10,263 research outputs found
Ozone control of biological activity during Earth's history, including the KT catastrophe
There have been brief periods since the beginning of the Cambrian some 600 m.y. ago when mass extinctions destroyed a significant fraction of living species. The most widely studied of these events is the catastrophe at the KT boundary that ended the long dominance of the dinosaurs. In addition to mass extinctions, there is another profound discontinuity in the history of Earth's biota, the explosion of life at the end of the Precambrian era which is an episode that is not explained well at all. For some 3 b.y. before the Cambrian, life had been present on Earth, but maintained a low level of activity which is an aspect of the biota that is puzzling, especially during the last two-thirds of that period. During the last 2 b.y. before the Cambrian, conditions at the Earth's surface were suitable for a burgeoning of the biota, according to most criteria: the oceans neither boiled nor were fozen solid during this time, and the atmosphere contained sufficient O for the development of animals. The purpose of this paper is to suggest that mass extinctions and the lackluster behavior of the Precambrian biota share a common cause: an inadequate amount of ozone in the atmosphere
A high pass, mechanical velocity filter for fast neutral molecular beams
Design and transmission characteristics of high pass, mechanical velocity filter for fast neutral molecular or atomic beam
Effects of P-wave Annihilation on the Angular Power Spectrum of Extragalactic Gamma-rays from Dark Matter Annihilation
We present a formalism for estimating the angular power spectrum of
extragalactic gamma-rays produced by dark matter annihilating with any general
velocity-dependent cross section. The relevant density and velocity
distribution of dark matter is modeled as an ensemble of smooth, universal,
rigid, disjoint, spherical halos with distribution and universal properties
constrained by simulation data. We apply this formalism to theories of dark
matter with p-wave annihilation, for which the relative-velocity-weighted
annihilation cross section is \sigma v=a+bv^2. We determine that this
significantly increases the gamma-ray power if b/a >> 10^6. The effect of
p-wave annihilation on the angular power spectrum is very similar for the
sample of particle physics models we explored, suggesting that the important
effect for a given b/a is largely determined by the cosmic dark matter
distribution. If the dark matter relic from strong p-wave theories is thermally
produced, the intensities of annihilation gamma-rays are strongly p-wave
suppressed, making them difficult to observe. If an angular power spectrum
consistent with a strong p-wave were to be observed, it would likely indicate
non-thermal production of dark matter in the early Universe.Comment: 20 pages, 3 figure
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HIV transmission networks among transgender women in Los Angeles County, CA, USA: a phylogenetic analysis of surveillance data.
BackgroundTransgender women are among the groups at highest risk for HIV infection, with a prevalence of 27·7% in the USA; and despite this known high risk, undiagnosed infection is common in this population. We set out to identify transgender women and their partners in a molecular transmission network to prioritise public health activities.MethodsSince 2006, HIV protease and reverse transcriptase gene (pol) sequences from drug resistance testing have been reported to the Los Angeles County Department of Public Health and linked to demographic data, gender, and HIV transmission risk factor data for each case in the enhanced HIV/AIDS Reporting System. We reconstructed a molecular transmission network by use of HIV-TRAnsmission Cluster Engine (with a pairwise genetic distance threshold of 0·015 substitutions per site) from the earliest pol sequences from 22 398 unique individuals, including 412 (2%) self-identified transgender women. We examined the possible predictors of clustering with multivariate logistic regression. We characterised the genetically linked partners of transgender women and calculated assortativity (the tendency for people to link to other people with the same attributes) for each transmission risk group.Findings8133 (36·3%) of 22 398 individuals clustered in the network across 1722 molecular transmission clusters. Transgender women who indicated a sexual risk factor clustered at the highest frequency in the network, with 147 (43%) of 345 being linked to at least one other person (adjusted odds ratio [aOR] 2·0, p=0·0002). Transgender women were assortative in the network (assortativity 0·06, p<0·001), indicating that they tended to link to other transgender women. Transgender women were more likely than expected to link to other transgender women (OR 4·65, p<0·001) and cisgender men who did not identify as men who have sex with men (MSM; OR 1·53, p<0·001). Transgender women were less likely than expected to link to MSM (OR 0·75, p<0·001), despite the high prevalence of HIV among MSM. Transgender women were distributed across 126 clusters, and cisgender individuals linked to one transgender woman were 9·2 times more likely to link to a second transgender woman than other individuals in the surveillance database. Reconstruction of the transmission network is limited by sample availability, but sequences were available for more than 40% of diagnoses.InterpretationClustering of transgender women and the observed tendency for linkage with cisgender men who did not identify as MSM, shows the potential to use molecular epidemiology both to identify clusters that are likely to include undiagnosed transgender women with HIV and to improve the targeting of public health prevention and treatment services to transgender women.FundingCalifornia HIV and AIDS Research Program and National Institutes of Health-National Institute of Allergy and Infectious Diseases
Draft Genome Sequence of Frankia sp. Strain CcI6, a Salt-Tolerant Nitrogen-Fixing Actinobacterium Isolated from the Root Nodule of Casuarina cunninghamiana
Members of the actinomycete genus Frankia form a nitrogen-fixing symbiosis with 8 different families of actinorhizal plants. We report a 5.57-Mbp draft genome sequence for Frankia sp. strain CcI6, a salt-tolerant nitrogen-fixing actinobacterium isolated from root nodules of Casurina cunninghamiana grown in Egyptian soils
Trigger, an active release experiment that stimulated auroral particle precipitation and wave emissions
The experiment design, including a description of the diagnostic and chemical release payload, and the general results are given for an auroral process simulation experiment. A drastic increase of the field aligned charged particle flux was observed over the approximate energy range 10 eV to more than 300 keV, starting about 150 ms after the release and lasting about one second. The is evidence of a second particle burst, starting one second after the release and lasting for tens of seconds, and evidence for a periodic train of particle bursts occurring with a 7.7 second period from 40 to 130 seconds after the release. A transient electric field pulse of 200 mv/m appeared just before the particle flux increase started. Electrostatic wave emissions around 2 kHz, as well as a delayed perturbation of the E-region below the plasma cloud were also observed. Some of the particle observations are interpreted in terms of field aligned electrostatic acceleration a few hundred kilometers above the injected plasma cloud. It is suggested that the acceleration electric field was created by an instability driven by field aligned currents originating in the plasma cloud
New Icing Cloud Simulation System at the NASA Glenn Research Center Icing Research Tunnel
A new spray bar system was designed, fabricated, and installed in the NASA Glenn Research Center's Icing Research Tunnel (IRT). This system is key to the IRT's ability to do aircraft in-flight icing cloud simulation. The performance goals and requirements levied on the design of the new spray bar system included increased size of the uniform icing cloud in the IRT test section, faster system response time, and increased coverage of icing conditions as defined in Appendix C of the Federal Aviation Regulation (FAR), Part 25 and Part 29. Through significant changes to the mechanical and electrical designs of the previous-generation spray bar system, the performance goals and requirements were realized. Postinstallation aerodynamic and icing cloud calibrations were performed to quantify the changes and improvements made to the IRT test section flow quality and icing cloud characteristics. The new and improved capability to simulate aircraft encounters with in-flight icing clouds ensures that the 1RT will continue to provide a satisfactory icing ground-test simulation method to the aeronautics community
Comparison of concurrent strain gage- and pressure transducer-measured flight loads on a lifting reentry vehicle and correlation with wind tunnel predictions
Concurrent strain gage and pressure transducer measured flight loads on a lifting reentry vehicle are compared and correlated with wind tunnel-predicted loads. Subsonic, transonic, and supersonic aerodynamic loads are presented for the left fin and control surfaces of the X-24B lifting reentry vehicle. Typical left fin pressure distributions are shown. The effects of variations in angle of attack, angle of sideslip, and Mach number on the left fin loads and rudder hinge moments are presented in coefficient form. Also presented are the effects of variations in angle of attack and Mach number on the upper flap, lower flap, and aileron hinge-moment coefficients. The effects of variations in lower flap hinge moments due to changes in lower flap deflection and Mach number are presented in terms of coefficient slopes
The L_X--M relation of Clusters of Galaxies
We present a new measurement of the scaling relation between X-ray luminosity
and total mass for 17,000 galaxy clusters in the maxBCG cluster sample.
Stacking sub-samples within fixed ranges of optical richness, N_200, we measure
the mean 0.1-2.4 keV X-ray luminosity, , from the ROSAT All-Sky Survey.
The mean mass, , is measured from weak gravitational lensing of SDSS
background galaxies (Johnston et al. 2007). For 9 <= N_200 < 200, the data are
well fit by a power-law, /10^42 h^-2 erg/s = (12.6+1.4-1.3 (stat) +/- 1.6
(sys)) (/10^14 h^-1 M_sun)^1.65+/-0.13. The slope agrees to within 10%
with previous estimates based on X-ray selected catalogs, implying that the
covariance in L_X and N_200 at fixed halo mass is not large. The luminosity
intercent is 30%, or 2\sigma, lower than determined from the X-ray flux-limited
sample of Reiprich & Bohringer (2002), assuming hydrostatic equilibrium. This
difference could arise from a combination of Malmquist bias and/or systematic
error in hydrostatic mass estimates, both of which are expected. The intercept
agrees with that derived by Stanek et al. (2006) using a model for the
statistical correspondence between clusters and halos in a WMAP3 cosmology with
power spectrum normalization sigma_8 = 0.85. Similar exercises applied to
future data sets will allow constraints on the covariance among optical and hot
gas properties of clusters at fixed mass.Comment: 5 pages, 1 figure, MNRAS accepte
Draft Genome Sequence of Frankia sp. Strain Thr, a Nitrogen-Fixing Actinobacterium Isolated from the Root Nodules of Casuarina cunninghamiana Grown in Egypt
Nitrogen-fixing actinobacteria of the genus Frankia are symbionts of woody dicotyledonous plants termed actinorhizal plants. We report here a 5.3-Mbp draft genome sequence for Frankia sp. stain Thr, a nitrogen-fixing actinobacterium isolated from root nodules of Casuarina cunninghamiana collected in Egypt
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