From: LARRY KLAES (ljk4_at_msn.com)
Date: Mon Sep 26 2005 - 14:40:51 UTC
Paper: astro-ph/0509701
Date: Thu, 22 Sep 2005 21:20:00 GMT (996kb)
Title: Planetesimal growth in turbulent discs before the onset of
gravitational instability
Authors: Alexander Hubbard (U. Rochester) and Eric G. Blackman (U.
Rochester)
Categories: astro-ph
Comments: Submitted to MNRAS
\\
Standard models for planet formation require gravitationally unstable discs.
Initially unstable gas-dust discs may form planets directly, but the high
surface density required has motivated the alternative that gravitational
instability occurs in a dust sub-layer only after grains have grown large
enough by electrostatic sticking. Although such growth up to the instability
stage is efficient for laminar discs, concern has mounted as to whether
realistic disc turbulence catastrophically increases the settling time,
thereby
requiring additional processes to facilitate planet formation on the needed
time scales. To evaluate this concern, we develop a model for grain growth
that
accounts for the influence of turbulence on the collisional velocity of
grains
and on the scale height of the dust layer. The relative effect on these
quantities depends on the grain size. The model produces a disc-radius
dependent time scale to reach the gravitationally unstable phase of planet
formation. For a range of dust sticking and disc parameters, we find that
for
viscosity parameters $\alpha < 10^{-3}$, this time scale is short enough
over a
significant range in radii $R$ that turbulence does not catastrophically
slow
the early phases of planet formation, even in the absence of agglomeration
enhancement agents like vortices.
\\ ( http://arXiv.org/abs/astro-ph/0509701 , 996kb)
Paper: astro-ph/0509711
Date: Fri, 23 Sep 2005 11:14:26 GMT (387kb)
Title: On the accumulation of solid bodies in global turbulent
protoplanetary
disc models
Authors: Sebastien Fromang, Richard P. Nelson
Categories: astro-ph
Comments: 5 pages, 4 figures, accepted in MNRAS. A version with full
resolution, colour figures is available at
http://www.maths.qmul.ac.uk/~rpn/preprints/
\\
We study the migration of solid bodies in turbulent protoplanetary accretion
discs by means of global MHD simulations. The bodies range in size from 5
centimetres up to 1 metre, and so include objects whose migration is
expected
to be the most rapid due to gas drag interaction with the disc. As they
drift
inward through the disc, some of them are trapped in regions where gas
pressure
maxima are created by long lived anticyclonic vortices. This accumulation is
very efficient, locally increasing the dust--to--gas ratio by a factor > 100
in
some cases. We discuss the possible implications of this result for theories
of
planet formation.
\\ ( http://arXiv.org/abs/astro-ph/0509711 , 387kb)
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