314 research outputs found
Accretion and evolution of solar system bodies
We use a combination of analytical and numerical methods to study dynamical processes involved in the formation of planets and smaller bodies in the solar system. Our goal was to identify and understand critical processes and to link them in a numerical model of planetesimal accretion. We study effects of these processes by applying them in the context of the standard model of solar system formation, which involves accretion of the terrestrial planets and cores of the giant planet from small planetesimals. The principal focus of our research effort is the numerical simulation of accretion of a swarm of planetesimals into bodies of planetary size. Our computer code uses a Monte Carlo method to determine collisional interactions within the swarm. These interactions are not determined simply by a relative velocity, but rather by explicit distributions of keplerian orbital elements. The planetesimal swarm is divided into a number of zones in semimajor axis, which are allowed to interact. The present version of our code has the capability of following detailed distributions of size, eccentricity, and inclination in each zone
Planet formation around stars of various masses: Hot super-Earths
We consider trends resulting from two formation mechanisms for short-period
super-Earths: planet-planet scattering and migration. We model scenarios where
these planets originate near the snow line in ``cold finger'' circumstellar
disks. Low-mass planet-planet scattering excites planets to low periastron
orbits only for lower mass stars. With long circularisation times, these
planets reside on long-period eccentric orbits. Closer formation regions mean
planets that reach short-period orbits by migration are most common around
low-mass stars. Above ~1 Solar mass, planets massive enough to migrate to
close-in orbits before the gas disk dissipates are above the critical mass for
gas giant formation. Thus, there is an upper stellar mass limit for
short-period super-Earths that form by migration. If disk masses are
distributed as a power law, planet frequency increases with metallicity because
most disks have low masses. For disk masses distributed around a relatively
high mass, planet frequency decreases with increasing metallicity. As icy
planets migrate, they shepherd interior objects toward the star, which grow to
~1 Earth mass. In contrast to icy migrators, surviving shepherded planets are
rocky. Upon reaching short-period orbits, planets are subject to evaporation
processes. The closest planets may be reduced to rocky or icy cores. Low-mass
stars have lower EUV luminosities, so the level of evaporation decreases with
decreasing stellar mass.Comment: Accepted to ApJ. 13 pages of emulateap
Planets Formed in Habitable Zones of M Dwarf Stars Probably are Deficient in Volatiles
Dynamical considerations, presented herein via analytic scalings and
numerical experiments, imply that Earth-mass planets accreting in regions that
become habitable zones of M dwarf stars form within several million years.
Temperatures in these regions during planetary accretion are higher than those
encountered by the material that formed the Earth. Collision velocities during
and after the prime accretionary epoch are larger than for Earth. These factors
suggest that planets orbiting low mass main sequence stars are likely to be
either too distant (and thus too cold) for carbon/water based life on their
surfaces or have abundances of the required volatiles that are substantially
less than on Earth.Comment: 11 pages, 1 figure, Astrophysical Journal Letters, in pres
Fast accretion of small planetesimals by protoplanetary cores
We explore the dynamics of small planetesimals coexisting with massive
protoplanetary cores in a gaseous nebula. Gas drag strongly affects the motion
of small bodies leading to the decay of their eccentricities and inclinations,
which are excited by the gravity of protoplanetary cores. Drag acting on larger
( km), high velocity planetesimals causes a mere reduction of their
average random velocity. By contrast, drag qualitatively changes the dynamics
of smaller ( km), low velocity objects: (1) small planetesimals
sediment towards the midplane of the nebula forming vertically thin subdisk;
(2) their random velocities rapidly decay between successive passages of the
cores and, as a result, encounters with cores typically occur at the minimum
relative velocity allowed by the shear in the disk. This leads to a drastic
increase in the accretion rate of small planetesimals by the protoplanetary
cores, allowing cores to grow faster than expected in the simple oligarchic
picture, provided that the population of small planetesimals contains more than
roughly 1% of the solid mass in the nebula. Fragmentation of larger
planetesimals ( km) in energetic collisions triggered by the
gravitational scattering by cores can easily channel this amount of material
into small bodies on reasonable timescales ( Myr in the outer Solar
System), providing a means for the rapid growth (within several Myr at 30 AU)
of rather massive protoplanetary cores. Effects of inelastic collisions between
planetesimals and presence of multiple protoplanetary cores are discussed.Comment: 17 pages, 8 figures, additional clarifications, 1 more figure and
table adde
Understanding asteroid collisional history through experimental and numerical studies
Asteroids can lose angular momentum due to so called splash effect, the analog to the drain effect for cratering impacts. Numerical code with the splash effect incorporated was applied to study the simultaneous evolution of asteroid sized and spins. Results are presented on the spin changes of asteroids due to various physical effects that are incorporated in the described model. The goal was to understand the interplay between the evolution of sizes and spins over a wide and plausible range of model parameters. A single starting population was used both for size distribution and the spin distribution of asteroids and the changes in the spins were calculated over solar system history for different model parameters. It is shown that there is a strong coupling between the size and spin evolution, that the observed relative spindown of asteroids approximately 100 km diameter is likely to be the result of the angular momentum splash effect
Experimental studies of collision and fragmentation phenomena
The reduction and publication of an extensive data set collected in experiments over several years at Ames and PSI is briefly examined. Hartmann has been assembling data sets from his experiments on catastrophic fragmentation of various materials, including basalt, other igneous rock, ice, and weak dirt clods. Weidenschilling and Davis have continued to gather and reduce data on oblique impacts. The data indicate a power law distribution of ejecta mass vs. velocity, with a slope that is independent of azimuth, and does not vary with impact angle from normal impacts to at least 75 deg from vertical. In order to improve models of coagulation of dust aggregates in the solar nebula, SJW developed an apparatus for drop tests of fragile projectiles. Davis and Weidenschilling continued to collect and analyze experimental data on collisional catastrophic disruption at the Ames Vertical Gun Range
Accretion in Protoplanetary Disks by Collisional Fusion
The formation of a solar system is believed to have followed a multi-stage
process around a protostar. Whipple first noted that planetesimal growth by
particle agglomeration is strongly influenced by gas drag; there is a
"bottleneck" at the meter scale with such bodies rapidly spiraling into the
central star, whereas much smaller or larger particles do not. Thus, successful
planetary accretion requires rapid planetesimal growth to km scale. A commonly
accepted picture is that for collisional velocities above a certain
threshold collisional velocity, 0.1-10 cm s, particle
agglomeration is not possible; elastic rebound overcomes attractive surface and
intermolecular forces. However, if perfect sticking is assumed for all
collisions the bottleneck can be overcome by rapid planetesimal growth. While
previous work has dealt explicitly with the influences of collisional pressures
and the possibility of particle fracture or penetration, the basic role of the
phase behavior of matter--phase diagrams, amorphs and polymorphs--has been
neglected. Here it is demonstrated that novel aspects of surface phase
transitions provide a physical basis for efficient sticking through collisional
melting or amphorph-/polymorphization and fusion to extend the collisional
velocity range of primary accretion to 1-100 m s,
which bound both turbulent RMS speeds and the velocity differences between
boulder sized and small grains 1-50 m s. Thus, as inspiraling
meter sized bodies collide with smaller particles in this high velocity
collisional fusion regime they grow rapidly to km scales and hence settle into
stable Keplerian orbits in 10 years before photoevaporative wind
clears the disk of source material.Comment: 11 pages, 7 figures, 1 tabl
Planetary geology: Impact processes on asteroids
The fundamental geological and geophysical properties of asteroids were studied by theoretical and simulation studies of their collisional evolution. Numerical simulations incorporating realistic physical models were developed to study the collisional evolution of hypothetical asteroid populations over the age of the solar system. Ideas and models are constrained by the observed distributions of sizes, shapes, and spin rates in the asteroid belt, by properties of Hirayama families, and by experimental studies of cratering and collisional phenomena. It is suggested that many asteroids are gravitationally-bound "rubble piles.' Those that rotate rapidly may have nonspherical quasi-equilibrium shapes, such as ellipsoids or binaries. Through comparison of models with astronomical data, physical properties of these asteroids (including bulk density) are determined, and physical processes that have operated in the solar system in primordial and subsequent epochs are studied
Constraints on the Formation of the Planet Around HD188753A
The claimed discovery of a Jupiter-mass planet in the close triple star
system HD 188753 poses a problem for planet formation theory. A circumstellar
disk around the planet's parent star would be truncated close to the star,
leaving little material available for planet formation. In this paper, we
attempt to model a protoplanetary disk around HD 188753A using a fairly simple
alpha-disk model, exploring a range of parameters constrained by observations
of T Tauri-type stars. The disk is truncated to within 1.5 to 2.7 AU, depending
on model parameters. We find that the in situ formation of the planet around HD
188753A is implausible.Comment: Accepted version, to appear in ApJ. 23 pages, 5 figures (3 in color
Phobos and deimos: Analysis of surface features, ejecta dynamics and a volatile loss mechanism
The question of whether the crater population on Phobos represents a production population or an equilibrium population is considered. The absolute ages of cratered surfaces are interpreted and analyzed. A computer program was developed to study the dynamics of material ejected from Martian satellites and to investigate the hypothesis that at least some of the extensive set of linear features discovered on the surface of Phobos could be the result of secondary cratering from the Stickney impact. The possibility that Deimos was catastrophically disrupted by a large impact but subsequently reaccreted is considered as well as the probability the Phobos had an impact nearly large enough to disrupt it are also discussed
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