253 research outputs found
Gauge Freedom in Orbital Mechanics
In orbital and attitude dynamics the coordinates and the Euler angles are
expressed as functions of the time and six constants called elements. Under
disturbance, the constants are endowed with time dependence. The Lagrange
constraint is then imposed to guarantee that the functional dependence of the
perturbed velocity on the time and constants stays the same as in the
undisturbed case. Constants obeying this condition are called osculating
elements. The constants chosen to be canonical are called Delaunay elements, in
the orbital case, or Andoyer elements, in the spin case. (As some Andoyer
elements are time dependent even in the free-spin case, the role of constants
is played by their initial values.) The Andoyer and Delaunay sets of elements
share a feature not readily apparent: in certain cases the standard equations
render them non-osculating. In orbital mechanics, elements furnished by the
standard planetary equations are non-osculating when perturbations depend on
velocities. To preserve osculation, the equations must be amended with extra
terms that are not parts of the disturbing function. In the case of Delaunay
parameterisation, these terms destroy canonicity. So under velocity-dependent
disturbances, osculation and canonicity are incompatible. (Efroimsky and
Goldreich 2003, 2004) Similarly, the Andoyer elements turn out to be
non-osculating under angular-velocity-dependent perturbation. Amendment of only
the Hamiltonian makes the equations render nonosculating elements. To make them
osculating, more terms must enter the equations (and the equations will no
longer be canonical). In practical calculations, is often convenient to
deliberately deviate from osculation by substituting the Lagrange constraint
with a condition that gives birth to a family of nonosculating elements.Comment: Talk at the annual Princeton conference ``New Trends in
Astrodynamics" 2005 http://www.math.princeton.edu/astrocon
Mechanical Alignment of Suprathermal Paramagnetic Cosmic-Dust Granules: the Cross-Section Mechanism
We develop a comprehensive quantitative description of the cross-section
mechanism discovered several years ago by Lazarian. This is one of the
processes that determine grain orientation in clouds of suprathermal cosmic
dust. The cross-section mechanism manifests itself when an ensemble of
suprathermal paramagnetic granules is placed in a magnetic field and is subject
to ultrasonic gas bombardment. The mechanism yields dust alignment whose
efficiency depends upon two factors: the geometric shape of the granules, and
the angle Phi between the magnetic line and the gas flow. We calculate the
quantitative measure of this alignment, and study its dependence upon the said
factors. It turns out that, irrelevant of the grain shape, the action of a flux
does not lead to alignment if Phi = arccos(1/sqrt{3}).Comment: 9 figure
Stress field and spin axis relaxation for inelastic triaxial ellipsoids
A compact formula for the stress tensor inside a self-gravitating, triaxial
ellipsoid in an arbitrary rotation state is given. It contains no singularity
in the incompressible medium limit. The stress tensor and the quality factor
model are used to derive a solution for the energy dissipation resulting in the
damping (short axis mode) or excitation (long axis) of wobbling. In the limit
of an ellipsoid of revolution, we compare our solution with earlier ones and
show that, with appropriate corrections, the differences in damping times
estimates are much smaller than it has been claimed.
This version implements corrections of misprints found in the MNRAS published
text.Comment: 14 pages, 6 figures, published in Monthly Notices RAS (containing
misprints
Cross-Section Alignment of Oblate Grains
This paper provides a quantitative account of a recently introduced mechanism
of mechanical alignment of suprathermally rotating grains. These rapidly
rotating grains are essentially not susceptible to random torques arising from
gas-grain collisions, as the timescales for such torques to have significant
effect are orders of magnitude greater than the mean time between crossovers.
Such grains can be aligned by gaseous torques during the short periods of
crossovers and/or due to the difference in the rate at which atoms arrive at
grain surface. The latter is a result of the difference in orientation of a
grain in respect to the supersonic flow. This process, which we call
cross-section alignment, is the subject of our present paper. We derive
expressions for the measure of cross-section alignment for oblate grains and
study how this measure depends upon the angle between the interstellar magnetic
field and the gaseous flow and upon the grain shape.Comment: 24 pages, Post Script file. To appear in The Astrophysical Journal,
Vol. 466, p. 274 - 281, July 199
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