581 research outputs found
Rapid Formation of Ice Giant Planets
The existence of Uranus and Neptune presents severe difficulties for the core
accretion model for the formation of ice giant planets. We suggest an
alternative mechanism, namely disk instability leading to the formation of gas
giant protoplanets, coagulation and settling of dust grains to form ice/rock
cores at their centers, and photoevaporation of their gaseous envelopes by a
nearby OB star, as a possible means of forming ice giant planets.Comment: TeX file, 20 pages, no figures, 1 table, accepted by Icarus,
Preprints available at http://www.ciw.edu/boss/ftp/planets/icaice.p
Planet formation in highly inclined binaries
We explore planet formation in binary systems around the central star where
the protoplanetary disk plane is highly inclined with respect to the companion
star orbit. This might be the most frequent scenario for binary separations
larger than 40 AU, according to Hale (1994). We focus on planetesimal accretion
and compute average impact velocities in the habitable region and up to 6 AU
from the primary.Comment: Accepted for publication on A&
Shape models and physical properties of asteroids
Despite the large amount of high quality data generated in recent space
encounters with asteroids, the majority of our knowledge about these objects
comes from ground based observations. Asteroids travelling in orbits that are
potentially hazardous for the Earth form an especially interesting group to be
studied. In order to predict their orbital evolution, it is necessary to
investigate their physical properties. This paper briefly describes the data
requirements and different techniques used to solve the lightcurve inversion
problem. Although photometry is the most abundant type of observational data,
models of asteroids can be obtained using various data types and techniques. We
describe the potential of radar imaging and stellar occultation timings to be
combined with disk-integrated photometry in order to reveal information about
physical properties of asteroids.Comment: From Assessment and Mitigation of Asteroid Impact Hazards boo
The formation of Kuiper-belt Binaries through Exchange Reactions
Recent observations have revealed an unexpectedly high binary fraction among
the Trans-Neptunian Objects (TNOs) that populate the Kuiper-belt. The
discovered binaries have four characteristics they comprise a few percent of
the TNOs, the mass ratio of their components is close to unity, their internal
orbits are highly eccentric, and the orbits are more than 100 times wider than
the primary's radius. In contrast, theories of binary asteroid formation tend
to produce close, circular binaries. Therefore, a new approach is required to
explain the unique characteristics of the TNO binaries. Two models have been
proposed. Both, however, require extreme assumptions on the size distribution
of TNOs. Here we show a mechanism which is guaranteed to produces binaries of
the required type during the early TNO growth phase, based on only one
plausible assumption, namely that initially TNOs were formed through
gravitational instabilities of the protoplanetary dust layer.Comment: 12pages, 4 figure
Collisional Velocities and Rates in Resonant Planetesimal Belts
We consider a belt of small bodies around a star, captured in one of the
external or 1:1 mean-motion resonances with a massive perturber. The objects in
the belt collide with each other. Combining methods of celestial mechanics and
statistical physics, we calculate mean collisional velocities and collisional
rates, averaged over the belt. The results are compared to collisional
velocities and rates in a similar, but non-resonant belt, as predicted by the
particle-in-a-box method. It is found that the effect of the resonant lock on
the velocities is rather small, while on the rates more substantial. The
collisional rates between objects in an external resonance are by about a
factor of two higher than those in a similar belt of objects not locked in a
resonance. For Trojans under the same conditions, the collisional rates may be
enhanced by up to an order of magnitude. Our results imply, in particular,
shorter collisional lifetimes of resonant Kuiper belt objects in the solar
system and higher efficiency of dust production by resonant planetesimals in
debris disks around other stars.Comment: 31 pages, 11 figures (some of them heavily compressed to fit into
arxiv-maximum filesize), accepted for publication at "Celestial Mechanics and
Dynamical Astronomy
Formation of Kuiper Belt Binaries
It appears that at least several percent of large Kuiper belt objects are
members of wide binaries. Physical collisions are too infrequent to account for
their formation. Collisionless gravitational interactions are more promising.
These provide two channels for binary formation. In each, the initial step is
the formation of a transient binary when two large bodies penetrate each
other's Hill spheres. Stabilization of a transient binary requires that it lose
energy. Either dynamical friction due to small bodies or the scattering of a
third large body can be responsible. Our estimates favor the former, albeit by
a small margin. We predict that most objects of size comparable to those
currently observed in the Kuiper belt are members of multiple systems. More
specifically, we derive the probability that a large body is a member of a
binary with semi-major axis of order a. The probability depends upon sigma, the
total surface density, Sigma, the surface density of large bodies having radius
R, and theta=10^-4, the angle subtended by the solar radius as seen from the
Kuiper belt. For (sigma/Sigma)R<a< R/theta, the probability is just (Sigma/rho
R)theta^-2, the optical depth of the large bodies divided by the solid angle
subtended by the Sun. For R<a<r_u=(sigma/Sigma)R, it varies inversely with
semimajor axis and reaches (sigma/rho R)theta^-2 at a=R. Based on current
surveys of the Kuiper belt, we estimate Sigma/rho=3 10^-4cm and R=100km. We
obtain sigma/rho=0.3cm by extrapolating the surface density deduced for the
minimum mass solar nebula. Rough predictions are: outside of the critical
separation r_u/a_odot=3'', the binary probability is 0.3%; at separations of
0.2'', comparable to current resolving capabilities, it reaches 5%, in
agreement with results from the HST binary survey by Brown
Vertical structure of debris discs
The vertical thickness of debris discs is often used as a measure of these
systems' dynamical excitation and as clues to the presence of hidden massive
perturbers such as planetary embryos. However, this argument could be flawed
because the observed dust should be naturally placed on inclined orbits by the
combined effect of radiation pressure and mutual collisions. We critically
reinvestigate this issue and numerically estimate what the "natural" vertical
thickness of a collisionally evolving disc is, in the absence of any additional
perturbing body. We use a deterministic collisional code, following the
dynamical evolution of a population of indestructible test grains suffering
mutual inelastic impacts. Grain differential sizes as well as the effect of
radiation pressure are taken into account. We find that, under the coupled
effect of radiation pressure and collisions, grains naturally acquire
inclinations of a few degrees. The disc is stratified with respect to grain
sizes, with the smallest grains having the largest vertical dispersion and the
bigger ones clustered closer to the midplane. Debris discs should have a
minimum "natural" observed aspect ratio at visible to
mid-IR wavelengths where the flux is dominated by the smallest bound grains.
These values are comparable to the estimated thicknesses of many vertically
resolved debris discs, as is illustrated with the specific example of AU Mic.
For all systems with , the presence (or absence) of embedded
perturbing bodies cannot be inferred from the vertical dispersion of the discComment: accepted for publication in Astronomy and Astrophysics (full abstract
in the pdf file
Relative velocities among accreting planetesimals in binary systems: the circumbinary case
We numerically investigate the possibility of planetesimal accretion in
circumbinary disks, under the coupled influence of both stars' secular
perturbations and friction due to the gaseous component of the protoplanetary
disk. We focus on one crucial parameter: the distribution of encounter
velocities between planetesimals in the 0.5 to 100km size range. An extended
range of binary systems with differing orbital parameters is explored. The
resulting encounter velocities are compared to the threshold velocities below
which the net outcome of a collision is accumulation into a larger body instead
of mass erosion. For each binary configuration, we derive the critical radial
distance from the binary barycenter beyond which planetesimal accretion is
possible. This critical radial distance is smallest for equal-mass binaries on
almost circular orbits. It shifts to larger values for increasing
eccentricities and decreasing mass ratio. The importance of the planetesimals'
orbital alignments of planetesimals due to gas drag effects is discussed.Comment: accepted for publication in MNRA
Planetary Microlensing at High Magnification
Simulations of planetary microlensing at high magnification that were carried
out on a cluster computer are presented. It was found that the perturbations
due to two-thirds of all planets occur in the time interval [-0.5t_FWHM, 0.5t_
FWHM] with respect to the peak of the microlensing light curve, where t_FWHM is
typically about 14 hours. This implies that only this restricted portion of the
light curve need be intensively monitored for planets, a very significant
practical advantage. Nearly all planetary detections in high magnification
events will not involve caustic crossings. We discuss the issues involved in
determining the planetary parameters in high magnification microlensing events.
Earth mass planets may be detected with 1-m class telescopes if their projected
orbital radii lie within about 1.5 - 2.5 AU. Giant planets are detectable over
a much larger region. For multi-planet systems the perturbations due to
individual planets can be separated under certain conditions. The size of the
source star needs to be determined independently, but the presence of spots on
the source star is likely to be negligible, as is the effect of planetary
motion during an event.Comment: 12 pages, 13 embedded figures, accepted for publication by MNRA
On the dynamics and collisional growth of planetesimals in misaligned binary systems
Context. Abridged. Many stars are members of binary systems. During early
phases when the stars are surrounded by discs, the binary orbit and disc
midplane may be mutually inclined. The discs around T Tauri stars will become
mildly warped and undergo solid body precession around the angular momentum
vector of the binary system. It is unclear how planetesimals in such a disc
will evolve and affect planet formation. Aims. We investigate the dynamics of
planetesimals embedded in discs that are perturbed by a binary companion on a
circular, inclined orbit. We examine collisional velocities of the
planetesimals to determine when they can grow through accretion. We vary the
binary inclination, binary separation, D, disc mass, and planetesimal radius.
Our standard model has D=60 AU, inclination=45 deg, and a disc mass equivalent
to the MMSN. Methods. We use a 3D hydrodynamics code to model the disc.
Planetesimals are test particles which experience gas drag, the gravitational
force of the disc, the companion star gravity. Planetesimal orbit crossing
events are detected and used to estimate collisional velocities. Results. For
binary systems with modest inclination (25 deg), disc gravity prevents
planetesimal orbits from undergoing strong differential nodal precession (which
occurs in absence of the disc), and forces planetesimals to precess with the
disc on average. For bodies of different size the orbit planes become modestly
mutually inclined, leading to collisional velocities that inhibit growth. For
larger inclinations (45 degrees), the Kozai effect operates, leading to
destructively large relative velocities. Conclusions. Planet formation via
planetesimal accretion is difficult in an inclined binary system with
parameters similar to those considered in this paper. For systems in which the
Kozai mechanism operates, the prospects for forming planets are very remote.Comment: 24 pages, 16 figures, recently published in Astronomy and
Astrophysic
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