13 research outputs found
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
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
Explaining the variability of WD 1145+017 with simulations of asteroid tidal disruption
Post-main-sequence planetary science has been galvanised by the striking
variability, depth and shape of the photometric transit curves due to objects
orbiting white dwarf WD 1145+017, a star which also hosts a dusty debris disc
and circumstellar gas, and displays strong metal atmospheric pollution.
However, the physical properties of the likely asteroid which is discharging
disintegrating fragments remain largely unconstrained from the observations.
This process has not yet been modelled numerically. Here, we use the N-body
code PKDGRAV to compute dissipation properties for asteroids of different
spins, densities, masses, and eccentricities. We simulate both homogeneous and
differentiated asteroids, for up to two years, and find that the disruption
timescale is strongly dependent on density and eccentricity, but weakly
dependent on mass and spin. We find that primarily rocky differentiated bodies
with moderate (~3-4 g/cm^3) bulk densities on near-circular (e <~ 0.1) orbits
can remain intact while occasionally shedding mass from their mantles. These
results suggest that the asteroid orbiting WD 1145+017 is differentiated,
resides just outside of the Roche radius for bulk density but just inside the
Roche radius for mantle density, and is more akin physically to an asteroid
like Vesta instead of one like Itokawa.Comment: Accepted in MNRAS. Movies here!:
http://www.star.bris.ac.uk/pcarter/WD1145_asteroid_disruption
Hiding in the Shadows II: Collisional Dust as Exoplanet Markers
Observations of the youngest planets (1-10 Myr for a transitional disk)
will increase the accuracy of our planet formation models. Unfortunately,
observations of such planets are challenging and time-consuming to undertake
even in ideal circumstances. Therefore, we propose the determination of a set
of markers that can pre-select promising exoplanet-hosting candidate disks. To
this end, N-body simulations were conducted to investigate the effect of an
embedded Jupiter mass planet on the dynamics of the surrounding planetesimal
disk and the resulting creation of second generation collisional dust. We use a
new collision model that allows fragmentation and erosion of planetesimals, and
dust-sized fragments are simulated in a post process step including
non-gravitational forces due to stellar radiation and a gaseous protoplanetary
disk. Synthetic images from our numerical simulations show a bright double ring
at 850 m for a low eccentricity planet, whereas a high eccentricity planet
would produce a characteristic inner ring with asymmetries in the disk. In the
presence of first generation primordial dust these markers would be difficult
to detect far from the orbit of the embedded planet, but would be detectable
inside a gap of planetary origin in a transitional disk.Comment: Accepted for publication in Ap
Forming Circumbinary Planets: N-body Simulations of Kepler-34
Observations of circumbinary planets orbiting very close to the central stars
have shown that planet formation may occur in a very hostile environment, where
the gravitational pull from the binary should be very strong on the primordial
protoplanetary disk. Elevated impact velocities and orbit crossings from
eccentricity oscillations are the primary contributors towards high energy,
potentially destructive collisions that inhibit the growth of aspiring planets.
In this work, we conduct high resolution, inter-particle gravity enabled N-body
simulations to investigate the feasibility of planetesimal growth in the
Kepler-34 system. We improve upon previous work by including planetesimal disk
self-gravity and an extensive collision model to accurately handle
inter-planetesimal interactions. We find that super-catastrophic erosion events
are the dominant mechanism up to and including the orbital radius of
Kepler-34(AB)b, making in-situ growth unlikely. It is more plausible that
Kepler-34(AB)b migrated from a region beyond 1.5 AU. Based on the conclusions
that we have made for Kepler-34 it seems likely that all of the currently known
circumbinary planets have also migrated significantly from their formation
location with the possible exception of Kepler-47(AB)c.Comment: 6 pages, 5 figures, accepted for publication in ApJ
Explaining the variability of WD 1145+017 with simulations of asteroid tidal disruption
Post-main-sequence planetary science has been galvanised by the striking variability, depth and shape of the photometric transit curves due to objects orbiting white dwarf WD 1145+017, a star which also hosts a dusty debris disc and circumstellar gas, and displays strong metal atmospheric pollution. However, the physical properties of the
likely asteroid which is discharging disintegrating fragments remain largely unconstrained from the observations. This process has not yet been modelled numerically. Here, we use the N-body code PKDGRAV to compute dissipation properties for asteroids of different spins, densities, masses, and eccentricities. We simulate both homogeneous and differentiated asteroids, for up to two years, and find that the disruption timescale is strongly dependent on density and eccentricity, but weakly dependent on mass and spin. We find that primarily rocky differentiated bodies with moderate (⌠3â4 g/cm3 ) bulk densities on near-circular (e . 0.1) orbits can remain intact while occasionally
shedding mass from their mantles. These results suggest that the asteroid orbiting WD 1145+017 is differentiated, resides just outside of the Roche radius for bulk density but just inside the Roche radius for mantle density, and is more akin physically to an asteroid like Vesta instead of one like Itokawa
Planetary embryo collisions and the wiggly nature of extreme debris discs
In this paper, we present results from a multi-stage numerical campaign to
begin to explain and determine why extreme debris disk detections are rare,
what types of impacts will result in extreme debris disks and what we can learn
about the parameters of the collision from the extreme debris disks. We begin
by simulating many giant impacts using a smoothed particle hydrodynamical code
with tabulated equations of state and track the escaping vapour from the
collision. Using an -body code, we simulate the spatial evolution of the
vapour generated dust post-impact.
We show that impacts release vapour anisotropically not isotropically as has
been assumed previously and that the distribution of the resulting generated
dust is dependent on the mass ratio and impact angle of the collision. In
addition, we show that the anisotropic distribution of post-collision dust can
cause the formation or lack of formation of the short-term variation in flux
depending on the orientation of the collision with respect to the orbit around
the central star. Finally, our results suggest that there is a narrow region of
semi-major axis where a vapour generated disk would be observable for any
significant amount of time implying that giant impacts where most of the
escaping mass is in vapour would not be observed often but this does not mean
that the collisions are not occurring.Comment: 21 pages, 13 figures, 2 tables, accepted for publication in MNRA
Planetesimals to Protoplanets. I. Effect of Fragmentation on Terrestrial Planet Formation
How Not to Build Tatooine: The Difficulty of In Situ Formation of Circumbinary Planets Kepler 16b, Kepler 34b, and Kepler 35b
We study planetesimal evolution in circumbinary disks, focusing on the three
systems Kepler 16, 34 and 35 where planets have been discovered recently. We
show that for circumbinary planetesimals, in addition to secular forcing,
eccentricities evolve on a dynamical timescale, which leads to orbital
crossings even in the presence of gas drag. This makes the current locations of
the circumbinary Kepler planets hostile to planetesimal accretion. We then
present results from simulations including planetesimal formation and dust
accretion, and show that even in the most favourable case of 100% efficient
dust accretion, in situ growth starting from planetesimals smaller than ~10 km
is difficult for Kepler 16b, Kepler 34b and Kepler 35b. These planets were
likely assembled further out in the disk, and migrated inward to their current
location.Comment: 5 pages, 3 figures, accepted for publication in ApJ