2,625 research outputs found
Kinetic theory for nongeodesic particle motion: Selfinteracting equilibrium states and effective viscous fluid pressures
The particles of a classical relativistic gas are supposed to move under the
influence of a quasilinear (in the particle four-momenta), self-interacting
force inbetween elastic, binary collisions. This force which is completely
fixed by the equilibrium conditions of the gas, gives rise to an effective
viscous pressure on the fluid phenomenological level. Earlier results
concerning the possibility of accelerated expansion of the universe due to
cosmological particle production are reinterpreted. A phenomenon such as power
law inflation may be traced back to specific self-interacting forces keeping
the particles of a gas universe in states of generalized equilibrium.Comment: 16 pages, latex, uses ioplppt.sty. To appear in Class. Quantum Gra
Opacity and conductivity measurements in noble gases at conditions of planetary and stellar interiors
The noble gases are elements of broad importance across science and technology and are primary constituents of planetary and stellar atmospheres, where they segregate into droplets or layers that affect the thermal, chemical, and structural evolution of their host body. We have measured the optical properties of noble gases at relevant high pressures and temperatures in the laser-heated diamond anvil cell, observing insulator-to-conductor transformations in dense helium, neon, argon, and xenon at 4,000–15,000 K and pressures of 15–52 GPa. The thermal activation and frequency dependence of conduction reveal an optical character dominated by electrons of low mobility, as in an amorphous semiconductor or poor metal, rather than free electrons as is often assumed for such wide band gap insulators at high temperatures. White dwarf stars having helium outer atmospheres cool slower and may have different color than if atmospheric opacity were controlled by free electrons. Helium rain in Jupiter and Saturn becomes conducting at conditions well correlated with its increased solubility in metallic hydrogen, whereas a deep layer of insulating neon may inhibit core erosion in Saturn
Contrasting sensitivity of lake sediment n-alkanoic acids and n-alkanes to basin-scale vegetation and regional-scale precipitation δ2H in the Adirondack Mountains, NY (USA)
The hydrogen isotope values of plant waxes (δ2Hwax) primarily reflect plant source water. δ2Hwax preserved in lake sediments has therefore been widely used to investigate past hydroclimate. The processes by which plant waxes are integrated at regional and catchment scales are poorly understood and may affect the δ2Hwax values recorded in sediments. Here, we assess the variability of sedimentary δ2Hwax for two plant wax compound classes (n-alkanes and n-alkanoic acids) across 12 lakes in the Adirondack Mountains that receive similar regional precipitation δ2H but vary at the catchment-scale in terms of vegetation structure and basin morphology. Total long-chain (n-C27 to n-C35) alkane concentrations were similar across all sites (191 ± 53 µg/g TOC) while total long-chain (n-C28 and n-C30) alkanoic acid concentrations were more variable (117 ± 116 µg/g TOC) and may reflect shoreline vegetation composition. Lakes with shorelines dominated by evergreen gymnosperm plants had significantly higher concentrations of long-chain n-alkanoic acids relative to n-alkanes, consistent with our observations that deciduous angiosperms produced more long-chain n-alkanes than evergreen gymnosperms (471 and 33 µg/g TOC, respectively). In sediments, the most abundant chain lengths in each compound class were n-C29 alkane and n-C28 alkanoic acid, which had mean δ2H values of −188 ± 6‰ and −164 ± 9‰, respectively. Across sites, the range in sedimentary n-C29 alkane (22‰) and n-C28 alkanoic acid δ2H (35‰) was larger than expected based on the total range in modeled mean annual precipitation δ2H (4‰). We observed larger mean εapp (based on absolute values) for n-alkanes (−123‰) than for n-alkanoic acids (−97‰). Across sites, the δ2H offset between n-C29 alkane and the biosynthetic precursor n-C30 alkanoic acid (εC29-C30) ranged from −8 to −58‰, which was more variable than expected based on observations in temperate trees (−20 to −30‰). Sediments with greater aquatic organic matter contributions (lower C/N ratios) had significantly larger (absolute) εC29-C30 values, which may reflect long-chain n-alkanoic acids from aquatic sources. Concentration and δ2Hwax data in Adirondack lakes suggest that long-chain n-alkanes are more sensitive to regional-scale precipitation signals, while n-alkanoic acids are more sensitive to basin-scale differences in catchment vegetation and wax sourcing
Thioredoxin is a metabolic rheostat controlling regulatory B cells
Metabolic programming is important for B cell fate, but the bioenergetic requirement for regulatory B (B reg) cell differentiation and function is unknown. Here we show that B reg cell differentiation, unlike non-B reg cells, relies on mitochondrial electron transport and homeostatic levels of reactive oxygen species (ROS). Single-cell RNA sequencing analysis revealed that TXN, encoding the metabolic redox protein thioredoxin (Trx), is highly expressed by B reg cells, unlike Trx inhibitor TXNIP which was downregulated. Pharmacological inhibition or gene silencing of TXN resulted in mitochondrial membrane depolarization and increased ROS levels, selectively suppressing B reg cell differentiation and function while favoring pro-inflammatory B cell differentiation. Patients with systemic lupus erythematosus (SLE), characterized by B reg cell deficiencies, present with B cell mitochondrial membrane depolarization, elevated ROS and fewer Trx + B cells. Exogenous Trx stimulation restored B reg cells and mitochondrial membrane polarization in SLE B cells to healthy B cell levels, indicating Trx insufficiency underlies B reg cell impairment in patients with SLE. </p
Infrared nanoscopy of Dirac plasmons at the graphene-SiO2 interface
We report on infrared (IR) nanoscopy of 2D plasmon excitations of Dirac
fermions in graphene. This is achieved by confining mid-IR radiation at the
apex of a nanoscale tip: an approach yielding two orders of magnitude increase
in the value of in-plane component of incident wavevector q compared to free
space propagation. At these high wavevectors, the Dirac plasmon is found to
dramatically enhance the near-field interaction with mid-IR surface phonons of
SiO2 substrate. Our data augmented by detailed modeling establish graphene as a
new medium supporting plasmonic effects that can be controlled by gate voltage.Comment: 12 pages, 4 figure
Curvature force and dark energy
A curvature self-interaction of the cosmic gas is shown to mimic a
cosmological constant or other forms of dark energy, such as a rolling tachyon
condensate or a Chaplygin gas. Any given Hubble rate and deceleration parameter
can be traced back to the action of an effective curvature force on the gas
particles. This force self-consistently reacts back on the cosmological
dynamics. The links between an imperfect fluid description, a kinetic
description with effective antifriction forces, and curvature forces, which
represent a non-minimal coupling of gravity to matter, are established.Comment: 14 pages; references added, to appear in New Journal of Physics (v3
A Youth Compendium of Physical Activities: Activity Codes and Metabolic Intensities
A Youth Compendium of Physical Activities (Youth Compendium) was developed to estimate the energy costs of physical activities using data on youth only
High precision X-ray logN-logS distributions: implications for the obscured AGN population
We have constrained the extragalactic source count distributions over a broad
range of X-ray fluxes and in various energy bands to test whether the
predictions from X-ray background synthesis models agree with the observational
constraints provided by our measurements. We have used 1129 XMM-Newton
observations at |b|>20 deg covering a sky area of 132.3 deg^2 to compile the
largest complete samples of X-ray objects to date in the 0.5-1 keV, 1-2 keV,
2-4.5 keV, 4.5-10 keV, 0.5-2 keV and 2-10 keV energy bands. Our survey includes
in excess of 30,000 sources down to ~10^-15 erg/cm^2/s below 2 keV and down to
~10^{-14} erg/cm^2/s above 2 keV. A break in the source count distributions was
detected in all energy bands except the 4.5-10 keV band. An analytical model
comprising 2 power-law components cannot adequately describe the curvature seen
in the source count distributions. The shape of the logN(>S)-logS is strongly
dependent on the energy band with a general steepening apparent as we move to
higher energies. This is due to non-AGN populations, comprised mainly of stars
and clusters of galaxies, contribute up to 30% of the source population at
energies 10^{-13} erg/cm^2/s, and these populations of
objects have significantly flatter source count distributions than AGN. We find
a substantial increase in the relative fraction of hard X-ray sources at higher
energies, from >55% below 2 keV to >77% above 2 keV. However the majority of
sources detected above 4.5 keV still have significant flux below 2 keV.
Comparison with predictions from the synthesis models suggest that the models
might be overpredicting the number of faint absorbed AGN, which would call for
fine adjustment of some model parameters such as the obscured to unobscured AGN
ratio and/or the distribution of column densities at intermediate obscuration.Comment: Accepted for publication in Astronomy and Astrophysics. Abridged
Abstract. 23 pages, 47 figures, 8 table
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