Overview of the scientific background
Planet formation in protoplanetary disks
After a star is born it usually remains surrounded by the remnant of its
nascent cloud for a few million years. This disk of gas and small
(sub-micron) solid particles ('soot') is the birthplace of planets.
One of the two leading theories states that these solid particles
collide and stick to form ever bigger agglomerates, growing from
sub-micron scale all the way to 100 kilometer size objects called
'planetesimals'. These are the building blocks of planets. This
growth process is the topic of this group.
From fine 'dust' to full-grown planets
The growth process from the tiny particles to full-blown planets is
a growth process covering 13 orders of magnitude in size and
40 orders of magnitude in mass. On the way various processes become
important and subsequently lose importance to new ones. This is shown
in the image below. The red box shows the domain that is studied
in this group, although the group also studies qualitatively the
consequences of this early growth process on the final stages (to
the right of the diagram).
Different growth paths
One of the things we wish to learn is whether the growth is a hierarchical
growth process (shown left in the below image) or a linear sweep-up growth
(shown right). Presumably it will turn out to be a combination of both.
The hierarchical growth process is typical for aggregation due to
random motions such as turbulence or Brownian Motion. You can demonstrate
in your own kitchen that such a growth due to a random motion and hit-and-stick
proccess leads to very fluffy aggregates with open structures: see
this AVI movie CoagAtHome.avi ;-). In
this home-experiment a set of checker discs were supplied with bisided
sticky tape so that they would stick when they collide. These discs were
then placed on a tablet and the tablet was shaken. Eventually a nice and
fluffy aggregate (in 2D) was formed.
The 'meter-size barrier'
One of the biggest puzzles in the field is the so-called 'meter-size
barrier'. As particles grow bigger, they tend to move faster in the
disk. The peak velocity lies around a size of 1 meter, where velocities of
100 meter per second can be reached. This poses two problems. First, once
such objects are formed, they quickly drift toward the star and evaporate.
Secondly, when they reach such high velocities any collisions will become
destructive. Both effects stand in the way of growing bigger bodies.
However, if in some way they manage to cross the meter-size barrier
(shown as the colored area in the belo diagram), the velocities go down
again and they are 'saved'. We wish to study whether there are ways to
overcome this meter-size barrier.
Maintained by C.P. Dullemond
Last modified: October 31, 2008
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