The simulation of the outer Oort cloud formation^*

The first giga-year of the evolution

¹ Astronomical Observatory of the A. Mickiewicz University, Słoneczna 36, 60-286 Poznań, Poland e-mail: dybol@amu.edu.pl
² Catania Astrophysical Observatory, via Santa Sofia 78, 95123 Catania, Italy e-mail: gle@astrct.oact.inaf.it
³ Astronomical Institute of the Slovak Academy of Sciences, 05960 Tatranská Lomnica, Slovakia e-mail: [mjakubik;ne]@ta3.sk
⁴ Astronomical Institute of the Slovak Academy of Sciences, Dúbravská cesta 9, 84504 Bratislava, Slovakia e-mail: astrotom@savba.sk

Received: 17 September 2007
Accepted: 4 April 2008

Abstract

Aims. Considering a model of an initial disk of planetesimals that consists of 10 038 test particles, we simulate the formation of distant-comet reservoirs for the first 1 Gyr. Since only the outer part of the Oort cloud can be formed within this period, we analyse the efficiency of the formation process and describe approximately the structure of the part formed.

Methods. The dynamical evolution of the particles is followed by numerical integration of their orbits. We consider the perturbations by four giant planets on their current orbits and with their current masses, in addition to perturbations by the Galactic tide and passing stars.

Results. In our simulation, the population size of the outer Oort cloud reaches its maximum value at about 210 Myr. After a subsequent, rapid decrease, it becomes almost stable (with only a moderate decrease) from about 500 Myr. At 1 Gyr, the population size decreases to about 40% of its maximum value. The efficiency of the formation is low. Only about 0.3% of the particles studied still reside in the outer Oort cloud after 1 Gyr. The space density of particles in the comet cloud, beyond the heliocentric distance, r, of 25 000 AU is proportional to , where s = 4.08 ± 0.34. From about 50 Myr to the end of the simulation, the orbits of the Oort cloud comets are not distributed randomly, but high galactic inclinations of the orbital planes are strongly dominant. Among all of the outer perturbers considered, this is most likely caused by the dominant, disk component of the Galactic tide.