48 research outputs found
Growing into and out of the bouncing barrier in planetesimal formation
In recent laboratory studies the robustness of a bouncing barrier in
planetesimal formation was studied with an ensemble of preformed compact
mm-sized aggregates. Here we show that a bouncing barrier indeed evolves
self-consistently by hit-and-stick from an ensemble of smaller dust aggregates.
In addition, we feed small aggregates to an ensemble of larger bouncing
aggregates. The stickiness temporarily increases, but the final number of
aggregates still bouncing remains the same. However, feeding on the small
particle supply, the size of the bouncing aggregates increases. This suggests
that in the presence of a dust reservoir aggregates grow into but also out of a
bouncing barrier at larger size
Photophoresis boosts giant planet formation
In the core accretion model of giant planet formation, a solid protoplanetary
core begins to accrete gas directly from the nebula when its mass reaches about
5 earth masses. The protoplanet has at most a few million years to reach
runaway gas accretion, as young stars lose their gas disks after 10 million
years at the latest. Yet gas accretion also brings small dust grains entrained
in the gas into the planetary atmosphere. Dust accretion creates an optically
thick protoplanetary atmosphere that cannot efficiently radiate away the
kinetic energy deposited by incoming planetesimals. A dust-rich atmosphere
severely slows down atmospheric cooling, contraction, and inflow of new gas, in
contradiction to the observed timescales of planet formation. Here we show that
photophoresis is a strong mechanism for pushing dust out of the planetary
atmosphere due to the momentum exchange between gas and dust grains. The
thermal radiation from the heated inner atmosphere and core is sufficient to
levitate dust grains and to push them outward. Photophoresis can significantly
accelerate the formation of giant planets.Comment: accepted in Astronomy and Astrophysics, 201
Preplanetary scavengers: Growing tall in dust collisions
Dust collisions in protoplanetary disks are one means to grow planetesimals,
but the destructive or constructive nature of high speed collisions is still
unsettled. In laboratory experiments, we study the self-consistent evolution of
a target upon continuous impacts of submm dust aggregates at collision
velocities of up to 71m/s. Earlier studies analyzed individual collisions,
which were more speculative for high velocities and low projectile masses.
Here, we confirm earlier findings that high speed collisions result in mass
gain of the target. We also quantify the accretion efficiency for the used SiO2
(quartz) dust sample. For two different average masses of dust aggregates
(0.29g and 2.67g) accretion efficiencies are decreasing with velocity from 58%
to 18% and from 25% to 7% at 27m/s to 71m/s, respectively. The accretion
efficiency decreases approximately as logarithmic with impact energy. At the
impact velocity of 49m/s the target acquires a volume filling factor of 38%.
These data extend earlier work that pointed to the filling factor leveling off
at 8m/s to a value of 33%. Our results imply that high speed collisions are an
important mode of particle evolution. It especially allows existing large
bodies to grow further by scavenging smaller aggregates with high efficiency.Comment: This paper has been replaced by the author due to a transmission
error of references. Now the citations and references are give
Understanding planet formation using microgravity experiments
In 2018, images were released of a planet being formed around the star PDS
70, offering a tantalizing glimpse into how planets come into being. However,
many questions remain about how dust evolves into planets, and astrophysical
observations are unable to provide all the answers. It is therefore necessary
to perform experiments to reveal key details and, to avoid unwanted effects
from the Earth's gravitational pull, it is often necessary to perform such
experiments in microgravity platforms. This Review sketches current models of
planet formation and describes the experiments needed to test the models