1,006 research outputs found
Collisions Between Gravity-Dominated Bodies: 1. Outcome Regimes and Scaling Laws
Collisions are the core agent of planet formation. In this work, we derive an
analytic description of the dynamical outcome for any collision between
gravity-dominated bodies. We conduct high-resolution simulations of collisions
between planetesimals; the results are used to isolate the effects of different
impact parameters on collision outcome. During growth from planetesimals to
planets, collision outcomes span multiple regimes: cratering, merging,
disruption, super-catastrophic disruption, and hit-and-run events. We derive
equations (scaling laws) to demarcate the transition between collision regimes
and to describe the size and velocity distributions of the post-collision
bodies. The scaling laws are used to calculate maps of collision outcomes as a
function of mass ratio, impact angle, and impact velocity, and we discuss the
implications of the probability of each collision regime during planet
formation.
The analytic collision model presented in this work will significantly
improve the physics of collisions in numerical simulations of planet formation
and collisional evolution. (abstract abridged)Comment: Version 3, accepted to ApJ in Nov. 2011 published online Dec. 2011.
Abstract abridge
The Validity of the Super-Particle Approximation during Planetesimal Formation
The formation mechanism of planetesimals in protoplanetary discs is hotly
debated. Currently, the favoured model involves the accumulation of meter-sized
objects within a turbulent disc, followed by a phase of gravitational
instability. At best one can simulate a few million particles numerically as
opposed to the several trillion meter-sized particles expected in a real
protoplanetary disc. Therefore, single particles are often used as
super-particles to represent a distribution of many smaller particles. It is
assumed that small scale phenomena do not play a role and particle collisions
are not modeled. The super-particle approximation can only be valid in a
collisionless or strongly collisional system, however, in many recent numerical
simulations this is not the case.
In this work we present new results from numerical simulations of
planetesimal formation via gravitational instability. A scaled system is
studied that does not require the use of super-particles. We find that the
scaled particles can be used to model the initial phases of clumping if the
properties of the scaled particles are chosen such that all important
timescales in the system are equivalent to what is expected in a real
protoplanetary disc. Constraints are given for the number of particles needed
in order to achieve numerical convergence.
We compare this new method to the standard super-particle approach. We find
that the super-particle approach produces unreliable results that depend on
artifacts such as the gravitational softening in both the requirement for
gravitational collapse and the resulting clump statistics. Our results show
that short range interactions (collisions) have to be modelled properly.Comment: 10 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Museums, conversations, and learning
In this study, 178 groups of visitors were interviewed and recorded during their visits to museums. Three clusters of elements were shown to influence learning: the identity of the visitors, their response to the learning environment, and their explanatory engagement during the visit. A structural equation model using these variables fit well. Further examination revealed that not all conversational behavior was supportive of learning; some actions, such as making frequent personal connections, were detrimental to learning; additionally, silent contemplation was modestly associated with learning. This paper discusses these findings through the experiences of four couples whose outcome measures placed them at the extreme high or low end of the learning distribution
Erosive Hit-and-Run Impact Events: Debris Unbound
Erosive collisions among planetary embryos in the inner solar system can lead
to multiple remnant bodies, varied in mass, composition and residual velocity.
Some of the smaller, unbound debris may become available to seed the main
asteroid belt. The makeup of these collisionally produced bodies is different
from the canonical chondritic composition, in terms of rock/iron ratio and may
contain further shock-processed material. Having some of the material in the
asteroid belt owe its origin from collisions of larger planetary bodies may
help in explaining some of the diversity and oddities in composition of
different asteroid groups.Comment: 7 pages, 3 figure
WRITING AND PUBLISHING AS CONVERSATION/ LA ESCRITURA Y LA PUBLICACIÓN COMO CONVERSACIÓN/ A ESCRITA E A PUBLICAÇÃO COMO CONVERSAÇÃO
This article asserts that the publishing process will be better understood as a conversational task that connects empirical precedents and theoretical debates in a given domain to produce new claims. It proposes that the lack of a conversational focus on the current academic environment and an excessive emphasis on impact indexes have created a citational inflation, in which authors and journals try to artificially increase the impact indexes without contributing to disciplinary progress. Based on current literature in psychology and conversational analysis, the article suggests that conversations are collaborative efforts that, in the case of publishing, must respond to two principles: quantity and quality. Quantity refers to making contributions as informative as possible, and quality refers to providing strong support for every claim. To explain this perspective, the author presents two case studies regarding the elaboration of seminal papers on educational psychology and education
Direct N-body Simulations of Rubble Pile Collisions
There is increasing evidence that many km-sized bodies in the Solar System
are piles of rubble bound together by gravity. We present results from a
project to map the parameter space of collisions between km-sized spherical
rubble piles. The results will assist in parameterization of collision outcomes
for Solar System formation models and give insight into fragmentation scaling
laws. We use a direct numerical method to evolve the positions and velocities
of the rubble pile particles under the constraints of gravity and physical
collisions. We test the dependence of the collision outcomes on impact
parameter and speed, impactor spin, mass ratio, and coefficient of restitution.
Speeds are kept low (< 10 m/s, appropriate for dynamically cool systems such as
the primordial disk during early planet formation) so that the maximum strain
on the component material does not exceed the crushing strength. We compare our
results with analytic estimates and hydrocode simulations. Off-axis collisions
can result in fast-spinning elongated remnants or contact binaries while fast
collisions result in smaller fragments overall. Clumping of debris escaping
from the remnant can occur, leading to the formation of smaller rubble piles.
In the cases we tested, less than 2% of the system mass ends up orbiting the
remnant. Initial spin can reduce or enhance collision outcomes, depending on
the relative orientation of the spin and orbital angular momenta. We derive a
relationship between impact speed and angle for critical dispersal of mass in
the system. We find that our rubble piles are relatively easy to disperse, even
at low impact speed, suggesting that greater dissipation is required if rubble
piles are the true progenitors of protoplanets.Comment: 30 pages including 4 tables, 8 figures. Revised version to be
published in Icarus
Tidal disruption of satellites and formation of narrow rings
In this paper we investigate the formation of narrow planetary rings such as
those found around Uranus and Saturn through the tidal disruption of a weak,
gravitationally bound satellite that migrates within its Roche limit. Using
-body simulations, we study the behaviour of rubble piles placed on circular
orbits at different distances from a central planet. We consider both
homogeneous satellites and differentiated bodies containing a denser core. We
show that the Roche limit for a rubble pile is closer to the planet than for a
fluid body of the same mean density. The Roche limit for a differentiated body
is also closer to the planet than for a homogeneous satellite of the same mean
density. Within its Roche limit, a homogeneous satellite totally disrupts and
forms a narrow ring. The initial stages of the disruption are similar to the
evolution of a viscous fluid ellipsoid, which can be computed
semi-analytically. On the other hand, when a differentiated satellite is just
within the Roche limit only the mantle is disrupted. This process is similar to
Roche-lobe overflow in interacting binary stars and produces two narrow rings
on either side of a remnant satellite. We argue that the Uranian rings, and
possibly their shepherd satellites, could have been formed through the tidal
disruption of a number of protomoons that were formed inside the corotation
radius of Uranus and migrated slowly inwards as a result of tidal interaction
with the planet.Comment: Accepted for publication in MNRAS. Some figures have been compressed
to fit into astro-ph size guidelines. Please contact authors if full
resolution images are require
Hiding in the Shadows: Searching for Planets in Pre--transitional and Transitional Disks
Transitional and pre--transitional disks can be explained by a number of
mechanisms. This work aims to find a single observationally detectable marker
that would imply a planetary origin for the gap and, therefore, indirectly
indicate the presence of a young planet. N-body simulations were conducted to
investigate the effect of an embedded planet of one Jupiter mass on the
production of instantaneous collisional dust derived from a background
planetesimal disk. Our new model allows us to predict the dust distribution and
resulting observable markers with greater accuracy than previous work.
Dynamical influences from a planet on a circular orbit are shown to enhance
dust production in the disk interior and exterior to the planet orbit while
removing planetesimals from the the orbit itself creating a clearly defined
gap. In the case of an eccentric planet the gap opened by the planet is not as
clear as the circular case but there is a detectable asymmetry in the dust
disk.Comment: Accepted to ApJL 25th September 2013. 4 figures, 1 tabl
Collisional stripping of planetary crusts
Geochemical studies of planetary accretion and evolution have invoked various
degrees of collisional erosion to explain differences in bulk composition
between planets and chondrites. Here we undertake a full, dynamical evaluation
of 'crustal stripping' during accretion and its key geochemical consequences.
We present smoothed particle hydrodynamics simulations of collisions between
differentiated rocky planetesimals and planetary embryos. We find that the
crust is preferentially lost relative to the mantle during impacts, and we have
developed a scaling law that approximates the mass of crust that remains in the
largest remnant. Using this scaling law and a recent set of N-body simulations,
we have estimated the maximum effect of crustal stripping on incompatible
element abundances during the accretion of planetary embryos. We find that on
average one third of the initial crust is stripped from embryos as they
accrete, which leads to a reduction of ~20% in the budgets of the heat
producing elements if the stripped crust does not reaccrete. Erosion of crusts
can lead to non-chondritic ratios of incompatible elements, but the magnitude
of this effect depends sensitively on the details of the crust-forming melting
process. The Lu/Hf system is fractionated for a wide range of crustal formation
scenarios. Using eucrites (the products of planetesimal silicate melting,
thought to represent the crust of Vesta) as a guide to the Lu/Hf of
planetesimal crust partially lost during accretion, we predict the Earth could
evolve to a superchondritic 176-Hf/177-Hf (3-5 parts per ten thousand) at
present day. Such values are in keeping with compositional estimates of the
bulk Earth. Stripping of planetary crusts during accretion can lead to
detectable changes in bulk composition of lithophile elements, but the
fractionation is relatively subtle, and sensitive to the efficiency of
reaccretion.Comment: 15 pages, 9 figures. Accepted for publication in EPSL. Abstract
shortened. Accompanying animations can be found at
http://www.star.bris.ac.uk/pcarter/crust_strip
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