The structure of the inner Oort cloud from the simulation of its formation for 2 Gyr

Abstract

Considering the model of the initial disc of planetesimals consisting of 10 038 test particles, we simulated the formation of the distant comet reservoirs up to 2 Gyr. The result concerning the outer part of the Oort cloud (OC) was described in our previous paper. Here, we deal with the evolution of the population and structure at 2 Gyr of the complementary inner part of the OC. The dynamical evolution of the massless test particles was followed via the numerical integration of their orbits. We considered the perturbations produced by four giant planets assuming they have their current orbits and masses, as well as the perturbations caused by the Galactic tide and passing stars. The efficiency of the formation of inner OC is found to be very low: only about 1.1 per cent of all considered particles ended in this part of the comet cloud. In particular, the particles originated from Uranus (35.5 per cent), Neptune (35.4 per cent) and Edgeworth–Kuiper belt (18.4 per cent) regions of the initial protoplanetary disc. At 2 Gyr, the dynamics of the inner cloud is mainly governed by the dominant z-term of the Galactic tide. The number density of the bodies is proportional to the heliocentric distance, r ,a s∝ r −3.53 . The directional distribution of orbits is still strongly inhomogeneous. There are, for example, large empty regions in the space angles around the Galactic Equator points with the galactic longitude 90 ◦ and 270 ◦ (non-rotating frame), or there are only few bodies having the ecliptical latitude higher than +60 ◦ or lower than −60 ◦ . A strong concentration of objects at the ecliptic is apparent up to ≈1000 au (with a possible, but not exactly proved extension to ≈1500 au). Beyond r ≈ 6000 au, the bodies directly above and below the Sun, with respect to the ecliptic, are absent.