20,132 research outputs found
Dusty plasma cavities: probe-induced and natural
A comprehensive exploration of regional dust evacuation in complex plasma
crystals is presented. Voids created in 3D crystals on the International Space
Station have provided a rich foundation for experiments, but cavities in dust
crystals formed in ground-based experiments have not received as much
attention. Inside a modified GEC RF cell, a powered vertical probe was used to
clear the central area of a dust crystal, producing a cavity with high
cylindrical symmetry. Cavities generated by three mechanisms are examined.
First, repulsion of micrometer-sized particles by a negatively charged probe is
investigated. A model of this effect developed for a DC plasma is modified and
applied to explain new experimental data in RF plasma. Second, the formation of
natural cavities is surveyed; a radial ion drag proposed to occur due to a
curved sheath is considered in conjunction with thermophoresis and a flattened
confinement potential above the center of the electrode. Finally, cavity
formation unexpectedly occurs upon increasing the probe potential above the
plasma floating potential. The cavities produced by these methods appear
similar, but each are shown to be facilitated by fundamentally different
processes.Comment: 10 pages, 12 figure
Charging and coagulation of dust in protoplanetary plasma environments
Combining a particle-particle, particle-cluster and cluster-cluster
agglomeration model with an aggregate charging model, the coagulation and
charging of dust particles in various plasma environments relevant for
proto-planetary disks have been investigated. The results show that charged
aggregates tend to grow by adding small particles and clusters to larger
particles and clusters, leading to greater sizes and masses as compared to
neutral aggregates, for the same number of monomers in the aggregate. In
addition, aggregates coagulating in a Lorentzian plasma (containing a larger
fraction of high-energy plasma particles) are more massive and larger than
aggregates coagulating in a Maxwellian plasma, for the same plasma densities
and characteristic temperature. Comparisons of the grain structure, utilizing
the compactness factor, {\phi}{\sigma}, demonstrate that a Lorentzian plasma
environment results in fluffier aggregates, with small {\phi}{\sigma}, which
exhibit a narrow compactness factor distribution. Neutral aggregates are more
compact, with larger {\phi}{\sigma}, and exhibit a larger variation in
fluffiness. Measurement of the compactness factor of large populations of
aggregates is shown to provide information on the disk parameters that were
present during aggregation
Near-Infrared Observations of Powerful High-Redshift Radio Galaxies: 4C 40.36 and 4C 39.37
We present near-infrared imaging and spectroscopic observations of two FR II
high-redshift radio galaxies (HzRGs), 4C 40.36 (z=2.3) and 4C 39.37 (z=3.2),
obtained with the Hubble, Keck, and Hale Telescopes. High resolution images
were taken with filters both in and out of strong emission lines, and together
with the spectroscopic data, the properties of the line and continuum emissions
were carefully analyzed. Our analysis of 4C 40.36 and 4C 39.37 shows that
strong emission lines (e.g., [O III] 5007 A and H alpha+[N II]) contribute to
the broad-band fluxes much more significantly than previously estimated (80%
vs. 20-40%), and that when the continuum sources are imaged through line-free
filters, they show an extremely compact morphology with a high surface
brightness. If we use the R^1/4-law parametrization, their effective radii
(r(e)) are only 2-3 kpc while their restframe B-band surface brightnesses at
r(e) are I(B) ~ 18 mag/arcsec^2. Compared with z ~ 1 3CR radio galaxies, the
former is x3-5 smaller, while the latter is 1-1.5 mag brighter than what is
predicted from the I(B)-r(e) correlation. Although exponential profiles produce
equally good fits for 4C 40.36 and 4C 39.37, this clearly indicates that with
respect to the z~1 3CR radio galaxies, the light distribution of these two
HzRGs is much more centrally concentrated. Spectroscopically, 4C 40.36 shows a
flat (fnu=const) continuum while 4C 39.37 shows a spectrum as red as that of a
local giant elliptical galaxy. Although this difference may be explained in
terms of a varying degree of star formation, the similarities of their surface
brightness profiles and the submillimeter detection of 4C 39.37 might suggest
that the intrinsic spectra is equally blue (young stars or an AGN), and that
the difference is the amount of reddening.Comment: 30 pages, 6 tables, 10 figures; Accepted for publication in
Astronomical Journa
Testing for periodicities in near-IR light curves of Sgr A
We present the results of near-infrared (2 ÎĽm) monitoring of Sgr A*-IR with 1 minute time sampling using laser guide star adaptive optics (LGS AO) system at the Keck II
telescope. Sgr A*-IR was observed continuously for up to three hours on each of seven nights, between 2006 May and 2007 August. Sgr A*-IR is detected at all times and is continuously variable. These observations allow us to investigate Nyquist sampled periods ranging from
about 2 minutes to an hour. Of particular interest are periods of ~20 min, which corresponds to a quasi-periodic (QPO) signal claimed based upon previous near-infrared observations and interpreted as the orbit of a ’hot spot’ at or near the last stable orbit of a spinning black hole.
We investigate these claims by comparing periodograms of the light curves with models for red noise and find no significant deviations that would indicate QPO activity at any time scale probed in the study. We find that the variability of Sgr A* is consistent with a model based on
correlated noise with a power spectrum having a frequency dependence of ~ f^(2.5), consistent with that observed in AGNs. Furthermore, the periodograms show power down to the minimum sampling time of 2 min, well below the period of the last stable orbit of a maximally spinning black hole, indicating that the Sgr A*-IR light curves observed in this study is unlikely to be from the Keplerian motion of a single ’hot spot’ of orbiting plasma
Growth inhibition of cytosolic Salmonella by caspase-1 and caspase-11 precedes host cell death
Sensing bacterial products in the cytosol of mammalian cells by NOD-like receptors leads to the activation of caspase-1 inflammasomes, and the production of the pro-inflammatory cytokines interleukin (IL)-18 and IL-1β. In addition, mouse caspase-11 (represented in humans by its orthologs, caspase-4 and caspase-5) detects cytosolic bacterial LPS directly. Activation of caspase-1 and caspase-11 initiates pyroptotic host cell death that releases potentially harmful bacteria from the nutrient-rich host cell cytosol into the extracellular environment. Here we use single cell analysis and time-lapse microscopy to identify a subpopulation of host cells, in which growth of cytosolic Salmonella Typhimurium is inhibited independently or prior to the onset of cell death. The enzymatic activities of caspase-1 and caspase-11 are required for growth inhibition in different cell types. Our results reveal that these proteases have important functions beyond the direct induction of pyroptosis and proinflammatory cytokine secretion in the control of growth and elimination of cytosolic bacteria
Stochastic Chemical Reactions in Micro-domains
Traditional chemical kinetics may be inappropriate to describe chemical
reactions in micro-domains involving only a small number of substrate and
reactant molecules. Starting with the stochastic dynamics of the molecules, we
derive a master-diffusion equation for the joint probability density of a
mobile reactant and the number of bound substrate in a confined domain. We use
the equation to calculate the fluctuations in the number of bound substrate
molecules as a function of initial reactant distribution. A second model is
presented based on a Markov description of the binding and unbinding and on the
mean first passage time of a molecule to a small portion of the boundary. These
models can be used for the description of noise due to gating of ionic channels
by random binding and unbinding of ligands in biological sensor cells, such as
olfactory cilia, photo-receptors, hair cells in the cochlea.Comment: 33 pages, Journal Chemical Physic
Ocean temperature and salinity components of the Madden-Julian oscillation observed by Argo floats
New diagnostics of the Madden-Julian Oscillation (MJO) cycle in ocean temperature and, for the first time, salinity are presented. The MJO composites are based on 4 years of gridded Argo float data from 2003 to 2006, and extend from the surface to 1,400 m depth in the tropical Indian and Pacific Oceans. The MJO surface salinity anomalies are consistent with precipitation minus evaporation fluxes in the Indian Ocean, and with anomalous zonal advection in the Pacific. The Argo sea surface temperature and thermocline depth anomalies are consistent with previous studies using other data sets. The near-surface density changes due to salinity are comparable to, and partially offset, those due to temperature, emphasising the importance of including salinity as well as temperature changes in mixed-layer modelling of tropical intraseasonal processes. The MJO-forced equatorial Kelvin wave that propagates along the thermocline in the Pacific extends down into the deep ocean, to at least 1,400 m. Coherent, statistically significant, MJO temperature and salinity anomalies are also present in the deep Indian Ocean
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