Paper: astro-ph/0510007
Date: Fri, 30 Sep 2005 20:00:06 GMT (111kb)

Title: Induced planet formation in stellar clusters - a parameter study of
star-disk encounters

Authors: Ingo Thies (Bonn), Pavel Kroupa (Bonn) and Christian Theis (Vienna)

Categories: astro-ph

Comments: 11 pages, 9 figures, using aas_macros.sty. MNRAS, accepted
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We present a parameter study of the possibility of tidally triggered disk
instability. Using a restricted N-body model which allows for a survey of an
extended parameter space, we show that a passing dwarf star with a mass between
0.1 and 1 M_sun can probably induce gravitational instabilities in the
pre-planetary solar disk for prograde passages with minimum separations below
80-170 AU for isothermal or adiabatic disks. Inclined and retrograde encounters
lead to similar results but require slightly closer passages. Such encounter
distances are quite likely in young moderately massive star clusters (Scally &
Clarke 2001; Bonnell et al. 2001). The induced gravitational instabilities may
lead to enhanced planetesimal formation in the outer regions of the
protoplanetary disk, and could therefore be relevant for the existence of
Uranus and Neptune, whose formation timescale of about 100 Myr (Wuchterl,
Guillot & Lissauer 2000) is inconsistent with the disk lifetimes of about a few
Myr according to observational data by Haisch, Lada & Lada (2001). The
relatively small gas/solid ratio in Uranus and Neptune can be matched if the
perturbing fly-by occurred after early gas depletion of the solar system, i.e.
when the solar system was older than about 5 Myr.

We also confirm earlier results by Heller (1993) that the observed 7 degree
tilt of the solar equatorial plane relative to the ecliptic plane could be the
consequence of such a close encounter.

\\ ( http://arXiv.org/abs/astro-ph/0510007 , 111kb)