1,327 research outputs found
What drives the dust activity of comet 67P/Churyumov-Gerasimenko?
We use the gravitational instability formation scenario of cometesimals to
derive the aggregate size that can be released by the gas pressure from the
nucleus of comet 67P/Churyumov-Gerasimenko for different heliocentric distances
and different volatile ices. To derive the ejected aggregate sizes, we
developed a model based on the assumption that the entire heat absorbed by the
surface is consumed by the sublimation process of one volatile species. The
calculations were performed for the three most prominent volatile materials in
comets, namely, H_20 ice, CO_2 ice, and CO ice. We find that the size range of
the dust aggregates able to escape from the nucleus into space widens when the
comet approaches the Sun and narrows with increasing heliocentric distance,
because the tensile strength of the aggregates decreases with increasing
aggregate size. The activity of CO ice in comparison to H_20 ice is capable to
detach aggregates smaller by approximately one order of magnitude from the
surface. As a result of the higher sublimation rate of CO ice, larger
aggregates are additionally able to escape from the gravity field of the
nucleus. Our model can explain the large grains (ranging from 2 cm to 1 m in
radius) in the inner coma of comet 67P/Churyumov-Gerasimenko that have been
observed by the OSIRIS camera at heliocentric distances between 3.4 AU and 3.7
AU. Furthermore, the model predicts the release of decimeter-sized aggregates
(trail particles) close to the heliocentric distance at which the gas-driven
dust activity vanishes. However, the gas-driven dust activity cannot explain
the presence of particles smaller than ~1 mm in the coma because the high
tensile strength required to detach these particles from the surface cannot be
provided by evaporation of volatile ices. These smaller particles can be
produced for instance by spin-up and centrifugal mass loss of ejected larger
aggregates
Massive fields tend to form highly oscillating self-similarly expanding shells
The time evolution of self-interacting spherically symmetric scalar fields in
Minkowski spacetime is investigated based on the use of Green's theorem. It is
shown that a massive Klein-Gordon field can be characterized by the formation
of certain expanding shell structures where all the shells are built up by very
high frequency oscillations. This oscillation is found to be modulated by the
product of a simple time decaying factor of the form and of an
essentially self-similar expansion. Apart from this self-similar expansion the
developed shell structure is preserved by the evolution. In particular, the
energy transported by each shell appears to be time independent.Comment: 10 pages, to appear in Phys. Rev.
Indirect Evidence for L\'evy Walks in Squeeze Film Damping
Molecular flow gas damping of mechanical motion in confined geometries, and
its associated noise, is important in a variety of fields, including precision
measurement, gravitational wave detection, and MEMS devices. We used two
torsion balance instruments to measure the strength and distance-dependence of
`squeeze film' damping. Measured quality factors derived from free decay of
oscillation are consistent with gas particle superdiffusion in L\'evy walks and
inconsistent with those expected from traditional Gaussian random walk particle
motion. The distance-dependence of squeeze film damping observed in our
experiments is in agreement with a parameter-free Monte Carlo simulation. The
squeeze film damping of the motion of a plate suspended a distance d away from
a parallel surface scales with a fractional power between 1/d and 1/d^2.Comment: 5 pages 5 figures accepted for PRD; typo in equation 3 and figure 1
fixe
High Sensitivity Torsion Balance Tests for LISA Proof Mass Modeling
We have built a highly sensitive torsion balance to investigate small forces
between closely spaced gold coated surfaces. Such forces will occur between the
LISA proof mass and its housing. These forces are not well understood and
experimental investigations are imperative. We describe our torsion balance and
present the noise of the system. A significant contribution to the LISA noise
budget at low frequencies is the fluctuation in the surface potential
difference between the proof mass and its housing. We present first results of
these measurements with our apparatus.Comment: 6th International LISA Symposiu
Evolution equations for slowly rotating stars
We present a hyperbolic formulation of the evolution equations describing
non-radial perturbations of slowly rotating relativistic stars in the
Regge--Wheeler gauge. We demonstrate the stability preperties of the new
evolution set of equations and compute the polar w-modes for slowly rotating
stars.Comment: 27 pages, 2 figure
Tests of the Gravitational Inverse-Square Law below the Dark-Energy Length Scale
We conducted three torsion-balance experiments to test the gravitational
inverse-square law at separations between 9.53 mm and 55 micrometers, probing
distances less than the dark-energy length scale m. We find with 95% confidence
that the inverse-square law holds () down to a length scale
m and that an extra dimension must have a size m.Comment: 4 pages, 6 figure
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