2,654 research outputs found
Cryogenic systems for the large deployable reflector
There are five technologies which may have application for Large Deployable Reflector (LDR), one passive and four active. In order of maturity, they are passive stored cryogen systems, and mechanical, sorption, magnetic, and pulse-tube refrigerators. In addition, deep space radiators will be required to reject the heat of the active systems, and may be useful as auxiliary coolers for the stored cryogen systems. Hybrid combinations of these technologies may well be more efficient than any one alone, and extensive system studies will be required to determine the best trade-offs. Stored cryogen systems were flown on a number of missions. The systems are capable of meeting the temperature requirements of LDR. The size and weight of stored cryogen systems are proportional to heat load and, as a result, are applicable only if the low-temperature heat load can be kept small. Systems using chemisorption and physical adsorption for compressors and pumps have received considerable attention in the past few years. Systems based on adiabatic demagnetization of paramagnetic salts were used for refrigeration for many years. Pulse-tube refrigerators were recently proposed which show relatively high efficiency for temperatures in the 60 to 80 K range. The instrument heat loads and operating temperatures are critical to the selection and design of the cryogenic system. Every effort should be made to minimize heat loads, raise operating temperatures, and to define these precisely. No one technology is now ready for application to LDR. Substantial development efforts are underway in all of the technologies and should be monitored and advocated. Magnetic and pulse-tube refrigerators have high potential
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William C. Pilkington, wife, and three children died in airplane cras
On a class of second-order PDEs admitting partner symmetries
Recently we have demonstrated how to use partner symmetries for obtaining
noninvariant solutions of heavenly equations of Plebanski that govern heavenly
gravitational metrics. In this paper, we present a class of scalar second-order
PDEs with four variables, that possess partner symmetries and contain only
second derivatives of the unknown. We present a general form of such a PDE
together with recursion relations between partner symmetries. This general PDE
is transformed to several simplest canonical forms containing the two heavenly
equations of Plebanski among them and two other nonlinear equations which we
call mixed heavenly equation and asymmetric heavenly equation. On an example of
the mixed heavenly equation, we show how to use partner symmetries for
obtaining noninvariant solutions of PDEs by a lift from invariant solutions.
Finally, we present Ricci-flat self-dual metrics governed by solutions of the
mixed heavenly equation and its Legendre transform.Comment: LaTeX2e, 26 pages. The contents change: Exact noninvariant solutions
of the Legendre transformed mixed heavenly equation and Ricci-flat metrics
governed by solutions of this equation are added. Eq. (6.10) on p. 14 is
correcte
An Analysis of the Chemical Processes in the Smoke Plume from a Savanna Fire
[1] Photochemistry in young plumes from vegetation fires significantly transforms the initial fire emissions within the first hour after the emissions are injected into the atmosphere. Here we present an investigation of field measurements obtained in a smoke plume from a prescribed savanna fire during the SAFARI 2000 field experiment using a detailed photochemical box-dilution model. The dilution used in the model simulations was constrained by measurements of chemically passive tracers (e.g., CO) near and downwind of the fire. The emissions of the dominant carbonaceous compounds, including oxygenated ones, were taken into account. The field measurements revealed significant production of ozone and acetic acid in the gas phase. The photochemical model simulations also predict ozone production, but significantly less than the measurements. The underestimation of the ozone production in the model simulations is likely caused by shortcomings of our current understanding of ozone photochemistry under the polluted conditions in this young smoke plume. Several potential reasons for this discrepancy are discussed. One possible cause could be the neglect of unmeasured emissions or surface reactions of NO2 with methanol or other hydrocarbons. In contrast to the field measurements, no significant production of acetic acid was simulated by the model. We know of no gas-phase reactions that cause the production of acetic acid on the timescale considered here. Though many processes were well-simulated by the model, there is a need for further research on some key photochemical processes within young plumes from biomass burning and the potential interactions between gas and the particulate phases. These fundamental photochemical processes may also be of importance in other polluted environments
Early Ultraviolet, Optical and X-Ray Observations of the Type IIP SN 2005cs in M51 with Swift
We report early photospheric-phase observations of the Type IIP Supernova
(SN) 2005cs obtained by Swift's Ultraviolet-Optical and X-Ray Telescopes.
Observations started within two days of discovery and continued regularly for
three weeks. During this time the V-band magnitude remained essentially
constant, while the UV was initially bright but steadily faded until below the
brightness of an underlying UV-bright HII region. This UV decay is similar to
SNe II observed by the International Ultraviolet Explorer. UV grism spectra
show the P-Cygni absorption of MgII 2798A, indicating a photospheric origin of
the UV flux. Based on non-LTE model atmosphere calculations with the CMFGEN
code, we associate the rapid evolution of the UV flux with the cooling of the
ejecta, the peak of the spectral energy distribution (SED) shifting from ~700A
on June 30th to ~1200A on July 5th. Furthermore, the corresponding
recombination of the ejecta, e.g., the transition from FeIII to FeII, induces
considerable strengthening of metal line-blanketing at and above the
photosphere, blocking more effectively this fading UV flux. SN2005cs was not
detected in X-rays, and the upper limit to the X-ray luminosity yields a limit
to the mass loss rate of the progenitor of about 10^-5 solar masses per year.
Overall, Swift presents a unique opportunity to capture the early and fast
evolution of Type II SNe in the UV, providing additional constraints on the
reddening, the SED shortward of 4000A, and the ionization state and temperature
of the photon-decoupling regions.Comment: 15 pages, 6 figures. Accepted for publication by Astrophysical
Journa
Dynamic Network Centrality Summarizes Learning in the Human Brain
We study functional activity in the human brain using functional Magnetic
Resonance Imaging and recently developed tools from network science. The data
arise from the performance of a simple behavioural motor learning task.
Unsupervised clustering of subjects with respect to similarity of network
activity measured over three days of practice produces significant evidence of
`learning', in the sense that subjects typically move between clusters (of
subjects whose dynamics are similar) as time progresses. However, the high
dimensionality and time-dependent nature of the data makes it difficult to
explain which brain regions are driving this distinction. Using network
centrality measures that respect the arrow of time, we express the data in an
extremely compact form that characterizes the aggregate activity of each brain
region in each experiment using a single coefficient, while reproducing
information about learning that was discovered using the full data set. This
compact summary allows key brain regions contributing to centrality to be
visualized and interpreted. We thereby provide a proof of principle for the use
of recently proposed dynamic centrality measures on temporal network data in
neuroscience
Absolute polarization angle calibration using polarized diffuse Galactic emission observed by BICEP
We present a method of cross-calibrating the polarization angle of a
polarimeter using BICEP Galactic observations. \bicep\ was a ground based
experiment using an array of 49 pairs of polarization sensitive bolometers
observing from the geographic South Pole at 100 and 150 GHz. The BICEP
polarimeter is calibrated to +/-0.01 in cross-polarization and less than +/-0.7
degrees in absolute polarization orientation. BICEP observed the temperature
and polarization of the Galactic plane (R.A= 100 degrees ~ 270 degrees and Dec.
= -67 degrees ~ -48 degrees). We show that the statistical error in the 100 GHz
BICEP Galaxy map can constrain the polarization angle offset of WMAP Wband to
0.6 degrees +\- 1.4 degrees. The expected 1 sigma errors on the polarization
angle cross-calibration for Planck or EPIC are 1.3 degrees and 0.3 degrees at
100 and 150 GHz, respectively. We also discuss the expected improvement of the
BICEP Galactic field observations with forthcoming BICEP2 and Keck
observations.Comment: 13 pages, 10 figures and 2 tables. To appear in Proceedings of SPIE
Astronomical Telescopes and Instrumentation 201
Rheological constitutive equation for model of soft glassy materials
We solve exactly and describe in detail a simplified scalar model for the low
frequency shear rheology of foams, emulsions, slurries, etc. [P. Sollich, F.
Lequeux, P. Hebraud, M.E. Cates, Phys. Rev. Lett. 78, 2020 (1997)]. The model
attributes similarities in the rheology of such ``soft glassy materials'' to
the shared features of structural disorder and metastability. By focusing on
the dynamics of mesoscopic elements, it retains a generic character.
Interactions are represented by a mean-field noise temperature x, with a glass
transition occurring at x=1 (in appropriate units). The exact solution of the
model takes the form of a constitutive equation relating stress to strain
history, from which all rheological properties can be derived. For the linear
response, we find that both the storage modulus G' and the loss modulus G''
vary with frequency as \omega^{x-1} for 1<x<2, becoming flat near the glass
transition. In the glass phase, aging of the moduli is predicted. The steady
shear flow curves show power law fluid behavior for x<2, with a nonzero yield
stress in the glass phase; the Cox-Merz rule does not hold in this
non-Newtonian regime. Single and double step strains further probe the
nonlinear behavior of the model, which is not well represented by the BKZ
relation. Finally, we consider measurements of G' and G'' at finite strain
amplitude \gamma. Near the glass transition, G'' exhibits a maximum as \gamma
is increased in a strain sweep. Its value can be strongly overestimated due to
nonlinear effects, which can be present even when the stress response is very
nearly harmonic. The largest strain \gamma_c at which measurements still probe
the linear response is predicted to be roughly frequency-independent.Comment: 24 pages, REVTeX, uses multicol, epsf and amssymp; 20 postscript
figures (included). Minor changes to text (relation to mode coupling theory,
update on recent foam simulations etc.) and figures (emphasis on low
frequency regime); typos corrected and reference added. Version to appear in
Physical Review
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