The influence of the Galactic tidal field on the Oort comet cloud

We examine the effect of the Galactic disk potential on the Oort cloud. The tidal force from the disk produces a slow drift of a comet's orbital angular momentum, J. If J is reduced below a critical value, J_*, then the comet enters the planetary system. Once this occurs the comet is lost from the Oort cloud, either to an escape orbit or a much more tightly bound orbit, within a few orbital period. We determine the rate of orbit evolution due to the Galactic tide by analyzing the orbit averaged Hamiltonian. We calculate the loss rate due to the Galactic tide and compare this with the loss rate due to stellar encounters. We find that when the loss cylinder is empty the loss rate of comets from the Oort cloud due to the Galactic tide is larger than the loss rate due to stellar perturbations by about a factor 1.5-2. The Galactic tide appears to be the dominant cause of orbital 4evolution in the Oort cloud.