483 research outputs found
Debris and micrometeorite impact measurements in the laboratory
A method was developed to simulate space debris in the laboratory. This method, which is an outgrowth of research in inertial confinement fusion (ICF), uses laser ablation to accelerate material. Using this method, single 60 micron aluminum spheres were accelerated to 15 km/sec and larger 500 micron aluminum spheres were accelerated to 2 km/sec. Also, many small (less than 10 micron diameter) irregularly shaped particles were accelerated to speeds of 100 km/sec
Probing Interstellar Dust With Space-Based Coronagraphs
We show that space-based telescopes such as the proposed Terrestrial Planet
Finder Coronagraph will be able to detect the light scattered by the
interstellar grains along lines of sight passing near stars in our Galaxy. The
relative flux of the scattered light within one arcsecond of a star at 100 pc
in a uniform interstellar medium of 0.1 H atoms cm^-3 is about 10^-7. The halo
increases in strength with the distance to the star and is unlikely to limit
the coronagraphic detection of planets around the nearest stars. Grains passing
within 100 AU of Sun-like stars are deflected by radiation, gravity and
magnetic forces, leading to features in the scattered light that can
potentially reveal the strength of the stellar wind, the orientation of the
stellar magnetic field and the relative motion between the star and the
surrounding interstellar medium.Comment: Accepted for publication in ApJ Supplement
Athermal annealing of Si-implanted GaAs and InP
GaAs and InP crystals ion implanted with Si were athermally annealed by exposing each crystal at
a spot of ~2 mm diameter to a high-intensity 1.06 Ī¼m wavelength pulsed laser radiation with ~4
J pulse energy for 35 ns in a vacuum chamber. As a result a crater is formed at the irradiated spot.
The crater is surrounded by a dark-colored ring-shaped region which is annealed by mechanical
energy generated by rapidly expanding hot plasma that formed on the exposed spot. The electrical
characteristics of this annealed region are comparable to those of a halogen-lamp annealed sample.
No redistribution of impurities due to transient diffusion is observed in the implant tail region. In
x-ray diffraction measurements, a high angle side satellite peak due to lattice strain was observed in
the crater and near crater regions of the athermally annealed sample in addition to the main Bragg
peak that corresponds to the pristine sample. This high angle side satellite peak is not observed in
regions away from the crater (ā„5 mm from the center of the crater in GaAs)
All-optical trion generation in single walled carbon nanotubes
We present evidence of all optical trion generation and emission in undoped
single walled carbon nanotubes (SWCNTs). Luminescence spectra, recorded on
individual SWCNTs over a large CW excitation intensity range, show trion
emission peaks red-shifted with respect to the bright exciton peak. Clear
chirality dependence is observed for 22 separate SWCNT species, allowing for
determination of electron-hole exchange interaction and trion binding energy
contributions. Luminescence data together with ultrafast pump probe experiments
on chirality sorted bulk samples suggest that exciton-exciton annihilation
processes generate dissociated carriers that allow for trion creation upon a
subsequent photon absorption event.Comment: 13 pages, 4 figure
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
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
Free induction signal from biexcitons and bound excitons
A theory of the free induction signal from biexcitons and bound excitons is
presented. The simultaneous existence of the exciton continuum and a bound
state is shown to result in a new type of time dependence of the free
induction. The optically detected signal increases in time and oscillates with
increasing amplitude until damped by radiative or dephasing processes.
Radiative decay is anomalously fast and can result in strong picosecond pulses.
The expanding area of a coherent exciton polarization (inflating antenna),
produced by the exciting pulse, is the underlying physical mechanism. The
developed formalism can be applied to different biexciton transients.Comment: RevTeX, 20 p. + 2 ps fig. To appear in Phys. Rev. B1
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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 Ī³ (PPARĪ³) 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
Analysis of "Midnight" Tracks in the Stardust Interstellar Dust Collector: Possible Discovery of a Contemporary Interstellar Dust Grain
In January 2006, the Stardust sample return capsule returned to Earth bearing the first solid samples from a primitive solar system body, Comet 81P/Wild2, and a collector dedicated to the capture and return of contemporary interstellar dust. Both collectors were approximately 0.1m(exp 2) in area and were composed of aerogel tiles (85% of the collecting area) and aluminum foils. The Stardust Interstellar Dust Collector (SIDC) was exposed to the interstellar dust stream for a total exposure factor of 20 m(exp 2) day. The Stardust Interstellar Preliminary Examination (ISPE) is a three-year effort to characterize the collection using nondestructive techniques
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