35 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
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
Post-Giant Impact Planetesimals Sustaining Extreme Debris Disks
Extreme debris disks can show short term behaviour through the evolution and
clearing of small grains produced in giant impacts, and potentially a longer
period of variability caused by a planetesimal population formed from giant
impact ejecta. In this paper, we present results of numerical simulations to
explain how a planetesimal populated disk can supply an observed extreme debris
disk with small grains. We simulated a sample of giant impacts from which we
form a planetesimal population. We then use the -body code {\sc Rebound} to
evolve the planetesimals spatially and collisionally. We adopt a simplistic
collision criteria in which we define destructive collisions to be between
planetesimals with a mutual impact velocity that exceeds two times the
catastrophic disruption threshold, . We find that for some configurations,
a planetesimal populated disk can produce a substantial amount of dust to
sustain an observable disk. The semi-major axis at which the giant impact
occurs changes the mass added to the observed disk substantially while the
orientation of the impact has less of an effect. We determine how the collision
rate at the collision point changes over time and show that changes in
semi-major axis and orientation only change the initial collision rate of the
disk. Collision rates across all disks evolve at a similar rate.Comment: 20 pages, 21 figures, 3 tables, accepted by MNRA
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
Compositional evolution during rocky protoplanet accretion
The Earth appears non-chondritic in its abundances of refractory lithophile
elements, posing a significant problem for our understanding of its formation
and evolution. It has been suggested that this non-chondritic composition may
be explained by collisional erosion of differentiated planetesimals of
originally chondritic composition. In this work, we present N-body simulations
of terrestrial planet formation that track the growth of planetary embryos from
planetesimals. We simulate evolution through the runaway and oligarchic growth
phases under the Grand Tack model and in the absence of giant planets. These
simulations include a state-of-the-art collision model which allows multiple
collision outcomes, such as accretion, erosion, and bouncing events, that
enables tracking of the evolving core mass fraction of accreting planetesimals.
We show that the embryos grown during this intermediate stage of planet
formation exhibit a range of core mass fractions, and that with significant
dynamical excitation, enough mantle can be stripped from growing embryos to
account for the Earth's non-chondritic Fe/Mg ratio. We also find that there is
a large diversity in the composition of remnant planetesimals, with both
iron-rich and silicate-rich fragments produced via collisions.Comment: 19 pages, 15 figures. Accepted for publication in ApJ. Accompanying
animations can be found at http://www.star.bris.ac.uk/pcarter/comp_evo_1
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
The Formation of the Collisional Family around the Dwarf Planet Haumea
Haumea, a rapidly rotating elongated dwarf planet (~ 1500 km in diameter),
has two satellites and is associated with a "family" of several smaller Kuiper
Belt objects (KBOs) in similar orbits. All members of the Haumea system share a
water ice spectral feature that is distinct from all other KBOs. The relative
velocities between the Haumea family members are too small to have formed by
catastrophic disruption of a large precursor body, which is the process that
formed families around much smaller asteroids in the Main Belt. Here we show
that all of the unusual characteristics of the Haumea system are explained by a
novel type of giant collision: a graze-and-merge impact between two comparably
sized bodies. The grazing encounter imparted the high angular momentum that
spun off fragments from the icy crust of the elongated merged body. The
fragments became satellites and family members. Giant collision outcomes are
extremely sensitive to the impact parameters. Compared to the Main Belt, the
largest bodies in the Kuiper Belt are more massive and experience slower
velocity collisions; hence, outcomes of giant collisions are dramatically
different between the inner and outer solar system. The dwarf planets in the
Kuiper Belt record an unexpectedly large number of giant collisions, requiring
a special dynamical event at the end of solar system formation.Comment: Accepted for publication in ApJ, 12 pages, 4 figures, 2 tables
A Collisional Origin to Earth's Non-chondritic Composition?
Several lines of evidence indicate a non-chondritic composition for Bulk
Earth. If Earth formed from the accretion of chondritic material, its
non-chondritic composition, in particular the super-chondritic 142Nd/144Nd and
low Mg/Fe ratios, might be explained by the collisional erosion of
differentiated planetesimals during its formation. In this work we use an
N-body code, that includes a state-of-the-art collision model, to follow the
formation of protoplanets, similar to proto-Earth, from differentiated
planetesimals (> 100 km) up to isolation mass (> 0.16 M_Earth). Collisions
between differentiated bodies have the potential to change the core-mantle
ratio of the accreted protoplanets. We show that sufficient mantle material can
be stripped from the colliding bodies during runaway and oligarchic growth,
such that the final protoplanets could have Mg/Fe and Si/Fe ratios similar to
that of bulk Earth, but only if Earth is an extreme case and the core is
assumed to contain 10% silicon by mass. This may indicate an important role for
collisional differentiation during the giant impact phase if Earth formed from
chondritic material.Comment: 14 pages, 9 figures, accepted for publication in Icaru
Extreme debris disk variability : exploring the diverse outcomes of large asteroid impacts during the era of terrestrial planet formation
The most dramatic phases of terrestrial planet formation are thought to be oligarchic and chaotic growth, on timescales of up to 100─200 Myr, when violent impacts occur between large planetesimals of sizes up to protoplanets. Such events are marked by the production of large amounts of debris, as has been observed in some exceptionally bright and young debris disks (termed extreme debris disks). Here we report five years of Spitzer measurements of such systems around two young solar-type stars: ID8 and P1121. The short-term (weekly to monthly) and long-term (yearly) disk variability is consistent with the aftermaths of large impacts involving large asteroid-sized bodies. We demonstrate that an impact-produced clump of optically thick dust, under the influence of the dynamical and viewing geometry effects, can produce short-term modulation in the disk light curves. The long-term disk flux variation is related to the collisional evolution within the impact-produced fragments once released into a circumstellar orbit. The time-variable behavior observed in the P1121 system is consistent with a hypervelocity impact prior to 2012 that produced vapor condensates as the dominant impact product. Two distinct short-term modulations in the ID8 system suggest two violent impacts at different times and locations. Its long-term variation is consistent with the collisional evolution of two different populations of impact-produced debris dominated by either vapor condensates or escaping boulders. The bright, variable emission from the dust produced in large impacts from extreme debris disks provides a unique opportunity to study violent events during the era of terrestrial planet formation