2,458 research outputs found
Pencil-Beam Surveys for Faint Trans-Neptunian Objects
We have conducted pencil-beam searches for outer solar system objects to a
limiting magnitude of R ~ 26. Five new trans-neptunian objects were detected in
these searches. Our combined data set provides an estimate of ~90
trans-neptunian objects per square degree brighter than ~ 25.9. This estimate
is a factor of 3 above the expected number of objects based on an extrapolation
of previous surveys with brighter limits, and appears consistent with the
hypothesis of a single power-law luminosity function for the entire
trans-neptunian region. Maximum likelihood fits to all self-consistent
published surveys with published efficiency functions predicts a cumulative sky
density Sigma(<R) obeying log10(Sigma) = 0.76(R-23.4) objects per square degree
brighter than a given magnitude R.Comment: Accepted by AJ, 18 pages, including 6 figure
A Possible Divot in the Size Distribution of the Kuiper Belt's Scattering Objects
Via joint analysis of a calibrated telescopic survey, which found scattering
Kuiper Belt objects, and models of their expected orbital distribution, we
measure the form of the scattering object's size distribution. Ruling out a
single power-law at greater than 99% confidence, we constrain the form of the
size distribution and find that, surprisingly, our analysis favours a very
sudden decrease (a divot) in the number distribution as diameters decrease
below 100 km, with the number of smaller objects then rising again as expected
via collisional equilibrium. Extrapolating at this collisional equilibrium
slope produced enough kilometer-scale scattering objects to supply the nearby
Jupiter-Family comets. Our interpretation is that this divot feature is a
preserved relic of the size distribution made by planetesimal formation, now
"frozen in" to portions of the Kuiper Belt sharing a "hot" orbital inclination
distribution, explaining several puzzles in Kuiper Belt science. Additionally,
we show that to match today's scattering-object inclination distribution, the
supply source that was scattered outward must have already been vertically
heated to of order 10 degrees.Comment: accepted 2013 January 8; published 2013 January 22 21 pages, 4
figure
Constraints on the Orbital Evolution of Triton
We present simulations of Triton's post-capture orbit that confirm the
importance of Kozai-type oscillations in its orbital elements. In the context
of the tidal orbital evolution model, these variations require average
pericenter distances much higher than previously published, and the timescale
for the tidal orbital evolution of Triton becomes longer than the age of the
Solar System. Recently-discovered irregular satellites present a new constraint
on Triton's orbital history. Our numerical integrations of test particles
indicate a timescale for Triton's orbital evolution to be less than yrs
for a reasonable number of distant satellites to survive Triton's passage. This
timescale is inconsistent with the exclusively tidal evolution (time scale of
yrs), but consistent with the interestion with the debris from
satellite-satellite collisions. Any major regular satellites will quickly
collide among themselves after being perturbed by Triton, and the resulting
debris disk would eventually be swept up by Triton; given that the total mass
of the Uranian satellite system is 40% of that of Triton, large scale evolution
is possible. This scenario could have followed either collisional or the
recently-discussed three-body-interaction-based capture.Comment: 10 pages, 4 figures, accepted for ApJ
Detection of radiation pressure acting on 2009 BD
We report the direct detection of radiation pressure on the asteroid 2009 BD,
one of the smallest multi-opposition near-Earth objects currently known, with H
~ 28.4. Under the purely gravitational model of NEODyS the object is currently
considered a possible future impactor, with impact solutions starting in 2071.
The detection of a radiation-related acceleration allows us to estimate an Area
to Mass Ratio (AMR) for the object, that can be converted (under some
assumptions) into a range of possible values for its average density. Our
result AMR = (2.97 \pm 0.33) x 10^(-4) m^2 kg^(-1) is compatible with the
object being of natural origin, and it is narrow enough to exclude a man-made
nature. The possible origin of this object, its future observability, and the
importance of radiation pressure in the impact monitoring process, are also
discussed.Comment: To be submitted for refereed publication. Preliminary analysis based
on the 2009-2010 data, published on arXiv because of the current
observability window of the targe
Costs of colour change in fish: food intake and behavioural decisions
Many animals, particularly reptiles, amphibians, fish and cephalopods, have the ability to change their body colour, for functions including thermoregulation, signalling and predator avoidance. Many fish plastically darken their body colouration in response to dark visual backgrounds, and this functions to reduce predation risk. Here, we tested the hypotheses that colour change in fish (1) carries with it an energetic cost and (2) affects subsequent shoal and habitat choice decisions. We demonstrate that guppies (Poecilia reticulata) change colour in response to dark and light visual backgrounds, and that doing so carries an energetic cost in terms of food consumption. By increasing food intake, however, guppies are able to maintain growth rates and meet the energetic costs of changing colour. Following colour change, fish preferentially choose habitats and shoals that match their own body colouration, and maximise crypsis, thus avoiding the need for further colour change but also potentially paying an opportunity cost associated with restriction to particular habitats and social associates. Thus, colour change to match the background is complemented by behavioural strategies, which should act to maximise fitness in variable environments. © 2013. Published by The Company of Biologists Ltd
Photometric Survey of the Irregular Satellites
We present BVRI colors of 13 Jovian and 8 Saturnian irregular satellites
obtained with the 2.56m Nordic Optical Telescope on La Palma, the 6.5m Magellan
Baade Telescope on La Campanas, and the 6m MMT on Mt. Hopkins. The observations
were performed between December 2001 to March 2002. Nearly all of the known
irregular satellites can be divided into two distinct classes based on their
colors. One, the grey color class, has the similar colors to the C-type
asteroid, and the other, the light red color class, has colors similar to
P/D-type asteroids. We also find at least one object, the Jovian irregular J
XXIII Kalyke, that has colors similar to the red colored Centaurs/TNOs,
although its classification is unsecure. We also find that there is a
correlation between the physical properties and dynamical properties of the
irregular satellites. Most of the dynamical clusters have homogeneous colors,
which points to single homogeneous progenitors being cratered or fragmented as
the source of each individual cluster. The heterogeneous colored clusters are
most easily explained by assuming that there are several dynamical clusters in
the area, rather than just one.Comment: Submitted to Icarus, 43 pages including 5 figure
Consequences of a Distant Massive Planet on the Large Semi-major Axis Trans-Neptunian Objects
We explore the distant giant planet hypothesis by integrating the large
semi-major axis, large pericenter Trans-Neptunian Objects (TNOs) in the
presence of the giant planets and an external perturber whose orbit is
consistent with the proposed distant, eccentric, and inclined giant planet, so
called planet 9. We find that TNOs with semi-major axes greater than 250 au
experience some longitude of perihelion shepherding, but that a generic outcome
of such evolutions is that the TNOs evolve to larger pericenter orbits, and
commonly get raised to retrograde inclinations. This pericenter and inclination
evolution requires a massive disk of TNOs (tens of M_\Earth) in order to
explain the detection of the known sample today. Some of the highly inclined
orbits produced by the examined perturbers will be inside of the orbital
parameter space probed by prior surveys, implying a missing signature of the
9th planet scenario. The distant giant planet scenarios explored in this work
do not reproduce the observed signal of simultaneous clustering in argument of
pericenter, longitude of the ascending node, and longitude of perihelion in the
region of the known TNOs
Utopia revisited - or is it better the second time around?
This article essays comparisons between nineteenth- and twentieth-century school management and its direction and control by central government. Its starting point is 1816 when Jeremy Bentham presented his utopian vision of a model school, to be managed by a school master exhibiting competences detailed in his Chrestomathic Table. This has similarities to the headteachers’ competences required in the late twentieth century by the government through the Teacher Training Agency. The article presents several areas for comparisons in addition to competences: definitions of effective management, governance and local community influence and the focus on quality assurance. Both periods have seen major changes in educational management and administration but will the lessons learnt from these innovations when first introduced in the nineteenth century be transferred to the late twentieth century
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