Coagulation of grains in static and collapsing protostellar clouds
S. J. Weidenschilling, T. V. Ruzmaikina;
ApJ, 1994, 430, 713
ABSTRACT:We simulate collisional evolution of grains in dense turbulent molecular cloud cores (or Bok globules) in static equilibrium and free-fall
collapse, assuming spherical symmetry.
Relative velocities are due to thermal
motions, differential settling, and turbulence, with the latter dominant for
sonic turbulence with an assumed Kolmogorov spectrum.
Realistic criteria
are used to determine outcomes of collisions (coagulation
vs.
destruction) as functions of particle size and velocity.
Results are presented for
a variety of cloud parameters (radial density profile, turbulent
velocity) and particle properties (density, impact strength).
Results are
sensitive to the assumed mechanical properties (density and impact strength) of
grain aggregates.
Particle growth is enhanced if aggregates have low
density or fractal structures.
On a timescale of a few Myr, an initial
population of 0.1 micrometers grains may produce dense compact particles
approximately 1 micrometer in size, or fluffy aggregates approximately 100
micrometers.
For impact strengths less than or equal to 106 ergs/g, a steady
state is reached between coagulation of small grains and collisional
disruption of larger aggregates.
Formation of macroscopic aggregates requires
high mechanical strengths and low aggregate densities.
We assume sonic
turbulence during collapse, with varied eddy size scales determining the
dissipation rate or turbulence strength.
The degree of collisional evolution
during collapse is sensitive to the assumed small-scale structure (inner sc
ale) of the turbulence.
Weak turbulence results in few collisions and
preserves the precollapse particle size distribution with little
change.
Strong turbulence tends to produce net destruction, rather than particle
growth, during infall, unless inpact strengths are greater than
106ergs/g.
KEYWORDS: cosmic dust, interstellar extinction, interstellar matter, molecular clouds, protostars, star formation, coagulation, collisional plasmas, cosmology, particle size distribution, plasma turbulence
PERSOKEY:dust, size distribution, ,
CODE: weidenschilling94