359 research outputs found
Pore evolution in interstellar ice analogues: simulating the effects of temperature increase
Context. The level of porosity of interstellar ices - largely comprised of
amorphous solid water (ASW) - contains clues on the trapping capacity of other
volatile species and determines the surface accessibility that is needed for
solid state reactions to take place. Aims. Our goal is to simulate the growth
of amorphous water ice at low temperature (10 K) and to characterize the
evolution of the porosity (and the specific surface area) as a function of
temperature (from 10 to 120 K). Methods. Kinetic Monte Carlo simulations are
used to mimic the formation and the thermal evolution of pores in amorphous
water ice. We follow the accretion of gas-phase water molecules as well as
their migration on surfaces with different grid sizes, both at the top growing
layer and within the bulk. Results. We show that the porosity characteristics
change substantially in water ice as the temperature increases. The total
surface of the pores decreases strongly while the total volume decreases only
slightly for higher temperatures. This will decrease the overall reaction
efficiency, but in parallel, small pores connect and merge, allowing trapped
molecules to meet and react within the pores network, providing a pathway to
increase the reaction efficiency. We introduce pore coalescence as a new solid
state process that may boost the solid state formation of new molecules in
space and has not been considered so far.Comment: 9 pages, 8 figures Accepted for publication in A&
The Bank of Westminster and Hyland Park construction contracts as engineering student classroom projects:construction phase.
http://archive.org/details/bankofwestminste00bossNAN
Porosity measurements of interstellar ice mixtures using optical laser interference and extended effective medium approximations
Aims. This article aims to provide an alternative method of measuring the
porosity of multi-phase composite ices from their refractive indices and of
characterising how the abundance of a premixed contaminant (e.g., CO2) affects
the porosity of water-rich ice mixtures during omni-directional deposition.
Methods. We combine optical laser interference and extended effective medium
approximations (EMAs) to measure the porosity of three astrophysically relevant
ice mixtures: H2O:CO2=10:1, 4:1, and 2:1. Infrared spectroscopy is used as a
benchmarking test of this new laboratory-based method. Results. By
independently monitoring the O-H dangling modes of the different water-rich ice
mixtures, we confirm the porosities predicted by the extended EMAs. We also
demonstrate that CO2 premixed with water in the gas phase does not
significantly affect the ice morphology during omni-directional deposition, as
long as the physical conditions favourable to segregation are not reached. We
propose a mechanism in which CO2 molecules diffuse on the surface of the
growing ice sample prior to being incorporated into the bulk and then fill the
pores partly or completely, depending on the relative abundance and the growth
temperature.Comment: 9 pages, 6 figures, 1 table. Accepted for publication in A&
Decaying neutron propagation in the Galaxy and the Cosmic Ray anisotropy at 1 EeV
We study the cosmic ray arrival distribution expected from a source of
neutrons in the galactic center at energies around 1 EeV and compare it with
the anisotropy detected by AGASA and SUGAR. Besides the point-like signal in
the source direction produced by the direct neutrons, an extended signal due to
the protons produced in neutron decays is expected. This associated proton
signal also leads to an excess in the direction of the spiral arm. For
realistic models of the regular and random galactic magnetic fields, the
resulting anisotropy as a function of the energy is obtained. We find that for
the anisotropy to become sufficiently suppressed below E\sim 10^{17.9}eV, a
significant random magnetic field component is required, while on the other
hand, this also tends to increase the angular spread of the associated proton
signal and to reduce the excess in the spiral arm direction. The source
luminosity required in order that the right ascension anisotropy be 4% for the
AGASA angular exposure corresponds to a prediction for the point-like flux from
direct neutrons compatible with the flux detected by SUGAR. We also analyse the
distinguishing features predicted for a large statistics southern observatory.Comment: 14 pages, 6 figures, minor changes to match published versio
Probiotics [LGG-BB12 or RC14-GR1] versus placebo as prophylaxis for urinary tract infection in persons with spinal cord injury [ProSCIUTTU]: a study protocol for a randomised controlled trial
© 2016 Lee et al. Background: Urinary tract infections [UTIs] are very common in people with Spinal Cord Injury [SCI]. UTIs are increasingly difficult and expensive to treat as the organisms that cause them become more antibiotic resistant. Among the SCI population, there is a high rate of multi-resistant organism [MRO] colonisation. Non-antibiotic prevention strategies are needed to prevent UTI without increasing resistance. Probiotics have been reported to be beneficial in preventing UTIs in post-menopausal women in several in vivo and in vitro studies. The main aim of this study is to determine whether probiotic therapy with combinations of Lactobacillus reuteri RC-14 + Lactobacillus rhamnosus GR-1 [RC14-GR1] and/or Lactobacillus rhamnosus GG + Bifidobacterium BB-12 [LGG-BB12] are effective in preventing UTI in people with SCI compared to placebo. Method: This is a multi-site randomised double-blind double-dummy placebo-controlled factorial design study conducted in New South Wales, Australia. All participants have a neurogenic bladder as a result of spinal injury. Recruitment started in April 2011. Participants are randomised to one of four arms, designed for factorial analysis of LGG-BB12 and/or RC14-GR1 v Placebo. This involves 24 weeks of daily oral treatment with RC14-GR1 + LGG-BB12, RC14-GR1 + placebo, LGG-BB12 + placebo or two placebo capsules. Randomisation is stratified by bladder management type and inpatient status. Participants are assessed at baseline, three months and six months for Short Form Health Survey [SF-36], microbiological swabs of rectum, nose and groin; urine culture and urinary catheters for subjects with indwelling catheters. A bowel questionnaire is administered at baseline and three months to assess effect of probiotics on bowel function. The primary outcome is time from randomisation to occurrence of symptomatic UTI. The secondary outcomes are change of MRO status and bowel function, quality of life and cost-effectiveness of probiotics in persons with SCI. The primary outcome will be analysed using survival analysis of factorial groups, with Cox regression modelling to test the effect of each treatment while allowing for the other, assuming no interaction effect. Hazard ratios and Kaplan-Meier survival curves will be used to summarise results. Discussion: If these probiotics are shown to be effective in preventing UTI and MRO colonisation, they would be a very attractive alternative for UTI prophylaxis and for combating the increasing rate of antibiotic resistance after SCI. Trial registration: Australian New Zealand Clinical Trials Registry [ ACTRN 12610000512022 ]. Date of registration: 21 June 2010
Spectrally-resolved UV photodesorption of CH4 in pure and layered ices
Context. Methane is among the main components of the ice mantles of
insterstellar dust grains, where it is at the start of a rich solid-phase
chemical network. Quantification of the photon-induced desorption yield of
these frozen molecules and understanding of the underlying processes is
necessary to accurately model the observations and the chemical evolution of
various regions of the interstellar medium. Aims. This study aims at
experimentally determining absolute photodesorption yields for the CH4 molecule
as a function of photon energy. The influence of the ice composition is also
investigated. By studying the methane desorption from layered CH4:CO ice,
indirect desorption processes triggered by the excitation of the CO molecules
is monitored and quantified. Methods. Tunable monochromatic VUV light from the
DESIRS beamline of the SOLEIL synchrotron is used in the 7 - 13.6 eV (177 - 91
nm) range to irradiate pure CH4 or layers of CH4 deposited on top of CO ice
samples. The release of species in the gas phase is monitored by quadrupole
mass spectrometry and absolute photodesorption yields of intact CH4 are
deduced. Results. CH4 photodesorbs for photon energies higher than ~9.1 eV
(~136 nm). The photodesorption spectrum follows the absorption spectrum of CH4,
which confirms a desorption mechanism mediated by electronic transitions in the
ice. When it is deposited on top of CO, CH4 desorbs between 8 and 9 eV with a
pattern characteristic of CO absorption, indicating desorption induced by
energy transfer from CO molecules. Conclusions. The photodesorption of CH4 from
the pure ice in various interstellar environments is around 2.0 x 10^-3
molecules per incident photon. Results on CO-induced indirect desorption of CH4
provide useful insights for the generalization of this process to other
molecules co-existing with CO in ice mantles
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Ulcerative colitis-risk loci on chromosomes 1p36 and 12q15 found by genome-wide association study.
Ulcerative colitis is a chronic inflammatory disease of the colon that presents as diarrhea and gastrointestinal bleeding. We performed a genome-wide association study using DNA samples from 1,052 individuals with ulcerative colitis and preexisting data from 2,571 controls, all of European ancestry. In an analysis that controlled for gender and population structure, ulcerative colitis loci attaining genome-wide significance and subsequent replication in two independent populations were identified on chromosomes 1p36 (rs6426833, combined P = 5.1 x 10(-13), combined odds ratio OR = 0.73) and 12q15 (rs1558744, combined P = 2.5 x 10(-12), combined OR = 1.35). In addition, combined genome-wide significant evidence for association was found in a region spanning BTNL2 to HLA-DQB1 on chromosome 6p21 (rs2395185, combined P = 1.0 x 10(-16), combined OR = 0.66) and at the IL23R locus on chromosome 1p31 (rs11209026, combined P = 1.3 x 10(-8), combined OR = 0.56; rs10889677, combined P = 1.3 x 10(-8), combined OR = 1.29)
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