384 research outputs found
On compressive radial tidal forces
Radial tidal forces can be compressive instead of disruptive, a possibility
that is frequently overlooked in high level physics courses. For example,
radial tidal compression can emerge in extended stellar systems containing a
smaller stellar cluster. For particular conditions the tidal field produced by
this extended mass distribution can exert on the cluster it contains
compressive effects instead of the common disruptive forces. This interesting
aspect of gravity can be derived from standard relations given in many
textbooks and introductory courses in astronomy and can serve as an opportunity
to look closer at some aspects of gravitational physics, stellar dynamics, and
differential geometry. The existence of compressive tides at the center of huge
stellar systems might suggest new evolutionary scenarios for the formation of
stars and primordial galactic formation processes.Comment: 22 pages, 2 figure
Lake Ontario Long Term Biological Monitoring Program: 1981, 1982 Data Base
The Bioindex, or Long Term Biological Monitoring Program, was developed to: 1) determine normal seasonal patterns and annual ranges of abundance, community structure, and when possible, productivity of the biological components - phytoplankton, zooplankton, and benthos; 2) relate the biological components to variations in the physical, nutrient, and biological environment; and, 3) assess the adopted sampling strategy for long term monitoring. The data bases from the first two years are summarized in this document
How Observations of Circumstellar Disk Asymmetries Can Reveal Hidden Planets: Pericenter Glow and its Application to the HR 4796 Disk
Recent images of the disks of dust around the young stars HR 4796A and
Fomalhaut show, in each case, a double-lobed feature that may be asymmetric
(one lobe may be brighter than the other). A symmetric double-lobed structure
is that expected from a disk of dust with a central hole that is observed
nearly edge-on (i.e., close to the plane of the disk). This paper shows how the
gravitational influence of a second body in the system with an eccentric orbit
would cause a brightness asymmetry in such a disk by imposing a "forced
eccentricity" on the orbits of the constituent dust particles, thus shifting
the center of symmetry of the disk away from the star and causing the dust near
the forced pericenter of the perturbed disk to glow. Dynamic modeling of the HR
4796 disk shows that its 5% brightness asymmetry could be the result of a
forced eccentricity as small as 0.02 imposed on the disk by either the binary
companion HR 4796B, or by an unseen planet close to the inner edge of the disk.
Since it is likely that a forced eccentricity of 0.01 or higher would be
imposed on a disk in a system in which there are planets, but no binary
companion, the corresponding asymmetry in the disk's structure could serve as a
sensitive indicator of these planets that might otherwise remain undetected.Comment: 61 pages, 10 figures, accepted for publication in the Astrophysical
Journal (scheduled for January 10, 2000
Origin and evolution of the zodiacal dust cloud
The astrophysical importance of the zodiacal cloud became more apparent. The most useful source of information on the structure of the zodiacal cloud is the Infrared Astronomical Satellite (IRAS) observations. A substantial fraction of the extensive IRAS data set was analyzed. Also, a numerical model was developed (SIMUL) that allows to calculate the distribution of night-sky brightness that would be produced by any particular distribution of dust particle orbits. This model includes the effects of orbital perturbations by both the planets and solar radiation, it reproduces the exact viewing geometry of the IRAS telescope, and allows for the eccentricity of the Earth's orbit. SIMUL now is used to model not just the solar system dust bands discovered by IRAS but the whole zodiacal cloud
Exchange of ejecta between Telesto and Calypso: Tadpoles, horseshoes, and passing orbits
We have numerically integrated the orbits of ejecta from Telesto and Calypso,
the two small Trojan companions of Saturn's major satellite Tethys. Ejecta were
launched with speeds comparable to or exceeding their parent's escape velocity,
consistent with impacts into regolith surfaces. We find that the fates of
ejecta fall into several distinct categories, depending on both the speed and
direction of launch.
The slowest ejecta follow sub-orbital trajectories and re-impact their source
moon in less than one day. Slightly faster debris barely escape their parent's
Hill sphere and are confined to tadpole orbits, librating about Tethys'
triangular Lagrange points L4 (leading, near Telesto) or L5 (trailing, near
Calypso) with nearly the same orbital semi-major axis as Tethys, Telesto, and
Calypso. These ejecta too eventually re-impact their source moon, but with a
median lifetime of a few dozen years. Those which re-impact within the first
ten years or so have lifetimes near integer multiples of 348.6 days (half the
tadpole period).
Still faster debris with azimuthal velocity components >~ 10 m/s enter
horseshoe orbits which enclose both L4 and L5 as well as L3, but which avoid
Tethys and its Hill sphere. These ejecta impact either Telesto or Calypso at
comparable rates, with median lifetimes of several thousand years. However,
they cannot reach Tethys itself; only the fastest ejecta, with azimuthal
velocities >~ 40 m/s, achieve "passing orbits" which are able to encounter
Tethys. Tethys accretes most of these ejecta within several years, but some 1 %
of them are scattered either inward to hit Enceladus or outward to strike
Dione, over timescales on the order of a few hundred years
The origin and evolution of the zodiacal dust cloud
We have now analyzed a substantial fraction of the IRAS observations of the zodiacal cloud, particularly in the 25 micron waveband. We have developed a gravitational perturbation theory that incorporates the effects of Poynting-Robertson light drag (Gomes and Dermott, 1992). We have also developed a numerical model, the SIMUL mode, that reproduces the exact viewing geometry of the IRAS telescope and calculates the distribution of thermal flux produced by any particular distribution of dust particle orbits (Dermott and Nicholson, 1989). With these tools, and using a distribution of orbits based on those of asteroidal particles with 3.4 micron radii whose orbits decay due to Poynting-Robertson light drag and are perturbed by the planets, we have been able to: (1) account for the inclination and node of the background zodiacal cloud observed by IRAS in the 25 micron waveband; (2) relate the distribution of orbits in the Hirayama asteroid families to the observed shapes of the IRAS solar system dustbands; and (3) show that there is observational evidence in the IRAS data for the transport of asteroidal particles from the main belt to the Earth by Poynting-Robertson light drag
Deep 10 and 18 micron Imaging of the HR 4796A Circumstellar Disk: Transient Dust Particles & Tentative Evidence for a Brightness Asymmetry
We present new 10.8 and 18.2 micron images of HR 4796A, a young A0V star that
was recently discovered to have a spectacular, nearly edge-on, circumstellar
disk prominent at ~20 microns (Jayawardhana et al. 1998; Koerner et al. 1998).
These new images, obtained with OSCIR at Keck II, show that the disk's size at
10 microns is comparable to its size at 18 microns. Therefore, the 18
micron-emitting dust may also emit some, or all, of the 10 micron radiation.
Using these multi-wavelength images, we determine a "characteristic" diameter
of 2-3 microns for the mid-infrared-emitting dust particles if they are
spherical and composed of astronomical silicates. Particles this small are
expected to be blown out of the system by radiation pressure in a few hundred
years, and therefore these particles are unlikely to be primordial. Dynamical
modeling of the disk (Wyatt et al. 2000) indicates that the disk surface
density is relatively sharply peaked near 70 AU, which agrees with the mean
annular radius deduced by Schneider et al. (1999) from their NICMOS images. We
present evidence (~1.8 sigma significance) for a brightness asymmetry that may
result from the presence of the hole and the gravitational perturbation of the
disk particle orbits by the low-mass stellar companion or a planet. This
"pericenter glow," which must still be confirmed, results from a very small (a
few AU) shift of the disk's center of symmetry relative to the central star HR
4796A; one side of the inner boundary of the annulus is shifted towards HR
4796A, thereby becoming warmer and more infrared-emitting. The possible
detection of pericenter glow implies that the detection of even complex
dynamical effects of planets on disks is within reach.Comment: 18 pages. 9 GIF images. Total size ~800 kB. High resolution images
available upon request. Accepted for publication in the Astrophysical Journal
(scheduled for January 10, 2000
Stable manifolds and homoclinic points near resonances in the restricted three-body problem
The restricted three-body problem describes the motion of a massless particle
under the influence of two primaries of masses and that circle
each other with period equal to . For small , a resonant periodic
motion of the massless particle in the rotating frame can be described by
relatively prime integers and , if its period around the heavier primary
is approximately , and by its approximate eccentricity . We give a
method for the formal development of the stable and unstable manifolds
associated with these resonant motions. We prove the validity of this formal
development and the existence of homoclinic points in the resonant region.
In the study of the Kirkwood gaps in the asteroid belt, the separatrices of
the averaged equations of the restricted three-body problem are commonly used
to derive analytical approximations to the boundaries of the resonances. We use
the unaveraged equations to find values of asteroid eccentricity below which
these approximations will not hold for the Kirkwood gaps with equal to
2/1, 7/3, 5/2, 3/1, and 4/1.
Another application is to the existence of asymmetric librations in the
exterior resonances. We give values of asteroid eccentricity below which
asymmetric librations will not exist for the 1/7, 1/6, 1/5, 1/4, 1/3, and 1/2
resonances for any however small. But if the eccentricity exceeds these
thresholds, asymmetric librations will exist for small enough in the
unaveraged restricted three-body problem
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