19,938 research outputs found
Fast accretion of small planetesimals by protoplanetary cores
We explore the dynamics of small planetesimals coexisting with massive
protoplanetary cores in a gaseous nebula. Gas drag strongly affects the motion
of small bodies leading to the decay of their eccentricities and inclinations,
which are excited by the gravity of protoplanetary cores. Drag acting on larger
( km), high velocity planetesimals causes a mere reduction of their
average random velocity. By contrast, drag qualitatively changes the dynamics
of smaller ( km), low velocity objects: (1) small planetesimals
sediment towards the midplane of the nebula forming vertically thin subdisk;
(2) their random velocities rapidly decay between successive passages of the
cores and, as a result, encounters with cores typically occur at the minimum
relative velocity allowed by the shear in the disk. This leads to a drastic
increase in the accretion rate of small planetesimals by the protoplanetary
cores, allowing cores to grow faster than expected in the simple oligarchic
picture, provided that the population of small planetesimals contains more than
roughly 1% of the solid mass in the nebula. Fragmentation of larger
planetesimals ( km) in energetic collisions triggered by the
gravitational scattering by cores can easily channel this amount of material
into small bodies on reasonable timescales ( Myr in the outer Solar
System), providing a means for the rapid growth (within several Myr at 30 AU)
of rather massive protoplanetary cores. Effects of inelastic collisions between
planetesimals and presence of multiple protoplanetary cores are discussed.Comment: 17 pages, 8 figures, additional clarifications, 1 more figure and
table adde
Some economic benefits of a synchronous earth observatory satellite
An analysis was made of the economic benefits which might be derived from reduced forecasting errors made possible by data obtained from a synchronous satellite system which can collect earth observation and meteorological data continuously and on demand. User costs directly associated with achieving benefits are included. In the analysis, benefits were evaluated which might be obtained as a result of improved thunderstorm forecasting, frost warning, and grain harvest forecasting capabilities. The anticipated system capabilities were used to arrive at realistic estimates of system performance on which to base the benefit analysis. Emphasis was placed on the benefits which result from system forecasting accuracies. Benefits from improved thunderstorm forecasts are indicated for the construction, air transportation, and agricultural industries. The effects of improved frost warning capability on the citrus crop are determined. The benefits from improved grain forecasting capability are evaluated in terms of both U.S. benefits resulting from domestic grain distribution and U.S. benefits from international grain distribution
Formation of laser plasma channels in a stationary gas
The formation of plasma channels with nonuniformity of about +- 3.5% has been
demonstrated. The channels had a density of 1.2x10^19 cm-3 with a radius of 15
um and with length >= 2.5 mm. The channels were formed by 0.3 J, 100 ps laser
pulses in a nonflowing gas, contained in a cylindrical chamber. The laser beam
passed through the chamber along its axis via pinholes in the chamber walls. A
plasma channel with an electron density on the order of 10^18 - 10^19 cm-3 was
formed in pure He, N2, Ar, and Xe. A uniform channel forms at proper time
delays and in optimal pressure ranges, which depend on the sort of gas. The
influence of the interaction of the laser beam with the gas leaking out of the
chamber through the pinholes was found insignificant. However, the formation of
an ablative plasma on the walls of the pinholes by the wings of the radial
profile of the laser beam plays an important role in the plasma channel
formation and its uniformity. A low current glow discharge initiated in the
chamber slightly improves the uniformity of the plasma channel, while a high
current arc discharge leads to the formation of overdense plasma near the front
pinhole and further refraction of the laser beam. The obtained results show the
feasibility of creating uniform plasma channels in non-flowing gas targets.Comment: 15 pages, 7 figures, submitted to Physics of Plasma
Microgravity Combustion Diagnostics Workshop
Through the Microgravity Science and Applications Division (MSAD) of the Office of Space Science and Applications (OSSA) at NASA Headquarters, a program entitled, Advanced Technology Development (ATD) was promulgated with the objective of providing advanced technologies that will enable the development of future microgravity science and applications experimental flight hardware. Among the ATD projects one, Microgravity Combustion Diagnostics (MCD), has the objective of developing advanced diagnostic techniques and technologies to provide nonperturbing measurements of combustion characteristics and parameters that will enhance the scientific integrity and quality of microgravity combustion experiments. As part of the approach to this project, a workshop was held on July 28 and 29, 1987, at the NASA Lewis Research Center. A small group of laser combustion diagnosticians met with a group of microgravity combustion experimenters to discuss the science requirements, the state-of-the-art of laser diagnostic technology, and plan the direction for near-, intermediate-, and long-term programs. This publication describes the proceedings of that workshop
Commercialization of the land remote sensing system: An examination of mechanisms and issues
In September 1982 the Secretary of Commerce was authorized (by Title II of H.R. 5890 of the 97th Congress) to plan and provide for the management and operation of the civil land remote sensing satellite systems, to provide for user fees, and to plan for the transfer of the ownership and operation of future civil operational land remote sensing satellite systems to the private sector. As part of the planning for transfer, a number of approaches were to be compared including wholly private ownership and operation of the system by an entity competitively selected, mixed government/private ownership and operation, and a legislatively-chartered privately-owned corporation. The results of an analysis and comparison of a limited number of financial and organizational approaches for either transfer of the ownership and operation of the civil operational land remote sensing program to the private sector or government retention are presented
The Energy Operator for a Model with a Multiparametric Infinite Statistics
In this paper we consider energy operator (a free Hamiltonian), in the
second-quantized approach, for the multiparameter quon algebras:
with
any hermitian matrix of deformation parameters. We obtain
an elegant formula for normally ordered (sometimes called Wick-ordered) series
expansions of number operators (which determine a free Hamiltonian). As a main
result (see Theorem 1) we prove that the number operators are given, with
respect to a basis formed by "generalized Lie elements", by certain normally
ordered quadratic expressions with coefficients given precisely by the entries
of the inverses of Gram matrices of multiparticle weight spaces. (This settles
a conjecture of two of the authors (S.M and A.P), stated in [8]). These Gram
matrices are hermitian generalizations of the Varchenko's matrices, associated
to a quantum (symmetric) bilinear form of diagonal arrangements of hyperplanes
(see [12]). The solution of the inversion problem of such matrices in [9]
(Theorem 2.2.17), leads to an effective formula for the number operators
studied in this paper. The one parameter case, in the monomial basis, was
studied by Zagier [15], Stanciu [11] and M{\o}ller [6].Comment: 24 pages. accepted in J. Phys. A. Math. Ge
The chameleon groups of Richard J. Thompson: automorphisms and dynamics
The automorphism groups of several of Thompson's countable groups of
piecewise linear homeomorphisms of the line and circle are computed and it is
shown that the outer automorphism groups of these groups are relatively small.
These results can be interpreted as stability results for certain structures of
PL functions on the circle. Machinery is developed to relate the structures on
the circle to corresponding structures on the line
Planetary geology: Impact processes on asteroids
The fundamental geological and geophysical properties of asteroids were studied by theoretical and simulation studies of their collisional evolution. Numerical simulations incorporating realistic physical models were developed to study the collisional evolution of hypothetical asteroid populations over the age of the solar system. Ideas and models are constrained by the observed distributions of sizes, shapes, and spin rates in the asteroid belt, by properties of Hirayama families, and by experimental studies of cratering and collisional phenomena. It is suggested that many asteroids are gravitationally-bound "rubble piles.' Those that rotate rapidly may have nonspherical quasi-equilibrium shapes, such as ellipsoids or binaries. Through comparison of models with astronomical data, physical properties of these asteroids (including bulk density) are determined, and physical processes that have operated in the solar system in primordial and subsequent epochs are studied
Superfield Realizations of Lorentz and CPT Violation
Superfield realizations of Lorentz-violating extensions of the Wess-Zumino
model are presented. These models retain supersymmetry but include terms that
explicitly break the Lorentz symmetry. The models can be understood as arising
from superspace transformations that are modifications of the familiar one in
the Lorentz-symmetric case.Comment: 10 page
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