6,128 research outputs found
Evolution of a ring around the Pluto-Charon binary
We consider the formation of satellites around the Pluto-Charon binary. An
early collision between the two partners likely produced the binary and a
narrow ring of debris, out of which arose the moons Styx, Nix, Kerberos and
Hydra. How the satellites emerged from the compact ring is uncertain. Here we
show that a particle ring spreads from physical collisions and collective
gravitational scattering, similar to migration. Around a binary, these
processes take place in the reference frames of "most circular" orbits, akin to
circular ones in a Keplerian potential. Ring particles damp to these orbits and
avoid destructive collisions. Damping and diffusion also help particles survive
dynamical instabilities driven by resonances with the binary. In some
situations, particles become trapped near resonances that sweep outward with
the tidal evolution of the Pluto-Charon binary. With simple models and
numerical experiments, we show how the Pluto-Charon impact ring may have
expanded into a broad disk, out of which grew the circumbinary moons. In some
scenarios, the ring can spread well beyond the orbit of Hydra, the most distant
moon, to form a handful of smaller satellites. If these small moons exist, New
Horizons will find them.Comment: 44 pages of text, 10 figures, ApJ, accepted. (In this version,
Section 2 and Figure 1 are new, outlining our approach to the problem of
satellite formation around the Pluto-Charon binary.
Terrestrial planet formation: Dynamical shake-up and the low mass of Mars
We consider a dynamical shake-up model to explain the low mass of Mars and
the lack of planets in the asteroid belt. In our scenario, a secular resonance
with Jupiter sweeps through the inner solar system as the solar nebula
depletes, pitting resonant excitation against collisional damping in the Sun's
protoplanetary disk. We report the outcome of extensive numerical calculations
of planet formation from planetesimals in the terrestrial zone, with and
without dynamical shake-up. If the Sun's gas disk within the terrestrial zone
depletes in roughly a million years, then the sweeping resonance inhibits
planet formation in the asteroid belt and substantially limits the size of
Mars. This phenomenon likely occurs around other stars with long-period massive
planets, suggesting that asteroid belt analogs are common.Comment: AJ, in press; 49 pages, 8 figure
Formation of Super-Earth Mass Planets at 125-250 AU from a Solar-type Star
We investigate pathways for the formation of icy super-Earth mass planets
orbiting at 125-250 AU around a 1 solar mass star. An extensive suite of
coagulation calculations demonstrates that swarms of 1 cm to 10 m planetesimals
can form super-Earth mass planets on time scales of 1-3 Gyr. Collisional
damping of 0.01-100 cm particles during oligarchic growth is a highlight of
these simulations. In some situations, damping initiates a second runaway
growth phase where 100-3000 km protoplanets grow to super-Earth sizes. Our
results establish the initial conditions and physical processes required for in
situ formation of super-Earth planets at large distances from the host star.
For nearby dusty disks in HD 107146, HD 202628, and HD 207129, ongoing
super-Earth formation at 80-150 AU could produce gaps and other structures in
the debris. In the solar system, forming a putative planet X at a
1000 AU) requires a modest (very massive) protosolar nebula.Comment: 48 pages of text, 23 figures, ApJ in press, revised version contains
new text on aspects of the calculations and a more comprehensive description
of the origin of the second phase of runaway growt
Coagulation Calculations of Icy Planet Formation Around 0.1--0.5~\msun\ Stars: Super-Earths From Large Planetestimals
We investigate formation mechanisms for icy super-Earth mass planets orbiting
at 2-20 AU around 0.1-0.5 solar mass stars. A large ensemble of coagulation
calculations demonstrates a new formation channel: disks composed of large
planetesimals with radii of 30-300 km form super-Earths on time scales of
roughly 1 Gyr. In other gas-poor disks, a collisional cascade grinds
planetesimals to dust before the largest planets reach super-Earth masses. Once
icy Earth-mass planets form, they migrate through the leftover swarm of
planetesimals at rates of 0.01-1 AU per Myr. On time scales of 10 Myr to 1 Gyr,
many of these planets migrate through the disk of leftover planetesimals from
semimajor axes of 5-10 AU to 1-2 AU. A few per cent of super-Earths might
migrate to semimajor axes of 0.1-0.2 AU. When the disk has an initial mass
comparable with the minimum mass solar nebula scaled to the mass of the central
star, the predicted frequency of super-Earths matches the observed frequency.Comment: 32 pages, 16 figures, ApJ accepte
Planet formation around binary stars: Tatooine made easy
We examine characteristics of circumbinary orbits in the context of current
planet formation scenarios. Analytical perturbation theory predicts the
existence of nested circumbinary orbits that are generalizations of circular
paths around a single star. These orbits have forced eccentric motion aligned
with the binary as well as higher frequency oscillations, yet they do not
cross, even in the presence of massive disks and perturbations from large
planets. For this reason, dissipative gas and planetesimals can settle onto
these "most circular" orbits, facilitating the growth of protoplanets. Outside
a region close to the binary where orbits are generally unstable, circumbinary
planets form in much the same way as their cousins around a single star. Here,
we review the theory and confirm its predictions with a suite of representative
simulations. We then consider the circumbinary planets discovered with NASA's
Kepler satellite. These Neptune- and Jupiter-size planets, or their
planetesimal precursors, may have migrated inward to reach their observed
orbits, since their current positions are outside of unstable zones caused by
overlapping resonances. In situ formation without migration seems less likely,
only because the surface density of the protoplanetary disks must be
implausibly high. Otherwise, the circumbinary environment is friendly to planet
formation, and we expect that many Earth-like "Tatooines" will join the growing
census of circumbinary planets.Comment: 45 pages of text, 1 table, 9 figures, as published in ApJ (2015, 806,
98
Making Planet Nine: Pebble Accretion at 250--750 AU in a Gravitationally Unstable Ring
We investigate the formation of icy super-Earth mass planets within a
gravitationally unstable ring of solids orbiting at 250-750 AU around a 1 solar
mass star. Coagulation calculations demonstrate that a system of a few large
oligarchs and a swarm of pebbles generates a super-Earth within 100-200 Myr at
250 AU and within 1-2 Gyr at 750 AU. Systems with more than ten oligarchs fail
to yield super-Earths over the age of the solar system. As these systems
evolve, destructive collisions produce detectable debris disks with
luminosities of to relative to the central star.Comment: 24 pages of text, 1 table, 8 figures, ApJ submitted, comments welcom
Numerical Simulations of Collisional Cascades at the Roche Limits of White Dwarf Stars
We consider the long-term collisional and dynamical evolution of solid
material orbiting in a narrow annulus near the Roche limit of a white dwarf.
With orbital velocities of 300 km/sec, systems of solids with initial
eccentricity generate a collisional cascade where objects
with radii 100--300 km are ground to dust. This process converts
1-100 km asteroids into 1 m particles in yr. Throughout this
evolution, the swarm maintains an initially large vertical scale height .
Adding solids at a rate enables the system to find an equilibrium
where the mass in solids is roughly constant. This equilibrium depends on
and , the radius of the largest solid added to the swarm. When
10 km, this equilibrium is stable. For larger , the mass
oscillates between high and low states; the fraction of time spent in high
states ranges from 100% for large to much less than 1% for small
. During high states, the stellar luminosity reprocessed by the solids
is comparable to the excess infrared emission observed in many metallic line
white dwarfs.Comment: 37 pages of text, 12 figures, ApJ, accepte
Numerical Simulations of Gaseous Disks Generated from Collisional Cascades at the Roche Limits of White Dwarf Stars
We consider the long-term evolution of gaseous disks fed by the vaporization
of small particles produced in a collisional cascade inside the Roche limit of
a 0.6 Msun white dwarf. Adding solids with radius \r0\ at a constant rate
into a narrow annulus leads to two distinct types of evolution.
When = ~g s, the cascade generates a fairly steady accretion disk
where the mass transfer rate of gas onto the white dwarf is roughly
and the mass in gas is ~g, where
is the temperature of the gas near the Roche limit and is the
dimensionless viscosity parameter. If , the
system alternates between high states with large mass transfer rates and low
states with negligible accretion. Although either mode of evolution adds
significant amounts of metals to the white dwarf photosphere, none of our
calculations yield a vertically thin ensemble of solids inside the Roche limit.
X-ray observations can place limits on the mass transfer rate and test this
model for metallic line white dwarfs.Comment: 30 pages and 8 figures, ApJ, accepte
Rapid Formation of Icy Super-Earths and the Cores of Gas Giant Planets
We describe a coagulation model that leads to the rapid formation of
super-Earths and the cores of gas giant planets. Interaction of collision
fragments with the gaseous disk is the crucial element of this model. The gas
entrains small collision fragments, which rapidly settle to the disk midplane.
Protoplanets accrete the fragments and grow to masses of at least 1 Earth mass
in roughly 1 Myr. Our model explains the mass distribution of planets in the
Solar System and predicts that super-Earths form more frequently than gas
giants in low mass disks.Comment: ApJLetters, accepted; 10 pages of text and 2 figure
Variations on Debris Disks IV. An Improved Analytical Model for Collisional Cascades
We derive a new analytical model for the evolution of a collisional cascade
in a thin annulus around a single central star. In this model, the
size of the largest object declines with time (t); , with = 0.1-0.2. Compared to standard models where
is constant in time, this evolution results in a more rapid decline
of , the total mass of solids in the annulus and , the luminosity of
small particles in the annulus: and . We demonstrate that the analytical model
provides an excellent match to a comprehensive suite of numerical coagulation
simulations for annuli at 1 AU and at 25 AU. If the evolution of real debris
disks follows the predictions of the analytical or numerical models, the
observed luminosities for evolved stars require up to a factor of two more mass
than predicted by previous analytical models.Comment: ApJ, in press, 22 pages of text and 14 figure
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