effector and memory T cells and the challenges for progression and clinical application of antigen-specific ‘tolerogenic’ HSCT therapy
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