Anisotropy of long-period comets explained by their formation process

<p>Long-period comets coming from the Oort cloud are thought to be<span class="Apple-converted-space"> </span>planetesimals formed in the planetary region on the ecliptic plane.<span class="Apple-converted-space"> </span>We have investigated the orbital evolution of these bodies<span class="Apple-converted-space"> </span>due to the Galactic tide.<span class="Apple-converted-space"> </span>We extended Higuchi et al. (2007) and derived the<span class="Apple-converted-space"> </span>analytical solutions to the Galactic longitude and latitude of<span class="Apple-converted-space"> </span>the direction of aphelion, <em>L</em> and <em>B</em>.<span class="Apple-converted-space"> </span>Using the analytical solutions,<span class="Apple-converted-space"> </span>we show that the ratio of the periods of the evolution of <em>L</em> and <em>B</em> is very close to either 2 or ∞ for initial eccentricities <em>e</em><sub>i</sub>∼1,<span class="Apple-converted-space"> </span>as is true for the Oort cloud comets.<span class="Apple-converted-space"> </span>From the relation between <em>L</em> and <em>B</em>, we predict that Oort cloud comets returning to the planetary region concentrate on the ecliptic plane<span class="Apple-converted-space"> </span>and a second plane, which we call the "empty ecliptic".<span class="Apple-converted-space"> </span>This consists in a rotation of 180<sup>°</sup> of the ecliptic around the Galactic pole. Our numerical integrations confirm that the radial component of the Galactic tide, which is neglected in the derivation of the analytical solutions,<span class="Apple-converted-space"> </span>is not strong enough to break the relation between <em>L</em> and <em>B</em> derived analytically. Brief examination of observational data shows<span class="Apple-converted-space"> </span>that there are concentrations near both the ecliptic and the empty ecliptic. We also show that the anomalies of the distribution of <em>B</em> of long-period comets mentioned by several authors are explained by the concentrations on the two planes more consistently than the previous explanation.</p>

Stability in the most external region of the Oort Cloud: evolution of the ejected comets

ABSTRACTIn this paper, we present a study of the dynamical effects of the Galaxy on the external region of the Oort Cloud. The aims of this paper are: (i) to determine an outer limit for the Oort Cloud; and (ii) to analyse the dynamical behaviour of the most external objects of the Oort Cloud and how they are ejected from the Solar system. To achieve these aims, we follow the temporal evolution of massless test particles in the Galactic environment of the solar neighbourhood. We show that the effect of the perturbations from the Galactic tide in the particles is similar to that found for the evolution of the population of wide binary stars. Moreover, in the Oort Cloud, we find a dynamical structure around 105 au conformed by objects unbound from the Sun. This structure allows us to define a transition region of stability and an outer boundary for the Oort Cloud, and it is also in agreement with previous results about the disruption of wide binary stars.