1,131 research outputs found
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
Why are the fastest runners of intermediate size? Contrasting scaling of mechanical demands and muscle supply of work and power
The fastest land animals are of intermediate size. Cheetah, antelope, greyhounds and racehorses have been measured running much faster than reported for elephants or elephant shrews. Can this be attributed to scaling of physical demands and explicit physiological constraints to supply? Here, we describe the scaling of mechanical work demand each stride, and the mechanical power demand each stance. Unlike muscle stress, strain and strain rate, these mechanical demands cannot be circumvented by changing the muscle gearing with minor adaptations in bone geometry or trivial adjustments to limb posture. Constraints to the capacity of muscle to supply work and power impose fundamental limitations to maximum speed. Given an upper limit to muscle work capacity each contraction, maximum speeds in big animals are constrained by the mechanical work demand each step. With an upper limit to instantaneous muscle power production, maximal speeds in small animals are limited by the high power demands during brief stance periods. The high maximum speed of the cheetah may therefore be attributed as much to its size as to its other anatomical and physiological adaptations
Prevalence of functional bowel disorders and faecal incontinence: an Australian primary care survey
Accepted Article online 30 October 2014. The copyright line for this article was changed on 14 February, 2017 after original online publication.Aim: Interest in functional bowel disorders (FBDs) and faecal incontinence (FI) has increased amongst coloproctologists. The study aimed to assess the prevalence of FBDs and FI (including its severity) among Australian primary healthcare seekers using objective criteria. Method: A cross-sectional survey was conducted in a primary care setting in Sydney, Australia. A self-administered questionnaire was used to collect demographic information and diagnose FBDs (irritable bowel syndrome, constipation, functional bloating and functional diarrhoea) based on Rome III criteria. The severity of FI was determined using the Vaizey incontinence score. Associations with medical/surgical history and healthcare utilization were assessed. Results: Of 596 subjects approached, 396 (66.4%) agreed to participate. Overall, 33% had FBD and/or FI. Irritable bowel syndrome was present in 11.1% and these participants were more likely to report anxiety/depression (P 8). Participants with FI were more likely to have irritable bowel syndrome, urinary incontinence and previous anal surgery (P < 0.01). Conclusion: FBDs and FI are prevalent conditions amongst primary healthcare seekers and the needs of those affected appear to be complex given their coexisting symptoms and conditions. Currently, the majority do not reach colorectal services, although increased awareness by primary care providers could lead to sufferers being referred for specialist management.K.-S. Ng, N. Nassar, K. Hamd, A. Nagarajah and M. A. Gladma
Formation of Kuiper Belt Binaries by Gravitational Collapse
A large fraction of 100-km-class low-inclination objects in the classical
Kuiper Belt (KB) are binaries with comparable mass and wide separation of
components. A favored model for their formation was capture during the
coagulation growth of bodies in the early KB. Instead, recent studies suggested
that large objects can rapidly form in the protoplanetary disks when swarms of
locally concentrated solids collapse under their own gravity. Here we examine
the possibility that KB binaries formed during gravitational collapse when the
excess of angular momentum prevented the agglomeration of available mass into a
solitary object. We find that this new mechanism provides a robust path toward
the formation of KB binaries with observed properties, and can explain wide
systems such as 2001 QW322 and multiples such as (47171) 1999 TC36. Notably,
the gravitational collapse is capable of producing 100% binary fraction for a
wide range of the swarm's initial angular momentum values. The binary
components have similar masses (80% have the secondary-over-primary radius
ratio >0.7) and their separation ranges from ~1,000 to ~100,000 km. The binary
orbits have eccentricities from e=0 to ~1, with the majority having e<0.6. The
binary orbit inclinations with respect to the initial angular momentum of the
swarm range from i=0 to ~90 deg, with most cases having i<50 deg. Our binary
formation mechanism implies that the primary and secondary components in each
binary pair should have identical bulk composition, which is consistent with
the current photometric data. We discuss the applicability of our results to
the Pluto-Charon, Orcus-Vanth, (617) Patroclus-Menoetius and (90) Antiope
binary systems.Comment: Astronomical Journal, in pres
The use of Systemic Lupus Erythematosus Disease Activity Index-2000 to define active disease and minimal clinically meaningful change based on data from a large cohort of systemic lupus erythematosus patients
Objectives. To examine SLEDAI-2000 cut-off scores for definition of active SLE and to determine the sensitivity to change of SLEDAI-2000 for the assessment of SLE disease activity and minimal clinically meaningful changes in score
Accretion in the Early Kuiper Belt II. Fragmentation
We describe new planetesimal accretion calculations in the Kuiper Belt that
include fragmentation and velocity evolution. All models produce two power law
cumulative size distributions, N_C propto r^{-q}, with q = 2.5 for radii less
than 0.3-3 km and q = 3 for radii exceeding 1-3 km. The power law indices are
nearly independent of the initial mass in the annulus, the initial eccentricity
of the planetesimal swarm, and the initial size distribution of the
planetesimal swarm. The transition between the two power laws moves to larger
radii as the initial eccentricity increases. The maximum size of objects
depends on their intrinsic tensile strength; Pluto formation requires a
strength exceeding 300 erg per gram. Our models yield formation timescales for
Pluto-sized objects of 30-40 Myr for a minimum mass solar nebula. The
production of several `Plutos' and more than 10^5 50 km radius Kuiper Belt
objects leaves most of the initial mass in 0.1-10 km radius objects that can be
collisionally depleted over the age of the solar system. These results resolve
the puzzle of large Kuiper Belt objects in a small mass Kuiper Belt.Comment: to appear in the Astronomical Journal (July 1999); 54 pages including
7 tables and 13 figure
Cognitive function, disease burden and the structural connectome in systemic lupus erythematosus
Asteroids in the Inner Solar System I - Existence
Ensembles of in-plane and inclined orbits in the vicinity of the Lagrange
points of the terrestrial planets are integrated for up to 100 million years.
The integrations incorporate the gravitational effects of Sun and the eight
planets (Pluto is neglected). Mercury is the least likely planet, as it is
unable to retain tadpole orbits over 100 million year timescales. Both Venus
and the Earth are much more promising, as they possess rich families of stable
tadpole and horseshoe orbits. Our survey of Trojans in the orbital plane of
Venus is undertaken for 25 million years. Some 40% of the survivors are on
tadpole orbits. For the Earth, the integrations are pursued for 50 million
years. The stable zones in the orbital plane are larger for the Earth than for
Venus, but fewer of the survivors are tadpoles. Both Venus and the Earth also
have regions in which inclined test particles can endure near the Lagrange
points. For Venus, only test particles close to the orbital plane are stable.
For the Earth, there are two bands of stability, one at low inclinations (i <
16 degrees) and one at moderate inclinations (between 24 degrees and 34
degrees). The inclined test particles that evade close encounters are primarily
moving on tadpole orbits. Our survey of in-plane test particles near the
Martian Lagrange points shows no survivors after 60 million years. Low
inclination test particles do not persist, as their inclinations are quickly
increased until the effects of a secular resonance with Jupiter cause
de-stabilisation. Numerical integrations of inclined test particles for
timespans of 25 million years show stable zones for inclinations between 14 and
40 degrees.Comment: 20 pages, 21 figures, Monthly Notices (in press
The Canada-France Ecliptic Plane Survey - Full Data Release: The orbital structure of the Kuiper belt
We report the orbital distribution of the trans-neptunian objects (TNOs)
discovered during the Canada-France Ecliptic Plane Survey, whose discovery
phase ran from early 2003 until early 2007. The follow-up observations started
just after the first discoveries and extended until late 2009. We obtained
characterized observations of 321 sq.deg. of sky to depths in the range g ~
23.5--24.4 AB mag. We provide a database of 169 TNOs with high-precision
dynamical classification and known discovery efficiency. Using this database,
we find that the classical belt is a complex region with sub-structures that go
beyond the usual splitting of inner (interior to 3:2 mean-motion resonance
[MMR]), outer (exterior to 2:1 MMR), and main (in between). The main classical
belt (a=40--47 AU) needs to be modeled with at least three components: the
`hot' component with a wide inclination distribution and two `cold' components
(stirred and kernel) with much narrower inclination distributions. The hot
component must have a significantly shallower absolute magnitude (Hg)
distribution than the other two components. With 95% confidence, there are
8000+1800-1600 objects in the main belt with Hg <= 8.0, of which 50% are from
the hot component, 40% from the stirred component and 10% from the kernel; the
hot component's fraction drops rapidly with increasing Hg. Because of this, the
apparent population fractions depend on the depth and ecliptic latitude of a
trans-neptunian survey. The stirred and kernel components are limited to only a
portion of the main belt, while we find that the hot component is consistent
with a smooth extension throughout the inner, main and outer regions of the
classical belt; the inner and outer belts are consistent with containing only
hot-component objects. The Hg <= 8.0 TNO population estimates are 400 for the
inner belt and 10,000 for the outer belt within a factor of two.Comment: 59 pages, 9 figures, 7 table
Cratering Experiments on the Self Armoring of Coarse-Grained Granular Targets
Recently published crater statistics on the small asteroids 25143 Itokawa and
433 Eros show a significant depletion of craters below approx. 100 m in
diameter. Possible mechanisms that were brought up to explain this lack of
craters were seismic crater erasure and self armoring of a coarse, boulder
covered asteroid surface. While seismic shaking has been studied in this
context, the concept of armoring lacks a deeper inspection and an experimental
ground truth. We therefore present cratering experiments of glass bead
projectiles impacting into granular glass bead targets, where the grain sizes
of projectile and target are in a similar range. The impact velocities are in
the range of 200 to 300 m/s. We find that craters become fainter and irregular
shaped as soon as the target grains are larger than the projectile sizes and
that granular craters rarely form when the size ratio between projectile and
target grain is around 1:10 or smaller. In that case, we observe a formation of
a strength determined crater in the first struck target grain instead. We
present a simple model based on the transfer of momentum from the projectile to
this first target grain, which is capable to explain our results with only a
single free parameter, which is moreover well determined by previous
experiments. Based on estimates of typical projectile size and boulder size on
Itokawa and Eros, given that our results are representative also for km/s
impact velocities, armoring should play an important role for their evolution.Comment: accepted for publication in Icaur
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