472 research outputs found
Cosmic Dust Collection Facility: Scientific objectives and programmatic relations
The science objectives are summarized for the Cosmic Dust Collection Facility (CDCF) on Space Station Freedom and these objectives are related to ongoing science programs and mission planning within NASA. The purpose is to illustrate the potential of the CDCF project within the broad context of early solar system sciences that emphasize the study of primitive objects in state-of-the-art analytical and experimental laboratories on Earth. Current knowledge about the sources of cosmic dust and their associated orbital dynamics is examined, and the results are reviewed of modern microanalytical investigations of extraterrestrial dust particles collected on Earth. Major areas of scientific inquiry and uncertainty are identified and it is shown how CDCF will contribute to their solution. General facility and instrument concepts that need to be pursued are introduced, and the major development tasks that are needed to attain the scientific objectives of the CDCF project are identified
The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background: IV. Cosmological Implications
In this paper we examine the cosmological constraints of the recent DIRBE and
FIRAS detection of the extragalactic background light between 125-5000 microns
on the metal and star formation histories of the universe.Comment: 38 pages and 9 figures. Accepted for publications in The
Astrophysical Journa
Perspectives on Interstellar Dust Inside and Outside of the Heliosphere
Measurements by dust detectors on interplanetary spacecraft appear to
indicate a substantial flux of interstellar particles with masses exceeding
10^{-12}gram. The reported abundance of these massive grains cannot be typical
of interstellar gas: it is incompatible with both interstellar elemental
abundances and the observed extinction properties of the interstellar dust
population. We discuss the likelihood that the Solar System is by chance
located near an unusual concentration of massive grains and conclude that this
is unlikely, unless dynamical processes in the ISM are responsible for such
concentrations. Radiation pressure might conceivably drive large grains into
"magnetic valleys". If the influx direction of interstellar gas and dust is
varying on a ~10 yr timescale, as suggested by some observations, this would
have dramatic implications for the small-scale structure of the interstellar
medium.Comment: 13 pages. To appear in Space Science Review
Wolf-Rayet and LBV Nebulae as the Result of Variable and Non-Spherical Stellar Winds
The physical basis for interpreting observations of nebular morphology around
massive stars in terms of the evolution of the central stars is reviewed, and
examples are discussed, including NGC 6888, OMC-1, and eta Carinae.Comment: To be published in the Proceedings of IAU Colloquium 169 on Variable
and Non-Spherical Stellar Winds in Luminous Hot Stars, ed. B. Wolf
(Springer-Verlag, Berlin, Heidelberg). 7 pages, including 5 figures. A
full-resolution version of fig 4 is available in the version at
http://www.mpia-hd.mpg.de/theory/preprints.html#maclo
Dynamical models of Kuiper Belt dust in the inner and outer Solar System
We report several results related to the dynamical evolution of dust produced
in the Kuiper Belt (KB). We show that its particle size frequency distribution
in space is greatly changed from the distribution at production, as a results
of the combined effects of radiation forces and the perturbations of the
planets. We estimate the contribution of KB dust to the zodiacal cloud by
calculating the radial profile of its number density near the ecliptic. We also
study the contribution of KB dust to the population of interplanetary dust
particles (IDPs) collected at Earth, by calculating geocentric encounter
velocities and capture rates. Our models show, in contrast with previous
studies, that KB dust grains on Earth-crossing orbits have high eccentricities
and inclinations and, therefore, their encounter velocities are similar to
those of cometary grains and not to asteroidal grains. We estimate that at most
25% in number of captured IDPs have cometary or KB origin; the KB contribution
may be as low as 1%-2%. We present the velocity field of KB dust throughout the
solar system; this, together with the number density radial profile, is
potentially useful for planning spacecraft missions to the outer solar system.Comment: 25 pages (pre-print format), including 5 figures. Published in AJ
(2003
The Cosmic Infrared Background at 1.25 microns and 2.2 microns using DIRBE and 2MASS: a contribution not due to galaxies ?
Using the 2MASS 2nd Incremental Data Release and the Zodiacal-Subtracted
Mission Average maps of COBE/DIRBE, we estimate the cosmic background in the J
(1.25 micron) and K (2.2 microns) bands using selected areas representing 550
square degrees of sky. We find a J background of 22.9 \pm 7.0 kJy/sr (54.0 \pm
16.8 nW/m2/sr) and a K background of 20.4 \pm 4.9 kJy/sr (27.8 \pm 6.7
nW/m2/sr). This large scale study shows that the main uncertainty comes from
the residual zodiacal emission. The cosmic background we obtain is
significantly higher than integrated galaxy counts (3.6 \pm 0.8 kJy/sr and 5.3
\pm 1.2 kJy/sr for J and K, respectively), suggesting either an increase of the
galaxy luminosity function for magnitudes fainter than 30 or the existence of
another contribution to the cosmic background from primeval stars, black holes,
or relic particle decay.Comment: 20 pages, 6 figures, accepted in Ap
On the Dynamical Overstability of Radiative Blast Waves: The Atomic Physics of Shock Stability
Atomic physics calculations of radiative cooling are used to calculate
criteria for the overstability of radiating shocks. Our calculations explain
the measurement of shock overstability by Grun et al. and explain why the
overstability was not observed in other experiments. The methodology described
here can be especially useful in astrophysical situations where the relevant
properties leading to an overstability can be measured spectroscopically, but
the effective adiabatic index is harder to determine.Comment: 11 pages including 3 figures, accepted for publication in Physical
Review Letter
Well posedness of an isothermal diffusive model for binary mixtures of incompressible fluids
We consider a model describing the behavior of a mixture of two
incompressible fluids with the same density in isothermal conditions. The model
consists of three balance equations: continuity equation, Navier-Stokes
equation for the mean velocity of the mixture, and diffusion equation
(Cahn-Hilliard equation). We assume that the chemical potential depends upon
the velocity of the mixture in such a way that an increase of the velocity
improves the miscibility of the mixture. We examine the thermodynamic
consistence of the model which leads to the introduction of an additional
constitutive force in the motion equation. Then, we prove existence and
uniqueness of the solution of the resulting differential problem
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Combined transcriptomic-(1)H NMR metabonomic study reveals yhat monoethylhexyl phthalate stimulates adipogenesis and glyceroneogenesis in human adipocytes
Adipose tissue is a major storage site for lipophilic environmental contaminants. The environmental metabolic disruptor hypothesis postulates that some pollutants can promote obesity or metabolic disorders by activating nuclear receptors involved in the control of energetic homeostasis. In this context, monoethylhexyl phthalate (MEHP) is of particular concern since it was shown to activate the peroxisome proliferator-activated receptor γ (PPARγ) in 3T3-L1 murine preadipocytes. In the present work, we used an untargeted, combined transcriptomic-(1)H NMR-based metabonomic approach to describe the overall effect of MEHP on primary cultures of human subcutaneous adipocytes differentiated in vitro. MEHP stimulated rapidly and selectively the expression of genes involved in glyceroneogenesis, enhanced the expression of the cytosolic phosphoenolpyruvate carboxykinase, and reduced fatty acid release. These results demonstrate that MEHP increased glyceroneogenesis and fatty acid reesterification in human adipocytes. A longer treatment with MEHP induced the expression of genes involved in triglycerides uptake, synthesis, and storage; decreased intracellular lactate, glutamine, and other amino acids; increased aspartate and NAD, and resulted in a global increase in triglycerides. Altogether, these results indicate that MEHP promoted the differentiation of human preadipocytes to adipocytes. These mechanisms might contribute to the suspected obesogenic effect of MEHP
Weak pairwise correlations imply strongly correlated network states in a neural population
Biological networks have so many possible states that exhaustive sampling is
impossible. Successful analysis thus depends on simplifying hypotheses, but
experiments on many systems hint that complicated, higher order interactions
among large groups of elements play an important role. In the vertebrate
retina, we show that weak correlations between pairs of neurons coexist with
strongly collective behavior in the responses of ten or more neurons.
Surprisingly, we find that this collective behavior is described quantitatively
by models that capture the observed pairwise correlations but assume no higher
order interactions. These maximum entropy models are equivalent to Ising
models, and predict that larger networks are completely dominated by
correlation effects. This suggests that the neural code has associative or
error-correcting properties, and we provide preliminary evidence for such
behavior. As a first test for the generality of these ideas, we show that
similar results are obtained from networks of cultured cortical neurons.Comment: Full account of work presented at the conference on Computational and
Systems Neuroscience (COSYNE), 17-20 March 2005, in Salt Lake City, Utah
(http://cosyne.org
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