3,717 research outputs found
Champagne Flutes and Brandy Snifters: Modelling Protostellar Outflow-Cloud Chemical Interfaces
A rich variety of molecular species has now been observed towards hot cores
in star forming regions and in the interstellar medium. An increasing body of
evidence from millimetre interferometers suggests that many of these form at
the interfaces between protostellar outflows and their natal molecular clouds.
However, current models have remained unable to explain the origin of the
observational bias towards wide-angled "brandy snifter" shaped outflows over
narrower "champagne flute" shapes in carbon monoxide imaging. Furthermore,
these wide-angled systems exhibit unusually high abundances of the molecular
ion HCO. We present results from a chemo-dynamic model of such regions
where a rich chemistry arises naturally as a result of turbulent mixing between
cold, dense molecular gas and the hot, ionized outflow material. The injecta
drives a rich and rapid ion-neutral chemistry in qualitative and quantitative
agreement with the observations. The observational bias towards wide-angled
outflows is explained naturally by the geometry-dependent ion injection rate
causing rapid dissociation of CO in the younger systems.Comment: Accepted to MNRAS. 12 pages, 8 Figure
Laser driven launch vehicles for continuous access to space
The availability of megawatt laser systems in the next century will make laser launch systems from ground to orbit feasible and useful. Systems studies indicate launch capabilities of 1 ton payload per gigawatt laser power. Recent research in ground to orbit laser propulsion has emphasized laser supported detonation wave thrusters driven by repetitively pulsed infrared lasers. In this propulsion concept each laser repetition cycle consists of two pulses. A lower energy first pulse is used to vaporize a small amount of solid propellant and then after a brief expansion period, a second and higher energy laser pulse is used to drive a detonation wave through the expanded vapor. The results are reported of numerical studies comparing the detonation wave properties of various candidate propellants, and the simulation of thruster performance under realistic conditions. Experimental measurements designed to test the theoretical predictions are also presented. Measurements are discussed of radiance and opacity in absorption waves, and mass loss and momentum transfer. These data are interpreted in terms of specific impulse and energy conversion efficiency
Plasduino: an inexpensive, general purpose data acquisition framework for educational experiments
Based on the Arduino development platform, Plasduino is an open-source data
acquisition framework specifically designed for educational physics
experiments. The source code, schematics and documentation are in the public
domain under a GPL license and the system, streamlined for low cost and ease of
use, can be replicated on the scale of a typical didactic lab with minimal
effort. We describe the basic architecture of the system and illustrate its
potential with some real-life examples.Comment: 11 pages, 10 figures, presented at the XCIX conference of the
Societ\`a Italiana di Fisic
Post Launch Calibration and Testing of the Advanced Baseline Imager on the GOES-R Satellite
The Geostationary Operational Environmental Satellite R (GOES-R) series is the planned next generation of operational weather satellites for the United State's National Oceanic and Atmospheric Administration. The first launch of the GOES-R series is planned for October 2016. The GOES-R series satellites and instruments are being developed by the National Aeronautics and Space Administration (NASA). One of the key instruments on the GOES-R series is the Advance Baseline Imager (ABI). The ABI is a multi-channel, visible through infrared, passive imaging radiometer. The ABI will provide moderate spatial and spectral resolution at high temporal and radiometric resolution to accurately monitor rapidly changing weather. Initial on-orbit calibration and performance characterization is crucial to establishing baseline used to maintain performance throughout mission life. A series of tests has been planned to establish the post launch performance and establish the parameters needed to process the data in the Ground Processing Algorithm. The large number of detectors for each channel required to provide the needed temporal coverage presents unique challenges for accurately calibrating ABI and minimizing striping. This paper discusses the planned tests to be performed on ABI over the six-month Post Launch Test period and the expected performance as it relates to ground tests
Characterization of the stretched exponential trap-time distributions in one-dimensional coupled map lattices
Stretched exponential distributions and relaxation responses are encountered
in a wide range of physical systems such as glasses, polymers and spin glasses.
As found recently, this type of behavior occurs also for the distribution
function of certain trap time in a number of coupled dynamical systems. We
analyze a one-dimensional mathematical model of coupled chaotic oscillators
which reproduces an experimental set-up of coupled diode-resonators and
identify the necessary ingredients for stretched exponential distributions.Comment: 8 pages, 8 figure
Liquefaction source layer for sand blows induced by the 2016 megathrust earthquake (Mw 7.8) in Ecuador (Boca de Briceño)
Numerous sand boils were generated in the alluvial plain at the mouth of the Rio Brice˜no valley (Ecuador) during
the Mw 7.8 earthquake of April 2016. The area is characterized by a series of raised marine terraces formed as a
consequence of the rapid tectonic coastal uplift during the Quaternary. Boreholes and geotechnical investigations
were carried during post-earthquake surveys and for the purpose of mitigating the liquefaction effects. Five
lithological units were identified at a site of embankment, which represented continental-marine and transitional
sedimentation since the Last Glacial Maximum. A comprehensive study of texture and petrographic composition
of sand boils has been performed and compared with sandy silts and silty sands of the buried sedimentary
sequence in order to identify the source levels for liquefaction. The petrographic components, in particular the
low content of bioclasts and carbonate fragments of the sand boils, allow to pinpoint a source layer made up of
fine-grained silty sands located between 2 and 4.5 m depth (Unit 2) whereas the deeper marine sands, richer in
bioclasts, were not involved. The results support the idea that earthquake-induced liquefaction phenomena are
not restricted to clean sands and well-sorted deposits, but may affect sand layers with significant amount of nonplastic
silt
Low energy polarization sensitivity of the Gas Pixel Detector
An X-ray photoelectric polarimeter based on the Gas Pixel Detector has been
proposed to be included in many upcoming space missions to fill the gap of
about 30 years from the first (and to date only) positive measurement of
polarized X-ray emission from an astrophysical source. The estimated
sensitivity of the current prototype peaks at an energy of about 3 keV, but the
lack of readily available polarized sources in this energy range has prevented
the measurement of detector polarimetric performances.
In this paper we present the measurement of the Gas Pixel Detector
polarimetric sensitivity at energies of a few keV and the new, light, compact
and transportable polarized source that was devised and built to this aim.
Polarized photons are produced, from unpolarized radiation generated with an
X-ray tube, by means of Bragg diffraction at nearly 45 degrees.
The employment of mosaic graphite and flat aluminum crystals allow the
production of nearly completely polarized photons at 2.6, 3.7 and 5.2 keV from
the diffraction of unpolarized continuum or line emission. The measured
modulation factor of the Gas Pixel Detector at these energies is in good
agreement with the estimates derived from a Monte Carlo software, which was up
to now employed for driving the development of the instrument and for
estimating its low energy sensitivity. In this paper we present the excellent
polarimetric performance of the Gas Pixel Detector at energies where the peak
sensitivity is expected. These measurements not only support our previous
claims of high sensitivity but confirm the feasibility of astrophysical X-ray
photoelectric polarimetry.Comment: 15 pages, 12 figures. Accepted for publication in NIM
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