1,112 research outputs found
Formation of planetesimals
Formation of planetesimals is discussed. The following subject areas are covered: (1) nebular structure; (2) aerodynamics of the solid bodies in the nebula; (3) problems with gravitational instability; (4) particle growth by coagulation; properties of fractal aggregates; and (5) coagulation and settling of fractal aggregates
Dust retention in protoplanetary disks
Context: Protoplanetary disks are observed to remain dust-rich for up to
several million years. Theoretical modeling, on the other hand, raises several
questions. Firstly, dust coagulation occurs so rapidly, that if the small dust
grains are not replenished by collisional fragmentation of dust aggregates,
most disks should be observed to be dust poor, which is not the case. Secondly,
if dust aggregates grow to sizes of the order of centimeters to meters, they
drift so fast inwards, that they are quickly lost.
Aims: We attempt to verify if collisional fragmentation of dust aggregates is
effective enough to keep disks 'dusty' by replenishing the population of small
grains and by preventing excessive radial drift.
Methods: With a new and sophisticated implicitly integrated coagulation and
fragmentation modeling code, we solve the combined problem of coagulation,
fragmentation, turbulent mixing and radial drift and at the same time solve for
the 1-D viscous gas disk evolution.
Results: We find that for a critical collision velocity of 1 m/s, as
suggested by laboratory experiments, the fragmentation is so effective, that at
all times the dust is in the form of relatively small particles. This means
that radial drift is small and that large amounts of small dust particles
remain present for a few million years, as observed. For a critical velocity of
10 m/s, we find that particles grow about two orders of magnitude larger, which
leads again to significant dust loss since larger particles are more strongly
affected by radial drift.Comment: Letter accepted 3 July 2009, included comments of language edito
The Physics of Protoplanetesimal Dust Agglomerates. Vi. Erosion of Large Aggregates as a Source of Micrometer-Sized Particles
Observed protoplanetary disks consist of a large amount of micrometer-sized
particles. Dullemond and Dominik (2005) pointed out for the first time the
difficulty in explaining the strong mid-IR excess of classical T-Tauri stars
without any dust-retention mechanisms. Because high relative velocities in
between micrometer-sized and macroscopic particles exist in protoplanetary
disks, we present experimental results on the erosion of macroscopic
agglomerates consisting of micrometer-sized spherical particles via the impact
of micrometer-sized particles. We find that after an initial phase, in which an
impacting particle erodes up to 10 particles of an agglomerate, the impacting
particles compress the agglomerate's surface, which partly passivates the
agglomerates against erosion. Due to this effect the erosion halts within our
error bars for impact velocities up to ~30 m/s. For larger velocities, the
erosion is reduced by an order of magnitude. This outcome is explained and
confirmed by a numerical model. In a next step we build an analytical disk
model and implement the experimentally found erosive effect. The model shows
that erosion is a strong source of micrometer-sized particles in a
protoplanetary disk. Finally we use the stationary solution of this model to
explain the amount of micrometer-sized particles in observational infrared data
of Furlan et al. (2006)
High Velocity Dust Collisions: Forming Planetesimals in a Fragmentation Cascade with Final Accretion
In laboratory experiments we determine the mass gain and loss in central
collisions between cm to dm-size SiO2 dust targets and sub-mm to cm-size SiO2
dust projectiles of varying mass, size, shape, and at different collision
velocities up to ~56.5 m/s. Dust projectiles much larger than 1 mm lead to a
small amount of erosion of the target but decimetre targets do not break up.
Collisions produce ejecta which are smaller than the incoming projectile.
Projectiles smaller than 1 mm are accreted by a target even at the highest
collision velocities. This implies that net accretion of decimetre and larger
bodies is possible. Independent of the original size of a projectile
considered, after several collisions all fragments will be of sub-mm size which
might then be (re)-accreted in the next collision with a larger body. The
experimental data suggest that collisional growth through fragmentation and
reaccretion is a viable mechanism to form planetesimals
Coagulation of small grains in disks: the influence of residual infall and initial small-grain content
Turbulent coagulation in protoplanetary disks is known to operate on
timescale far shorter than the lifetime of the disk. In the absence of
mechanisms that replenish the small dust grain population, protoplanetary disks
would rapidly lose their continuum opacity-bearing dust. This is inconsistent
with infrared observations of disks around T Tauri stars and Herbig Ae/Be
stars, which are usually optically thick at visual wavelengths and show
signatures of small (a<~ 3um) grains. A plausible replenishing mechanism of
small grains is collisional fragmentation or erosion of large dust aggregates,
which model calculations predict to play an important role in protoplanetary
disks. If optically thick disks are to be seen as proof for ongoing
fragmentation or erosion, then alternative explanations for the existence of
optically thick disks must be studied carefully. In this study we explore two
scenarios. First, we study the effect of residual, low-level infall of matter
onto the disk surface. We find that infall rates as low as 10^{-11} Msun/yr
can, in principle, replenish the small grain population to a level that keeps
the disk marginally optically thick. However, it remains to be seen if the
assumption of such inflow is realistic for star+disk systems at the age of
several Myrs, at which winds and jets are expected to have removed any residual
envelope. In summary, fragmentation or erosion still appear to be the most
promising processes to explain the abundant presence of small grains in old
disks.Comment: 10 pages, 4 figures, A&A in pres
Chondrules and Nebular Shocks
Beneath the fusion-encrusted surfaces of the most primitive stony meteorites
lies not homogeneous rock, but a profusion of millimeter-sized igneous spheres.
These chondrules, and their centimeter-sized counterparts, the
calcium-aluminum-rich inclusions, comprise more than half of the volume
fraction of chondritic meteorites. They are the oldest creations of the solar
system. Their chemical composition matches that of the solar photosphere in all
but the most volatile of elements, reflecting their condensation from the same
pristine gas that formed the sun. In this invited editorial, we review the
nebular shock wave model of Desch and Connolly (Meteoritics and Planetary
Science 2002, 37, 183) that seeks to explain their origin. While the model
succeeds in reproducing the unique petrological signatures of chondrules, the
origin of the required shock waves in protoplanetary disks remains a mystery.
Outstanding questions are summarized, with attention paid briefly to competing
models.Comment: Invited editorial in Meteoritics and Planetary Science, vol. 37, no.
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
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
The Progressive Self-Culture of a Lutheran Pastor
During his three years at the seminary the theological student is taught the qualifications and requirements of a Lutheran pastor, and he is informed of the tremendous responsibilities and the exacting demands which a call into the ministry will lay upon him when he has become a pastor of a congregation. But the seminary training is not able to convert the student into a full-fledged pastor. Only long years of hard study and practical experience and divine grace can equip a man successfully and completely for this greatest and hardest calling and work in this world. Unless the ministerial candidate, upon the entry into his life’s work, continues, with firm determination and fixed purpose all through his life, to erect a well-built superstructure upon the foundation which was laid with the help of his teachers at the seminary, he will soon find himself unable to cope with the problems of his calling, he will fail to render efficient service as a steward over God\u27s household, and he will forfeit the joy and the reward of a faithful shepherd of the f lock over which God has placed him
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
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