4,517 research outputs found
Planetary Trojans - the main source of short period comets?
We present a short review of the impact regime experienced by the terrestrial
planets within our own Solar system, describing the three populations of
potentially hazardous objects which move on orbits that take them through the
inner Solar system. Of these populations, the origins of two (the Near-Earth
Asteroids and the Long-Period Comets) are well understood, with members
originating in the Asteroid belt and Oort cloud, respectively. By contrast, the
source of the third population, the Short-Period Comets, is still under debate.
The proximate source of these objects is the Centaurs, a population of
dynamically unstable objects that pass perihelion between the orbits of Jupiter
and Neptune. However, a variety of different origins have been suggested for
the Centaur population. Here, we present evidence that at least a significant
fraction of the Centaur population can be sourced from the planetary Trojan
clouds, stable reservoirs of objects moving in 1:1 mean-motion resonance with
the giant planets (primarily Jupiter and Neptune). Focusing on simulations of
the Neptunian Trojan population, we show that an ongoing flux of objects should
be leaving that region to move on orbits within the Centaur population. With
conservative estimates of the flux from the Neptunian Trojan clouds, we show
that their contribution to that population could be of order ~3%, while more
realistic estimates suggest that the Neptune Trojans could even be the main
source of fresh Centaurs. We suggest that further observational work is needed
to constrain the contribution made by the Neptune Trojans to the ongoing flux
of material to the inner Solar system, and believe that future studies of the
habitability of exoplanetary systems should take care not to neglect the
contribution of resonant objects (such as planetary Trojans) to the impact flux
that could be experienced by potentially habitable worlds.Comment: 16 pages, 4 figures, published in the International Journal of
Astrobiology (the arXiv.org's abstract was shortened, but the original one
can be found in the manuscript file
(1173) Anchises - Thermophysical and Dynamical Studies of a Dynamically Unstable Jovian Trojan
We have performed detailed thermophysical and dynamical modelling of Jovian
Trojan (1173) Anchises. Our results reveal a most unusual object. By examining
observational data taken by IRAS, Akari and WISE between 11.5 and 60 microns,
along with variations in its optical lightcurve, we find Anchises is most
likely an elongated body, with an axes-ratio of ~1.4. This yields calculated
best-fit dimensions of 170x121x121km (an equivalent diameter of 136+18/-11km).
We find the observations are best fit by Anchises having a retrograde sense of
rotation, and an unusually high thermal inertia (25 to 100 Jm-2s-0.5K-1). The
geometric albedo is found to be 0.027 (+0.006/-0.007). Anchises therefore has
one of the highest published thermal inertias of any object larger than 100km
in diameter, at such large heliocentric distances, and is one of the lowest
albedo objects ever observed. More observations are needed to see if there is a
link between the very shallow phase curve, with almost no opposition effect,
and the derived thermal properties for this large Trojan asteroid. Our
dynamical investigation of Anchises' orbit has revealed it to be dynamically
unstable on timescales of hundreds of Myr, similar to the unstable Neptunian
Trojans 2001 QR322 and 2008 LC18. Unlike those objects, we find that Anchises'
dynamical stability is not a function of its initial orbital elements, the
result of the exceptional precision with which its orbit is known. This is the
first time that a Jovian Trojan has been shown to be dynamically unstable, and
adds weight to the idea that planetary Trojans represent a significant ongoing
contribution to the Centaur population, the parents of the short-period comets.
The observed instability does not rule out a primordial origin for Anchises,
but when taken in concert with the result of our thermophysical analysis,
suggest that it would be a fascinating target for future study.Comment: 5 figures, 3 tables, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Predictors of discordance among Chilean families
Parent-youth agreement on parental behaviors can characterize effective parenting. Although
discordance in families may be developmentally salient and harmful to youth outcomes, predictors
of discordance have been understudied, and existing research in this field has been mostly limited
to North American samples. This paper addressed this literature gap by using data from a
community-based study of Chilean adolescents. Analysis was based on 1,068 adolescents in
Santiago, Chile. The dependent variable was discordance which was measured by the difference
between parent and youth’s assessment of parental monitoring. Major independent variables for
this study were selected based on previous research findings that underscore youth’s
developmental factors, positive parental and familial factors and demographic factors. Descriptive
and multivariate analyses were conducted to examine the prevalence and associations between
youth, parental and familial measures with parent-youth discordance. There was a sizable level of
discordance between parent and youth’s report of parental monitoring. Youth’s gender and
externalizing behavior were significant predictors of discordance. Warm parenting and family
involvement were met with decreases in discordance. The negative interaction coefficients
between parental warmth and youth’s gender indicated that positive parental and familial
measures have a greater effect on reducing parent-youth discordance among male youths. Results
support the significance of positive family interactions in healthy family dynamics. Findings from
this study inform the importance of services and interventions for families that aim to reduce
youth’s problem behavior and to create a warm and interactive family environment.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4181713/Accepted manuscrip
Incidence of pelvic inflammatory disease associated with mycoplasma genitalium infection: Evidence synthesis of cohort study data
We synthesized evidence from the POPI sexual-health cohort study, and estimated that 4.9% (95% credible interval 0.4-14.1%) of Mycoplasma genitalium infections in women progress to pelvic inflammatory disease, versus 14.4% (5.9-24.6%) of chlamydial infections. For validation, we predicted PID rates in four age groups that agree well with surveillance data
The Capture of Trojan Asteroids by the Giant Planets During Planetary Migration
Of the four giant planets in the Solar system, only Jupiter and Neptune are
currently known to possess swarms of Trojan asteroids - small objects that
experience a 1:1 mean motion resonance with their host planet. In Lykawka et
al. (2009), we performed extensive dynamical simulations, including planetary
migration, to investigate the origin of the Neptunian Trojan population.
Utilising the vast amount of simulation data obtained for that work, together
with fresh results from new simulations, we here investigate the dynamical
capture of Trojans by all four giant planets from a primordial trans-Neptunian
disk. We find the likelihood of a given planetesimal from this region being
captured onto an orbit within Jupiter's Trojan cloud lies between several times
10^-6 and 10^-5. For Saturn, the probability is found to be in the range <10^-6
to 10^-5, whilst for Uranus the probabilities range between 10^-5 and 10^-4.
Finally, Neptune displays the greatest probability of Trojan capture, with
values ranging between 10^-4 and 10^-3. Our results suggest that all four giant
planets are able to capture and retain a significant population of Trojan
objects from the disk by the end of planetary migration. As a result of
encounters with the giant planets prior to Trojan capture, these objects tend
to be captured on orbits that are spread over a wide range of orbital
eccentricities and inclinations. The bulk of captured objects are to some
extent dynamically unstable, and therefore the populations of these objects
tend to decay over the age of the Solar System, providing an important ongoing
source of new objects moving on dynamically unstable orbits among the giant
planets. Given that a huge population of objects would be displaced by
Neptune's outward migration (with a potential cumulative mass a number of times
that of the Earth), we conclude that the surviving remnant of the Trojans
captured during the migration of the outer planets might be sufficient to
explain the currently known Trojan populations in the outer Solar system.Comment: 18 pages, 4 figures, MNRAS (in press
The Neptune Trojans - a new source for the Centaurs?
The fact that the Centaurs are the primary source of the Short Period Comets
is well established. However, the origin of the Centaurs themselves is still
under some debate, with a variety of different source reservoirs being proposed
in the last decade. In this work, we suggest that the Neptune Trojans (together
with the Jovian Trojans) could represent an additional significant source of
Centaurs. Using dynamical simulations of the first Neptune Trojan discovered
(2001 QR322), together with integrations following the evolution of clouds of
theoretical Neptune Trojans obtained during simulations of planetary migration,
we show that the Neptune Trojan population contains a great number of objects
which are unstable on both Myr and Gyr timescales. Using individual examples,
we show how objects that leave the Neptunian Trojan cloud evolve onto orbits
indistinguishable from those of the known Centaurs, before providing a range of
estimates of the flux from this region to the Centaur population. With only
moderate assumptions, it is shown that the Trojans can contribute a significant
proportion of the Centaur population, and may even be the dominant source
reservoir. This result is supported by past work on the colours of the Trojans
and the Centaurs, but it will take future observations to determine the full
scale of the contribution of the escaped Trojans to the Centaur population.Comment: 16 pages, 7 figure
A Dynamical Analysis of the Proposed Circumbinary HW Virginis Planetary System
In 2009, the discovery of two planets orbiting the evolved binary star system
HW Virginis was announced, based on systematic variations in the timing of
eclipses between the two stars. The planets invoked in that work were
significantly more massive than Jupiter, and moved on orbits that were mutually
crossing - an architecture which suggests that mutual encounters and strong
gravitational interactions are almost guaranteed. In this work, we perform a
highly detailed analysis of the proposed HW Vir planetary system. First, we
consider the dynamical stability of the system as proposed in the discovery
work. Through a mapping process involving 91,125 individual simulations, we
find that the system is so unstable that the planets proposed simply cannot
exist, due to mean lifetimes of less than a thousand years across the whole
parameter space. We then present a detailed re-analysis of the observational
data on HW Vir, deriving a new orbital solution that provides a very good fit
to the observational data. Our new analysis yields a system with planets more
widely spaced, and of lower mass, than that proposed in the discovery work, and
yields a significantly greater (and more realistic) estimate of the uncertainty
in the orbit of the outermost body. Despite this, a detailed dynamical analysis
of this new solution similarly reveals that it also requires the planets to
move on orbits that are simply not dynamically feasible. Our results imply that
some mechanism other than the influence of planetary companions must be the
principal cause of the observed eclipse timing variations for HW Vir. If the
sys- tem does host exoplanets, they must move on orbits differing greatly from
those previously proposed. Our results illustrate the critical importance of
performing dynamical analyses as a part of the discovery process for
multiple-planet exoplanetary systems.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Societ
Origin and Dynamical Evolution of Neptune Trojans - II: Long Term Evolution
We present results examining the fate of the Trojan clouds produced in our
previous work. We find that the stability of Neptunian Trojans seems to be
strongly correlated to their initial post-migration orbital elements, with
those objects that survive as Trojans for billions of years displaying
negligible orbital evolution. The great majority of these survivors began the
integrations with small eccentricities (e < 0.2) and small libration amplitudes
(A < 30 - 40{\deg}). The survival rate of "pre-formed" Neptunian Trojans (which
in general survived on dynamically cold orbits (e < 0.1, i < 5 - 10{\deg}))
varied between ~5 and 70%. By contrast, the survival rate of "captured" Trojans
(on final orbits spread across a larger region of e-i element space) were
markedly lower, ranging between 1 and 10% after 4 Gyr. Taken in concert with
our earlier work, we note that planetary formation scenarios which involve the
slow migration (a few tens of millions of years) of Neptune from an initial
planetary architecture that is both resonant and compact (aN < 18 AU) provide
the most promising fit of those we considered to the observed Trojan
population. In such scenarios, we find that the current day Trojan population
would number ~1% of that which was present at the end of the planet's
migration, with the bulk being sourced from captured, rather than pre-formed
objects. We note, however, that even those scenarios still fail to reproduce
the currently observed portion of the Neptune Trojan population moving on
orbits with e 20{\deg}. Dynamical integrations of the currently
observed Trojans show that five out of the seven are dynamically stable on 4
Gyr timescales, while 2001 QR322, exhibits significant dynamical instability.
The seventh Trojan object, 2008 LC18, has such large orbital uncertainties that
only future studies will be able to determine its stability.Comment: 24 pages, 6 figures, accepted for publication in MNRAS (The abstract
was shortened. Original version can be found in the pdf file
Consolidation of complex events via reinstatement in posterior cingulate cortex
It is well-established that active rehearsal increases the efficacy of memory consolidation. It is also known that complex events are interpreted with reference to prior knowledge. However, comparatively little attention has been given to the neural underpinnings of these effects. In healthy adult humans, we investigated the impact of effortful, active rehearsal on memory for events by showing people several short video clips and then asking them to recall these clips, either aloud (Experiment 1) or silently while in an MRI scanner (Experiment 2). In both experiments, actively rehearsed clips were remembered in far greater detail than unrehearsed clips when tested a week later. In Experiment 1, highly similar descriptions of events were produced across retrieval trials, suggesting a degree of semanticization of the memories had taken place. In Experiment 2, spatial patterns of BOLD signal in medial temporal and posterior midline regions were correlated when encoding and rehearsing the same video. Moreover, the strength of this correlation in the posterior cingulate predicted the amount of information subsequently recalled. This is likely to reflect a strengthening of the representation of the video's content. We argue that these representations combine both new episodic information and stored semantic knowledge (or "schemas"). We therefore suggest that posterior midline structures aid consolidation by reinstating and strengthening the associations between episodic details and more generic schematic information. This leads to the creation of coherent memory representations of lifelike, complex events that are resistant to forgetting, but somewhat inflexible and semantic-like in nature
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