657 research outputs found
Effect of turbulence on collisions of dust particles with planetesimals in protoplanetary disks
Planetesimals in gaseous protoplanetary disks may grow by collecting dust
particles. Hydrodynamical studies show that small particles generally avoid
collisions with the planetesimals because they are entrained by the flow around
them. This occurs when , the Stokes number, defined as the ratio of the
dust stopping time to the planetesimal crossing time, becomes much smaller than
unity. However, these studies have been limited to the laminar case, whereas
these disks are believed to be turbulent. We want to estimate the influence of
gas turbulence on the dust-planetesimal collision rate and on the impact
speeds. We used three-dimensional direct numerical simulations of a fixed
sphere (planetesimal) facing a laminar and turbulent flow seeded with small
inertial particles (dust) subject to a Stokes drag. A no-slip boundary
condition on the planetesimal surface is modeled via a penalty method. We find
that turbulence can significantly increase the collision rate of dust particles
with planetesimals. For a high turbulence case (when the amplitude of turbulent
fluctuations is similar to the headwind velocity), we find that the collision
probability remains equal to the geometrical rate or even higher for , i.e., for dust sizes an order of magnitude smaller than in the laminar
case. We derive expressions to calculate impact probabilities as a function of
dust and planetesimal size and turbulent intensity
Student Showcase Concert
Ida GotkovskyRobert PlanelEdith Piaf & Louis GuglielmiJohnny MandelKen ThomsonBob Becke
The effect of gas drag on the growth of protoplanets -- Analytical expressions for the accretion of small bodies in laminar disks
Planetary bodies form by accretion of smaller bodies. It has been suggested
that a very efficient way to grow protoplanets is by accreting particles of
size <<km (e.g., chondrules, boulders, or fragments of larger bodies) as they
can be kept dynamically cold. We investigate the effects of gas drag on the
impact radii and the accretion rates of these particles. As simplifying
assumptions we restrict our analysis to 2D settings, a gas drag law linear in
velocity, and a laminar disk characterized by a smooth (global) pressure
gradient that causes particles to drift in radially. These approximations,
however, enable us to cover an arbitrary large parameter space. The framework
of the circularly restricted three body problem is used to numerically
integrate particle trajectories and to derive their impact parameters. Three
accretion modes can be distinguished: hyperbolic encounters, where the 2-body
gravitational focusing enhances the impact parameter; three-body encounters,
where gas drag enhances the capture probability; and settling encounters, where
particles settle towards the protoplanet. An analysis of the observed behavior
is presented; and we provide a recipe to analytically calculate the impact
radius, which confirms the numerical findings. We apply our results to the
sweepup of fragments by a protoplanet at a distance of 5 AU. Accretion of
debris on small protoplanets (<50 km) is found to be slow, because the
fragments are distributed over a rather thick layer. However, the newly found
settling mechanism, which is characterized by much larger impact radii, becomes
relevant for protoplanets of ~10^3 km in size and provides a much faster
channel for growth.Comment: accepted for publication in Astronomy & Astrophysic
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