Non-gravitational effects change the original 1/a-distribution of near-parabolic comets

Abstract

Context.The original 1∕a-distribution is the only observational basis for the origin of long-period comets (LPCs) and the dynamical properties of the Oort Cloud. Although they are very subtle in the motion of these comets, non-gravitational effects can cause major changes in the original semimajor axis, 1∕aori.Aims.We obtained reliable non-gravitational orbits for as many LPCs with small perihelion distances ofq< 3.1 au as possible, and determined the corresponding shape of the Oort spike.Methods.We determined the osculating orbits of each comet using several data-processing methods, and selected the preferred orbit using a few specific criteria. The distribution of 1∕aorifor the whole comet sample was constructed using the individual Gaussian distribution we obtained for the preferred solution of each comet.Results.The derived distribution of 1∕aorifor almost all known small-perihelion Oort spike comets was based on 64% of the non-gravitational orbits. This was compared with the distribution based on purely gravitational orbits, as well as with 1∕aoriconstructed earlier for LPCs withq> 3.1 au. We present a statistical analysis of the magnitudes of the non-gravitational acceleration for about 100 LPCs.Conclusions.The 1∕aori-distribution, which is based mainly on the non-gravitational orbits of small-perihelion Oort spike comets, is shifted by about 10 × 10−6au−1to higher values of 1∕aoricompared with the distribution that is obtained when the non-gravitational effects on comet motion are ignored. We show the differences in the 1∕aori-distributions between LPCs withq< 3.1 au and those withq> 3.1 au. These findings indicate the important role of non-gravitational acceleration in the motion and origin of LPCs and in the formation of the Oort Cloud.