PERIODIC MODULATION OF THE OORT CLOUD COMET FLUX BY THE ADIABATICALLY CHANGING GALACTIC TIDE

The time variation of the flux of new Jupiter-dominated Oort cloud comets is considered here. It has previously been demonstrated that the major perturbation of these comets during the present epoch is due to the tidal field of the relatively smooth distribution of matter in the galactic disk. Over long time scales, secondary sources of the near-parabolic comet flux are stellar and molecular cloud impulses, both of which create brief comet showers from the inner Oort cloud. Substantial showers occur approximately every 50-500 Myr depending on the depth of the stellar penetration or the size of the molecular cloud. In contrast to these infrequent stochastic shower phenomena is the continuously varying tidal-induced flux due to the galaxy. As the Sun orbits the galactic center it undergoes quasi-harmonic (T-Z = 70 +/- 15 Myr) motion about the galactic midplane which is superimposed on the small eccentricity, near-Keplerian motion in the plane having radial period T-R = 170 +/- 10 Myr and orbital period T-phi = 250 +/- 15 Myr. In the process the galactic tidal field on the Sun/cloud system will adiabatically vary, causing a modulation of the observable Oort cloud flux. We have created a model of the galactic matter distribution as it affects the solar motion over a time interval ranging from 300 Myr in the past to 100 Myr into the future. As constraints on the disk's dark matter component we require (1) a fit to the observed galactic rotation curve, (2) consistency with the studies of K-giant and K-dwarf stellar velocity distributions, and (3) agreement with the observed energy distribution of new Oort cloud comets. The acceptable range of dark disk matter parameters is then determined and used to predict the related uncertainty in oscillation period and flux variability. We find that a model in which less than or equal to 40% of the disk matter is dark is consistent with these constraints. Under such circumstances the peak-to-trough Oort cloud comet flux variation will be as much as 4 to 1 with a full width of 9 Myr. The complete absence of dark disk matter will still lead to an Oort cloud comet flux variation of 2.5 to 1. It is important that we look for evidence of this periodicity in the cratering record since it contains significant information about the amount and distribution of dark disk matter, as well as the population and origins of long-period and short-period comets. (C) 1995 Academic Press, Inc.