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The Subaru Seminar is
usually held in Room 104 of the Hilo Base Facility, adjacent
to the main lobby. Everyone is welcome to attend. If you are
interested in giving a seminar, please contact Subaru seminar organizers
(Tomonori Usuda, Kumiko S. Usuda, Masayuki Akiyama)
by email : sseminar_at_subaru.naoj.org (please change"_at" to @).
March 19, Wednesday at 11:00 am
" Evolution of the Oort cloud due to the external forces "
Arika Higuchi (NAOJ)
We have investigated the evolution of the structure of the Oort
cloud by stellar encounters. Oort cloud comets are believed to be
planetesimals, which are remnants of planet formation. The
external forces such as the galactic tide and perturbations from
passing stars and/or giant molecular clouds pulls up perihelia and
randomizes inclinations of planetesimals with large aphelion
distances produced by planet scattering. These processes play
important roles in forming the spherical Oort cloud, which is
predicted by observations of long-period comets.
First, we considered the effect of the vertical component of the
tidal force from the galactic disk on the structure of the Oort
cloud. Due to the galactic tide, the eccentricity and inclination
of some planetesimals alternately oscillate a great deal. The
large change of eccentricity and inclination is effective to form
the Oort Cloud. We found that due to the galactic tide,
planetesimals with the semimajor axes >~1,000 AU raise the
perihelion distances outside the planetary region and
planetesimals with the semimajor axes >~20,000 AU obtain the
inclination to the ecliptic plane from 0 to153 deg in 5 Gyr. We
also found that the galactic tide alone can not realize the
isotropic distribution of inclination of new comets from the Oort
cloud, which is expected from the observation of long period
comets.
Second, we considered the effect of the sucsessive stellar
encounters on the formation of the Oort cloud. Passing stars give
the velocity changes to the planetesimals and the planetesimals
gain or lose their energies and/or angular momentum. These changes
induce the diffusion of the planetesimal disk i.e., the perihelion
distances and inclinations of the planetesimals are redistributed.
We used the impulse approximation to calculate the velocity change
of planetesimals.When the velocity change is large, some
planetesimals escape from the Solar system to the interstellar
space. We found that the spherical Oort cloud can be formed in 5
billion years by stellar encounters with the parameters derived
from the observation of the solar neighborhood. However, the
isotropic distributions of eccentricity and inclination are not
produced. We also found that, in the Sun-like parameter ranges,
the evolution rate of the planetesimal disk is scaled by the
masses, velocities, and total number of the stars.