15,357 research outputs found
Capabilities and applications of the Program to Optimize Simulated Trajectories (POST). Program summary document
The capabilities and applications of the three-degree-of-freedom (3DOF) version and the six-degree-of-freedom (6DOF) version of the Program to Optimize Simulated Trajectories (POST) are summarized. The document supplements the detailed program manuals by providing additional information that motivates and clarifies basic capabilities, input procedures, applications and computer requirements of these programs. The information will enable prospective users to evaluate the programs, and to determine if they are applicable to their problems. Enough information is given to enable managerial personnel to evaluate the capabilities of the programs and describes the POST structure, formulation, input and output procedures, sample cases, and computer requirements. The report also provides answers to basic questions concerning planet and vehicle modeling, simulation accuracy, optimization capabilities, and general input rules. Several sample cases are presented
Twisted K-theory and K-theory of bundle gerbes
In this note we introduce the notion of bundle gerbe K-theory and investigate
the relation to twisted K-theory. We provide some examples. Possible
applications of bundle gerbe K-theory to the classification of D-brane charges
in non-trivial backgrounds are discussed.Comment: 29 pages, corrected typos, added references, included new section on
twisted Chern character in non-torsion cas
On the Existence of the Quantum Action
We have previously proposed a conjecture stating that quantum mechanical
transition amplitudes can be parametrized in terms of a quantum action. Here we
give a proof of the conjecture and establish the existance of a local quantum
action in the case of imaginary time in the Feynman-Kac limit (when temperature
goes to zero). Moreover we discuss some symmetry properties of the quantum
action.Comment: revised version, Text (LaTeX
Contribution of brown dwarfs and white dwarfs to recent microlensing observations and to the halo mass budget
We examine the recent results of the MACHO collaboration towards the Large
Magellanic Cloud (Alcock et al. 1996) in terms of a halo brown dwarf or white
dwarf population. The possibility for most of the microlensing events to be due
to brown dwarfs is totally excluded by large-scale kinematic properties. The
white dwarf scenario is examined in details in the context of the most recent
white dwarf cooling theory (Segretain et al. 1994) which includes explicitely
the extra source of energy due to carbon-oxygen differentiation at
crystallization, and the subsequent Debye cooling. We show that the
observational constraints arising from the luminosity function of high-velocity
white dwarfs in the solar neighborhood and from the recent HST deep field
counts are consistent with a white dwarf contribution to the halo missing mass
as large as 50 %, provided i) an IMF strongly peaked around 1.7 Msol and ii) a
halo age older than 18 Gyr.Comment: 14 pages, 2 Postscript figures, to be published in Astrophysical
Journal Letters, minor revision in tex
Hydromagnetic constraints on deep zonal flow in the giant planets
The observed zonal flows of the giant planets will, if they penetrate below the visible atmosphere, interact
significantly with the planetary magnetic field outside the metallized core. The appropriate measure of this
interaction is the Chandrasekhar number Q = H^2 /4ÏÏΜα^2 λ (H = radial component of the magnetic field,
Îœ = eddy viscosity, λ = magnetic diffusivity, α^-1 = length scale on which λ varies); at depths where Q âł 1, the velocity will be forced to oscillate on a small length scale or decay to zero. We estimate the conductivity due
to semiconduction in H_2 (Jupiter, Saturn) and ionization in H_(2)0 (Uranus, Neptune) as a function of depth;
the value λ â 10^10 cm^2 s^-1 needed for Q = 1 is readily obtained well outside the metallic core (where A â 10^2
cm^2 s^-1). These assertions are quantified by a simple model of the equatorial zonal jet in which the flow is assumed uniform on cylinders concentric with the spin axis, and viscous and magnetic torques on each cylinder are
balanced. We solve this "Taylor constraint" simultaneously with the dynamo equation to obtain the velocity and magnetic field in the equatorial plane. With this model we reproduce the widely differing jet widths of Jupiter and Saturn (though not the flow at very high or low latitudes) using v = 2500 cm^2 s^-1, consistent with the requirement that viscous dissipation not exceed the specific luminosity. A model Uranian jet consistent with the limited Voyager data can also be constructed, with appropriately smaller v, but only if one assumes a two-layer interior. We tentatively predict a wide Neptunian jet. For Saturn (but not Jupiter or Uranus) the model has a large magnetic Reynolds number where Q = 1 and hence exhibits substantial axisymmetrization of the field in the equatorial plane. This effect may or may not
persist at higher latitudes. The one-dimensional model presented is only a first step. Variation of the velocity
and magnetic field parallel to the spin axis must be modeled in order to answer several important questions,
including: (1) What is the behavior of flows at high latitudes, whose Taylor cylinders are interrupted by the
region with Q > 1? (2) To what extent is differential rotation in the envelope responsible for the spinaxisymmetry
of Saturn's magnetic field
A time-dependent Hartree-Fock study of triple-alpha dynamics
Time-dependent Hartree-Fock calculations have been performed for fusion
reactions of He-4 + He-4 -> Be*-8, followed by He-4 + Be*-8 . Depending on the
orientation of the initial state, a linear chain vibrational state or a
triangular vibration is found in 12C, with transitions between these states
observed. The vibrations of the linear chain state and the triangular state
occur at ~9 and 4 MeV respectively.Comment: Re-submission to SciPost Proceedings following referee comment
Evolutionary calculations of phase separation in crystallizing white dwarf stars
We present an exploration of the significance of Carbon/Oxygen phase
separation in white dwarf stars in the context of self-consistent evolutionary
calculations. Because phase separation can potentially increase the calculated
ages of the oldest white dwarfs, it can affect the age of the Galactic disk as
derived from the downturn in the white dwarf luminosity function. We find that
the largest possible increase in ages due to phase separation is 1.5 Gyr, with
a most likely value of approximately 0.6 Gyr, depending on the parameters of
our white dwarf models.
The most important factors influencing the size of this delay are the total
stellar mass, the initial composition profile, and the phase diagram assumed
for crystallization. We find a maximum age delay in models with masses of 0.6
solar masses, which is near the peak in the observed white dwarf mass
distribution. We find that varying the opacities (via the metallicity) has
little effect on the calculated age delays.
In the context of Galactic evolution, age estimates for the oldest Galactic
globular clusters range from 11.5 to 16 Gyr, and depend on a variety of
parameters. In addition, a 4 to 6 Gyr delay is expected between the formation
of the globular clusters and that of the Galactic thin disk, while the observed
white dwarf luminosity function gives an age estimate for the thin disk of 9.5
+/-1.0 Gyr, without including the effect of phase separation. Using the above
numbers, we see that phase separation could add between 0 to 3 Gyr to the white
dwarf ages and still be consistent with the overall picture of Galaxy
formation. Our calculated maximum value of 1.5 Gyr fits within these bounds, as
does our best guess value of 0.6 Gyr.Comment: 13 total pages, 8 figures, 3 tables, accepted for publication in the
Astrophysical Journal on May 25, 199
Infusing literacy and math into a socioculturally responsive summer science camp
Presentation given by Georgia Southern faculty members Alma Stevenson and Shelli Casler-Failing at National Youth At-Risk Conference, Savannah, GA
Program to Optimize Simulated Trajectories (POST). Volume 1: Formulation manual
A general purpose FORTRAN program for simulating and optimizing point mass trajectories (POST) of aerospace vehicles is described. The equations and the numerical techniques used in the program are documented. Topics discussed include: coordinate systems, planet model, trajectory simulation, auxiliary calculations, and targeting and optimization
- âŠ