996 research outputs found
A Cross-Match of 2MASS and SDSS: Newly-Found L and T Dwarfs and an Estimate of the Space Densitfy of T Dwarfs
We report new L and T dwarfs found in a cross-match of the SDSS Data Release
1 and 2MASS. Our simultaneous search of the two databases effectively allows us
to relax the criteria for object detection in either survey and to explore the
combined databases to a greater completeness level. We find two new T dwarfs in
addition to the 13 already known in the SDSS DR1 footprint. We also identify 22
new candidate and bona-fide L dwarfs, including a new young L2 dwarf and a
peculiar L2 dwarf with unusually blue near-IR colors: potentially the result of
mildly sub-solar metallicity. These discoveries underscore the utility of
simultaneous database cross-correlation in searching for rare objects. Our
cross-match completes the census of T dwarfs within the joint SDSS and 2MASS
flux limits to the 97% level. Hence, we are able to accurately infer the space
density of T dwarfs. We employ Monte Carlo tools to simulate the observed
population of SDSS DR1 T dwarfs with 2MASS counterparts and find that the space
density of T0-T8 dwarf systems is 0.0070 (-0.0030; +0.0032) per cubic parsec
(95% confidence interval), i.e., about one per 140 cubic parsecs. Compared to
predictions for the T dwarf space density that depend on various assumptions
for the sub-stellar mass function, this result is most consistent with models
that assume a flat sub-stellar mass function dN/dM ~ M^0. No >T8 dwarfs were
discovered in the present cross-match, though less than one was expected in the
limited area (2099 sq. degrees) of SDSS DR1.Comment: To appear in ApJ, Feb 10, 2008 issue. 37 pages, including 12 figures
and 14 table
Stochastic Modelling Approach to the Incubation Time of Prionic Diseases
Transmissible spongiform encephalopathies like the bovine spongiform
encephalopathy (BSE) and the Creutzfeldt-Jakob disease (CJD) in humans are
neurodegenerative diseases for which prions are the attributed pathogenic
agents. A widely accepted theory assumes that prion replication is due to a
direct interaction between the pathologic (PrPsc) form and the host encoded
(PrPc) conformation, in a kind of an autocatalytic process. Here we show that
the overall features of the incubation time of prion diseases are readily
obtained if the prion reaction is described by a simple mean-field model. An
analytical expression for the incubation time distribution then follows by
associating the rate constant to a stochastic variable log normally
distributed. The incubation time distribution is then also shown to be log
normal and fits the observed BSE data very well. The basic ideas of the
theoretical model are then incorporated in a cellular automata model. The
computer simulation results yield the correct BSE incubation time distribution
at low densities of the host encoded protein
Conformational properties of the prion octa-repeat and hydrophobic sequences
AbstractWe have used circular dichroism to study synthetic peptides from two important regions of the prion protein: the N-terminal octa-repeat domain and a highly conserved hydrophobic section. Our results show that the octa-repeat sequence in free solution can adopt a non-random, extended conformation with properties similar to the poly-l-proline type II left-handed helix. We also show that the conformation can be changed by temperature, organic solvents (e.g. acetonitrile) and on binding to phospholipid vesicles. We compared CD data from two peptides corresponding to the hydrophobic region between residues 106 and 136 which contained either methionine or valine at position 129. This variation represents a common polymorphism in humans which has been shown to influence predisposition towards iatrogenic and sporadic CJD. There was no detectable difference between the CD spectra of these peptides irrespective of the solvent conditions we used
On the Spectral Evolution of Cool, Helium-Atmosphere White Dwarfs: Detailed Spectroscopic and Photometric Analysis of DZ Stars
We present a detailed analysis of a large spectroscopic and photometric
sample of DZ white dwarfs based on our latest model atmosphere calculations. We
revise the atmospheric parameters of the trigonometric parallax sample of
Bergeron, Leggett, & Ruiz (12 stars) and analyze 147 new DZ white dwarfs
discovered in the Sloan Digital Sky Survey. The inclusion of metals and
hydrogen in our model atmosphere calculations leads to different atmospheric
parameters than those derived from pure helium models. Calcium abundances are
found in the range from log (Ca/He) = -12 to -8. We also find that fits of the
coolest objects show peculiarities, suggesting that our physical models may not
correctly describe the conditions of high atmospheric pressure encountered in
the coolest DZ stars. We find that the mean mass of the 11 DZ stars with
trigonometric parallaxes, = 0.63 Mo, is significantly lower than that
obtained from pure helium models, = 0.78 Mo, and in much better agreement
with the mean mass of other types of white dwarfs. We determine hydrogen
abundances for 27% of the DZ stars in our sample, while only upper limits are
obtained for objects with low signal-to-noise ratio spectroscopic data. We
confirm with a high level of confidence that the accretion rate of hydrogen is
at least two orders of magnitude smaller than that of metals (and up to five in
some cases) to be compatible with the observations. We find a correlation
between the hydrogen abundance and the effective temperature, suggesting for
the first time empirical evidence of a lower temperature boundary for the
hydrogen screening mechanism. Finally, we speculate on the possibility that the
DZA white dwarfs could be the result of the convective mixing of thin
hydrogen-rich atmospheres with the underlying helium convection zone.Comment: 67 pages, 32 figures, accepted for publication in Ap
HECTD2 Is Associated with Susceptibility to Mouse and Human Prion Disease
Prion diseases are fatal transmissible neurodegenerative disorders, which include Scrapie, Bovine Spongiform Encephalopathy (BSE), Creutzfeldt-Jakob Disease (CJD), and kuru. They are characterised by a prolonged clinically silent incubation period, variation in which is determined by many factors, including genetic background. We have used a heterogeneous stock of mice to identify Hectd2, an E3 ubiquitin ligase, as a quantitative trait gene for prion disease incubation time in mice. Further, we report an association between HECTD2 haplotypes and susceptibility to the acquired human prion diseases, vCJD and kuru. We report a genotype-associated differential expression of Hectd2 mRNA in mouse brains and human lymphocytes and a significant up-regulation of transcript in mice at the terminal stage of prion disease. Although the substrate of HECTD2 is unknown, these data highlight the importance of proteosome-directed protein degradation in neurodegeneration. This is the first demonstration of a mouse quantitative trait gene that also influences susceptibility to human prion diseases. Characterisation of such genes is key to understanding human risk and the molecular basis of incubation periods
Ex vivomammalian prions are formed of paired double helical prion protein fibrils
Mammalian prions are hypothesized to be fibrillar or amyloid forms of prion protein (PrP), but structures observed to date have not been definitively correlated with infectivity and the three-dimensional structure of infectious prions has remained obscure. Recently, we developed novel methods to obtain exceptionally pure preparations of prions from mouse brain and showed that pathogenic PrP in these high-titre preparations is assembled into rod-like assemblies. Here, we have used precise cell culture-based prion infectivity assays to define the physical relationship between the PrP rods and prion infectivity and have used electron tomography to define their architecture. We show that infectious PrP rods isolated from multiple prion strains have a common hierarchical assembly comprising twisted pairs of short fibres with repeating substructure. The architecture of the PrP rods provides a new structural basis for understanding prion infectivity and can explain the inability to systematically generate high-titre synthetic prions from recombinant PrP
Plant extracts as potential control agents of Black Sigatoka in banana
Aqueous extracts of Cinnamomum zeylanicum, Capsicum annuum and Azadirachta indica were tested for efficacy for management of Pseudocercospora fijiensis in banana (Musa spp.) when applied as foliar sprays. Extracts of C. annuum and A.indica demonstrated fungicidal effects in vitro, without showing phytotoxic effects. The two extracts protected tissue culture banana plantlets of cultivar Musakala to a similar degree as the traditional fungicide difenoconazole. Extracts of A. indica and C. annuum (0.3 g/ml) reduced Black Sigatoka severity by 69.3% and 65.6%, respectively, and were thus comparable to the fungicide difenoconazole (72.7%). Soil drenching of the extracts did not significantly reduce Black Sigatoka severity. In planta effects of the extracts mainly consisted of delayed symptom appearance and reduced lesion number. Symptom development was dependent on extract concentration and days between extract application and inoculation. Effect of the
tested extracts on height of plants and new leaves was not significant. Collectively, our data suggest that A. indica and C. annuum have interesting and unique properties as plant protection agents against Pseudocercospora fijiensis, but further research is needed to investigate their efficacy
An approximate model for cancellous bone screw fixation
This is the author's accepted manuscript. The final published article is available from the link below. Copyright @ 2013 Taylor & Francis.This paper presents a finite element (FE) model to identify parameters that affect the performance of an improved cancellous bone screw fixation technique, and hence potentially improve fracture treatment. In cancellous bone of low apparent density, it can be difficult to achieve adequate screw fixation and hence provide stable fracture fixation that enables bone healing. Data from predictive FE models indicate that cements can have a significant potential to improve screw holding power in cancellous bone. These FE models are used to demonstrate the key parameters that determine pull-out strength in a variety of screw, bone and cement set-ups, and to compare the effectiveness of different configurations. The paper concludes that significant advantages, up to an order of magnitude, in screw pull-out strength in cancellous bone might be gained by the appropriate use of a currently approved calcium phosphate cement
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