**DPS 2001 meeting, November 2001**

*Session 15. Solar System Origins Posters*

Displayed, 9:00am Tuesday - 3:00pm Saturday, Highlighted, Wednesday, November 28, 2001, 10:30am-12:30pm, French Market Exhibit Hall
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## [15.07] Gas-Drag Effect on the Orbital Instability of a Protoplanet System

*K. Iwasaki, H. Emori, H. Tanaka, K. Nakazawa (Tokyo Inst. of Tech., Dept. of Earth & Planetary Sci.)*

We investigated the effect of the gravitational or
hydrodynamical interaction with the gas disk on the orbital
instability of a protoplanet system. We consider five
protoplanets with the mass of 1 \times 10^{-7} \:
M_{\odot}, which are distributed with the same separation
distance, \Delta a, and their initial orbits are circular
and coplanar. The gravitational interaction between a
protoplanet and the gas disk is expressed as the drag force
which is proportional to the random velocity of a
protoplanet, while the hydrodynamical gas drag force caused
by the gas disk is proportional to the square of the random
velocity. Taking account of these two kinds of drag force
effects respectively, we calculated numerically the orbits
of the protoplanets and examined the orbital instability
time under the drag force, T_{\rm{inst}}^{\rm{df}}.
Regardless of the kind of the drag force, we found that
T_{\rm{inst}}^{\rm{df}} is enlarged in magnitude compared
with the instability time under the drag-free condition and
the orbital instability does not occur substantially when
\Delta a is larger than a critical separation distance,
{\Delta a}_{\rm{crit}}. We obtained the relation between
{\Delta a}_{\rm{crit}} and the surface density of the gas
disk. Applying this relation to a protoplanet system with a
typical orbital separation (i.e., \Delta a ~q 10 Hill
radii), we found that, for the formation of terrestrial
planets, in other words, the occurence of an orbital
instability of the protoplanet system, the surface density
of the nebular gas must decrease to about 0.1 % of the
minimum mass nebula model.

The author(s) of this abstract have provided an email address
for comments about the abstract:
kiwasaki@geo.titech.ac.jp

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