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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 : (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.

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