The “memory” of the Oort cloud

Abstract

Aims. Our aim in this paper is to try to discover if we can find any record of the Oort cloud formation process in the orbital distribution of currently observable long-periodic comets. Methods. Long-term simulations of tens of millions of comets from two different kinds of proto-Oort clouds (isotropic and disk-like) were performed. In these simulations we considered the Galactic tides, stellar passage, and planetary perturbations. Results. In the case of an initially disk-like proto-Oort cloud, the final Oort cloud remains anisotroic inside of about 13 200 au. A record of the initial shape is preserved, here referred to as the “memory”, even on the final distribution of observable comets. This memory is measurable in particular for observable comets for which the previous perihelion was beyond 10 au and that were significantly affected by Uranus or Neptune at that moment (the so-called Kaib-Quinn jumpers observable class). Indeed, these comets are strongly concentrated along an extended scattered disk that is the remnant of the initial population 1 Gyr before the comets are observable. In addition, for this class of comets, the distributions of ecliptic inclination and Galactic longitude of the ascending node at the previous perihelion preceding the observable perihelion highlight characteristics that are not present in the isotropic model. Furthermore, the disk-like model produces four times more observable comets than the isotropic one, and its flux is independent of the initial distribution of orbital energy. Also for the disk-like model, the region beyond Neptune up to ~40 au gives the major contribution to the final flux of observable comets. Conclusions. The disk-like model sustains a flux of observable comets that are more consistent with the actually observed flux than using the isotropic model. However, further investigations are needed to reveal whether a fingerprint of the initial proto-Oort cloud, such as those highlighted in the present article, is present in the sample of known long-period comets.