The key role of massive stars in Oort cloud comet dynamics

The effects of a sample of 1300 individual stellar encounters spanning a wide range of parameter values (mass, velocity and encounter distance) are investigated. Power law fits for the number of injected comets demonstrate the long range effect of massive stars, whereas light stars affect comets mainly along their tracks. Similarly, we show that the efficiency of a star to fill the phase space region of the Oort cloud where the Galactic tides are able to inject comets into the observable region – the so-called “tidally active zone” (TAZ) – is also strongly dependent on the stellar mass. Power laws similar to those for direct injection are obtained for the efficiency of stars to fill the TAZ. This filling of the tidally active zone is crucial for the long term flux of comets from the Oort cloud. Based on long-term Monte Carlo simulations using a constant Galactic tide and a constant flux of stellar encounters, but neglecting the detailed effects of planetary perturbations, we show that this flux essentially results from a two step mechanism: (i) the stellar injection of comets into the TAZ; and (ii) the tidal injection of TAZ comets into the loss cone. We find that single massive stars are able to induce “comet drizzles” – corresponding to an increase of the cometary flux of about 40% – which may last for more than 100 Myr by filling the TAZ to a higher degree than normal. It appears that the stars involved in this process are the same that cause comet showers.

► The effects of passing stars on the flux of observable comets is studied.
► Massive stars inject observable comets much more efficiently than light stars do.
► The long term synergy between stars and Galactic tide is explained.
► Stars send comets into orbits from which the tides may make them observable.
► Single massive stars may increase the flux significantly for more than 100 Myr.