22 research outputs found
The influence of high speed collisions and gas flow on the formation of planetesimals
Ein zentrales Problem der Planetologie ist die Frage nach der Entstehung der Planeten. Als Vorläufer der Planeten werden die so genannten Planetesimale betrachtet. Diese km-großen Körper wachsen in protoplanetaren Scheiben aus kleinen Staubteilchen. Verantwortlich für das Wachstum sind unter anderem die zahlreichen Stöße zwischen den Staubteilchen, die dabei entstehende Staubaggregate, und die Wechselwirkung dieser Körper mit dem Gas in der Scheibe. Wie genau die Entstehungsprozesse ablaufen und ob größere Körper in Stößen wachsen können, ist noch umstritten. In gezielten Laborexperimenten haben wir versucht diese Prozesse wiederzugeben. In Stoßexperimenten haben wir den Ausgang von Hochgeschwindigkeitskollisionen zwischen cm-großen Staubaggregaten mit verschiedener Struktur untersucht. Zusätzlich haben wir in einer Serie von Windtunnelexperimenten die Stabilität dieser Aggregate in starker Windströmung erforscht. Unsere Ergebnisse zeigen, dass bei bestimmten Bedingungen ein Wachstum von Planetesimalen möglich ist.thesi
A Mechanism to Produce the Small Dust Observed in Protoplanetary Disks
Small (sub)-micron dust is present over the entire lifetime of protoplanetary
disks. As aggregation readily depletes small particles, one explanation might
be that dust is continuously generated by larger bodies in the midplane and
transported to the surface of the disks. In general, in a first step of this
scenario, the larger bodies have to be destroyed again and different mechanisms
exist with the potential to accomplish this. Possible destructive mechanisms
are fragmentation in collisions, erosion by gas drag or light induced erosion.
In laboratory experiments we find that the latter, light induced erosion by
Knudsen compression and photophoresis, can provide small particles. It might be
a preferred candidate as the dust is released into a low particle density
region. The working principle of this mechanism prevents or decreases the
likelihood for instant re-accretion or re-growth of large dense aggregates.
Provided that there is a particle lift, e.g. turbulence, these particles might
readily reach the surface of the disk.Comment: 7 pages, 6 figure
Wind-shearing in gaseous protoplanetary disks and the evolution of binary planetesimals
One of the first stages of planet formation is the growth of small
planetesimals. This early stage occurs much before the dispersal of most of the
gas from the protoplanetary disk. Due to their different aerodynamic
properties, planetesimals of different sizes and shapes experience different
drag forces from the gas during this time. Such differential forces produce a
wind-shearing (WISH) effect between close by, different size planetesimals. For
any two planetesimals, a WISH radius can be considered, at which the
differential acceleration due to the wind becomes greater than the mutual
gravitational pull between the planetesimals. We find that the WISH radius
could be much smaller than the Hill radius, i.e. WISH could play a more
important role than tidal perturbations by the star. Here we study the WISH
radii for planetesimal pairs of different sizes and compare the effects of wind
and gravitational shearing (drag force vs. gravitational tidal force). We then
discuss the role of WISH for the stability and survival of binary
planetesimals. Binaries are sheared apart by the wind if they are wider than
their WISH radius. WISH-stable binaries can inspiral and possibly coalesce due
to gas drag. Here, we calculate the WISH radius and the gas drag-induced merger
timescale, providing stability and survival criteria for gas-embedded binary
planetesimals. Our results suggest that even WISH-stable binaries may merge in
times shorter than the lifetime of the gaseous disk. This may constrain
currently observed binary planetesimals to have formed far from the star or at
a late stage after the dispersal of most of the disk gas. We note that the WISH
radius may also be important for other processes such as planetesimal erosion
and planetesimal encounters and collisions in a gaseous environment.Comment: ApJ, in pres
Dust Ejection from Planetary Bodies by Temperature Gradients: Laboratory Experiments
Laboratory experiments show that dusty bodies in a gaseous environment eject
dust particles if they are illuminated. We find that even more intense dust
eruptions occur when the light source is turned off. We attribute this to a
compression of gas by thermal creep in response to the changing temperature
gradients in the top dust layers. The effect is studied at a light flux of 13
kW/(m*m) and 1 mbar ambient pressure. The effect is applicable to
protoplanetary disks and Mars. In the inner part of protoplanetary disks,
planetesimals can be eroded especially at the terminator of a rotating body.
This leads to the production of dust which can then be transported towards the
disk edges or the outer disk regions. The generated dust might constitute a
significant fraction of the warm dust observed in extrasolar protoplanetary
disks. We estimate erosion rates of about 1 kg/s for 100 m parent bodies. The
dust might also contribute to subsequent planetary growth in different
locations or on existing protoplanets which are large enough not to be
susceptible to particle loss by light induced ejection. Due to the ejections,
planetesimals and smaller bodies will be accelerated or decelerated and drift
outward or inward, respectively. The effect might also explain the entrainment
of dust in dust devils on Mars, especially at high altitudes where gas drag
alone might not be sufficient.Comment: 7 pages, 10 figure
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
The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? I. Mapping the zoo of laboratory collision experiments
The growth processes from protoplanetary dust to planetesimals are not fully
understood. Laboratory experiments and theoretical models have shown that
collisions among the dust aggregates can lead to sticking, bouncing, and
fragmentation. However, no systematic study on the collisional outcome of
protoplanetary dust has been performed so far so that a physical model of the
dust evolution in protoplanetary disks is still missing. We intend to map the
parameter space for the collisional interaction of arbitrarily porous dust
aggregates. This parameter space encompasses the dust-aggregate masses, their
porosities and the collision velocity. With such a complete mapping of the
collisional outcomes of protoplanetary dust aggregates, it will be possible to
follow the collisional evolution of dust in a protoplanetary disk environment.
We use literature data, perform own laboratory experiments, and apply simple
physical models to get a complete picture of the collisional interaction of
protoplanetary dust aggregates. In our study, we found four different types of
sticking, two types of bouncing, and three types of fragmentation as possible
outcomes in collisions among protoplanetary dust aggregates. We distinguish
between eight combinations of porosity and mass ratio. For each of these cases,
we present a complete collision model for dust-aggregate masses between 10^-12
and 10^2 g and collision velocities in the range 10^-4 to 10^4 cm/s for
arbitrary porosities. This model comprises the collisional outcome, the
mass(es) of the resulting aggregate(s) and their porosities. We present the
first complete collision model for protoplanetary dust. This collision model
can be used for the determination of the dust-growth rate in protoplanetary
disks.Comment: accepted by Astronomy and Astrophysic
Light-induced disassembly of dusty bodies in inner protoplanetary discs: implications for the formation of planets
Laboratory experiments show that a solid-state greenhouse effect in
combination with thermophoresis can efficiently erode a dust bed in a
low-pressure gaseous environment. The surface of an illuminated, light
absorbing dusty body is cooler than the dust below the surface (solidstate
greenhouse effect). This temperature gradient leads to a directed momentum
transfer between gas and dust particles and the dust particles are subject to a
force towards the surface(thermophoresis). If the thermophoretic force is
stronger than gravity and cohesion, dust particles are ejected. Applied to
protoplanetary discs, dusty bodies smaller than several kilometres in size
which are closer to a star than about 0.4 au are subject to a rapid and
complete disassembly to submillimetre size dust aggregates by this process.
While an inward-drifting dusty body is destroyed, the generated dust is not
lost for the disc by sublimation or subsequent accretion on to the star but can
be reprocessed by photophoresis or radiation pressure. Planetesimals cannot
originate through aggregation of dust inside the erosion zone. If objects
larger than several kilometres already exist, they prevail and further grow by
collecting dust from disassembled smaller bodies. The pile-up of solids in a
confined inner region of the disc, in general, boosts the formation of planets.
Erosion is possible in even strongly gas-depleted inner regions as observed for
TW Hya. Reprocessing of dust through light-induced erosion offers one possible
explanation for growth of large cores of gas-poor giant planets in a
gas-starved region as recently found around HD 149026b
Planet formation bursts at the borders of the dead zone in 2D numerical simulations of circumstellar disks
As accretion in protoplanetary disks is enabled by turbulent viscosity, the
border between active and inactive (dead) zones constitutes a location where
there is an abrupt change in the accretion flow. The gas accumulation that
ensues triggers the Rossby wave instability, that in turn saturates into
anticyclonic vortices. It was suggested that the trapping of solids within them
leads to a burst of planet formation on very short timescales. We perform
two-dimensional global simulations of the dynamics of gas and solids in a
non-magnetized thin protoplanetary disk with the Pencil Code. We use multiple
particle species of radius 1, 10, 30, and 100 cm, solving for the particles'
gravitational interaction by a particle-mesh method. The dead zone is modeled
as a region of low viscosity. Adiabatic and locally isothermal equations of
state are used. We find that the Rossby wave instability is triggered under a
variety of conditions, thus making vortex formation a robust process. Inside
the vortices, fast accumulation of solids occurs and the particles collapse
into objects of planetary mass in timescales as short as five orbits. Because
the drag force is size-dependent, aerodynamical sorting ensues within the
vortical motion, and the first bound structures formed are composed primarily
of similarly-sized particles. In addition to erosion due to ram pressure, we
identify gas tides from the massive vortices as a disrupting agent of formed
protoplanetary embryos. We also estimate the collisional velocity history of
the particles that compose the most massive embryo by the end of the
simulation, finding that the vast majority of them never experienced a
collision with another particle at speeds faster than 1 m/s.Comment: 19 pages, 15 figures + Appendices. Accepted by A&A. Nature of
replacement: included a missing referenc
Photophoretic separation of metals and silicates: the formation of Mercury like planets and metal depletion in chondrites
Mercury's high uncompressed mass density suggests that the planet is largely
composed of iron, either bound within metal (mainly Fe-Ni), or iron sulfide.
Recent results from the MESSENGER mission to Mercury imply a low temperature
history of the planet which questions the standard formation models of impact
mantle stripping or evaporation to explain the high metal content. Like
Mercury, the two smallest extrasolar rocky planets with mass and size
determination, CoRoT-7b and Kepler-10b, were found to be of high density. As
they orbit close to their host stars this indicates that iron rich inner
planets might not be a nuisance of the solar system but be part of a general
scheme of planet formation. From undifferentiated chondrites it is also known
that the metal to silicate ratio is highly variable which must be ascribed to
pre-planetary fractionation processes. Due to this fractionation most
chondritic parent bodies - most of them originated in the asteroid belt - are
depleted in iron relative to average solar system abundances. The astrophysical
processes leading to metal silicate fractionation in the solar nebula are
essentially unknown. Here, we consider photophoretic forces. As these forces
particularly act on irradiated solids, they might play a significant role for
the composition of planetesimals forming at the inner edge of protoplanetary
discs. Photophoresis can separate high thermal conductivity materials (iron)
from lower thermal conductivity solids (silicate). We suggest that the
silicates are preferentially pushed into the optical thick disk. Subsequent
planetesimal formation at the edge moving outwards leads to metal rich
planetesimals close to the star and metal depleted planetesimals further out in
the nebula
Planetesimal formation by sweep-up: How the bouncing barrier can be beneficial to growth
The formation of planetesimals is often accredited to collisional sticking of
dust grains. The exact process is unknown, as collisions between larger
aggregates tend to lead to fragmentation or bouncing rather than sticking.
Recent laboratory experiments have however made great progress in the
understanding and mapping of the complex physics involved in dust collisions.
We want to study the possibility of planetesimal formation using the results
from the latest laboratory experiments, particularly by including the
fragmentation with mass transfer effect, which might lead to growth even at
high impact velocities. We present a new experimentally and physically
motivated dust collision model capable of predicting the outcome of a collision
between two particles of arbitrary masses and velocities. It is used together
with a continuum dust-size evolution code that is both fast in terms of
execution time and able to resolve the dust well at all sizes, allowing for all
types of interactions to be studied without biases. We find that for the
general dust population, bouncing collisions prevent the growth above
millimeter-sizes. However, if a small number of cm-sized particles are
introduced, for example due to vertical mixing or radial drift, they can act as
a catalyst and start to sweep up the smaller particles. At a distance of 3 AU,
100-meter-sized bodies are formed on a timescale of 1 Myr. We conclude that
direct growth of planetesimals might be a possibility thanks to a combination
of the existence of a bouncing barrier and the fragmentation with mass transfer
effect. The bouncing barrier is here even beneficial, as it prevents the growth
of too many large particles that would otherwise only fragment among each
other, and creates a reservoir of small particles that can be swept up by
larger bodies. However, for this process to work, a few seeds of cm in size or
larger have to be introduced.Comment: 17 pages, 13 figures. Accepted for publication in Astronomy and
Astrophysic