26,184 research outputs found
Hydrodynamic crystals: collective dynamics of regular arrays of spherical particles in a parallel-wall channel
Simulations of over hydrodynamically coupled solid spheres are
performed to investigate collective motion of linear trains and regular square
arrays of particles suspended in a fluid bounded by two parallel walls. Our
novel accelerated Stokesian-dynamics algorithm relies on simplifications
associated with the Hele--Shaw asymptotic far-field form of the flow scattered
by the particles. The simulations reveal propagation of particle-displacement
waves, deformation and rearrangements of a particle lattice, propagation of
dislocation defects in ordered arrays, and long-lasting coexistence of ordered
and disordered regions.Comment: 4 pages 6 figure
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Endolithic colonization of fluid inclusion trails in mineral grains
Many scenarios for the colonization of planetary surfaces by microbial life involve endoliths. This study records microbial mass along fluid inclusion trails (healed microfractures) in quartz grains
Feasibility of an Incoherent-scatter Radar Aboard the Space Shuttle
The results of a preliminary study to investigate the feasibility of conducting an incoherent scatter radar experiment on board the space shuttle are presented. The results indicate that such an experiment is technically feasible. The more difficult questions to answer are whether the system can be made flexible enough to justify the problems and costs involved. The design parameters and the tradeoffs that are available in the consideration of these questions are evaluated. Some of the more serious limitations pertain to: (1) the presence of ground clutter and F region auroral clutter; (2) available average power; (3) weight and volume associated with required antenna size, transmitter, and energy storage devices; and (4) antenna breakdown associated with high power transmitter problems
Isospin and density dependences of nuclear matter symmetry energy coefficients II
Symmetry energy coefficients of explicitly isospin asymmetric nuclear matter
at variable densities (from .5 up to 2 ) are studied as
generalized screening functions. An extended stability condition for asymmetric
nuclear matter is proposed. We find the possibility of obtaining stable
asymmetric nuclear matter even in some cases for which the symmetric nuclear
matter limit is unstable. Skyrme-type forces are extensively used in analytical
expressions of the symmetry energy coefficients derived as generalized
screening functions in the four channels of the particle hole interaction
producing alternative behaviors at different and (respectively the
density and the asymmetry coefficient). The spin and spin-isospin coefficients,
with corrections to the usual Landau Migdal parameters, indicate the
possibility of occurring instabilities with common features depending on the
nuclear density and n-p asymmetry. Possible relevance for high energy heavy
ions collisions and astrophysical objects is discussed.Comment: 16 pages (latex) plus twelve figures in four eps files, to be
published in I.J.M.P.
High-overtone Bulk-Acoustic Resonator gravimetric sensitivity: towards wideband acoustic spectroscopy
In the context of direct detection sensors with compact dimensions, we
investigate the gravimetric sensitivity of High-overtone Bulk Acoustic
Resonators, through modeling of their acoustic characteristics and experiment.
The high frequency characterizing such devices is expected to induce a
significant effect when the acoustic field boundary conditions are modified by
a thin adlayer. Furthermore, the multimode spectral characteristics is
considered for wideband acoustic spectroscopy of the adlayer, once the
gravimetric sensitivity dependence of the various overtones is established.
Finally, means of improving the gravimetric sensitivity by confining the
acoustic field in a low acoustic-impedance layer is theoretically established.Comment: 9 pages, 10 figures in J. Appl. Phys. 201
Detailed Spectral Modeling of a 3-D Pulsating Reverse Detonation Model: Too Much Nickel
We calculate detailed NLTE synthetic spectra of a Pulsating Reverse
Detonation (PRD) model, a novel explosion mechanism for Type Ia supernovae.
While the hydro models are calculated in 3-D, the spectra use an angle averaged
hydro model and thus some of the 3-D details are lost, but the overall average
should be a good representation of the average observed spectra. We study the
model at 3 epochs: maximum light, seven days prior to maximum light, and 5 days
after maximum light. At maximum the defining Si II feature is prominent, but
there is also a prominent C II feature, not usually observed in normal SNe Ia
near maximum. We compare to the early spectrum of SN 2006D which did show a
prominent C II feature, but the fit to the observations is not compelling.
Finally we compare to the post-maximum UV+optical spectrum of SN 1992A. With
the broad spectral coverage it is clear that the iron-peak elements on the
outside of the model push too much flux to the red and thus the particular PRD
realizations studied would be intrinsically far redder than observed SNe Ia. We
briefly discuss variations that could improve future PRD models.Comment: 15 pages, 4 figures, submitted to Ap
Analytic Inversion of Emission Lines of Arbitrary Optical Depth for the Structure of Supernova Ejecta
We derive a method for inverting emission line profiles formed in supernova
ejecta. The derivation assumes spherical symmetry and homologous expansion
(i.e., ), is analytic, and even takes account of occultation by
a pseudo-photosphere. Previous inversion methods have been developed which are
restricted to optically thin lines, but the particular case of homologous
expansion permits an analytic result for lines of {\it arbitrary} optical
depth. In fact, we show that the quantity that is generically retrieved is the
run of line intensity with radius in the ejecta. This result is
quite general, and so could be applied to resonance lines, recombination lines,
etc. As a specific example, we show how to derive the run of (Sobolev) optical
depth with radius in the case of a pure resonance scattering
emission line.Comment: 6 pages, no figures, to appear in Astrophysical Journal Letters,
requires aaspp4.sty to late
Penetration rates over 30 years in the space age
Experimental data from spacecraft providing impact penetration rates and cratering for metallic targets is reviewed. Data includes NASA Explorers 16 and 23 and the Pegasus series, the second US-UK satellite Ariel 2, Space Shuttle STS-3 (MFE), recovered surfaces on Solar Max Satellite, The Long Duration Exposure Facility (LDEF) and EuReCa TiCCE. Factors concerning exposure to the environment are considered and, especially, material properties which affect the penetration resistance. Reference to a common material, Aluminum alloy 2024-T3, is effected and the data then compared to define firstly an average impact flux over the period. The data is examined, in the context of possible satellite and space debris growth rates, to determine the constancy of the flux. This also provides strong constraints on the current space debris component. It is found that the impact data are consistent with domination by natural meteoroid sources. Growth rates are not evident within the period 1980-1990 and Eureca TiCCE fluxes in 1993, for particles penetrating foils of around 10 microns thickness, supports the constancy of the flux. At larger dimensions the 1993 Eureca TiCCE fluxes show an 8-fold increase but this is considered not inconsistent with the selective exposure to meteoroid streams of a satellite stabilized in heliocentric co-ordinates for an 11 month period
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