Leena Valtaoja

Capture of comets during the evolution of a star cluster and the origin of the Oort Cloud

It is generally assumed that the Solar System is surrounded by a swarm of comets, the so-called Oort Cloud, which contains approximately 1011 members. The observed comets belong to a small subsection of the Cloud, and they have very elongated orbits. The origin of the Cloud is presently unclear. Here we consider the possibility that the comets were born in a star cluster together with the Sun. We follow the evolution of the star cluster with its embedded swarm of comets and calculate the rate at which stars accumulate stable comet companions. We conclude that if the Oort Cloud of comets was born in this process, then the present day density of comets in interstellar space has to be high, and that comets make a significant contribution to the overall mass density of the Galaxy.

Dynamical friction on a satellite of a disk galaxy: the circular orbit

In a previous paper, we have studied dynamical friction during a parabolic passage of a companion galaxy past a disk galaxy. This paper continues with the study of satellites in circular orbits around the disk galaxy. Simulations of orbit decay in a self gravitating disk are compared with estimates based on two-body scattering theories; the theories are found to give a satisfactory explanation of the orbital changes. The disk friction is strongly dependent on the sense of rotation of the companion relative to the rotation of the disk galaxy as well as on the amount of mass in a spherical halo. The greatest amount of dynamical friction occurs in direct motion if no spherical halo is present. Then the infall time from the edge of the disk is about one half of the orbital period of the disk edge. A halo twice as massive as the disk increases the infall time four fold. The results of Quinn and Goodman, obtained with a non-self-gravitating method, agree well with our experiments with massive halos (Q0 ≈ 1.5), but are not usable in a more general case. We give analytic expressions for calculating the disk friction in galaxies of different disk/halo mass ratios.

Tidal Triggering of Seyfert Galaxies and Quasars: Physical Sufficiency

Excess optical and radio emission in nuclei of Seyfert galaxies and quasars is observationally related to nearby companions. Dahari found Seyferts are more strongly perturbed tidally by companions than normal spirals are. QSO data suggests a similar relationship. Quasar and Seyfert activity are thought to be fueled by gas inflow to the nucleus. To study whether gravitational tides from companions cause inflow from the spiral’s disk, we use a self-gravitating 60,000 particle disk in Miller’s polar n-body program. The perturbation to produce levels of inflow required for activity matches Dahari’s Seyfert triggering level. QSO’s are also perturbed by observed companions more than necessary for high inflow. We reanalyze Dahari’s data to find that at least the majority of Seyferts are in multiple systems. Tidal triggering thus could be the main cause of Seyfert activity.