ENCOUNTERS WITH PROTOSTELLAR DISKS .1. DISK TILT AND THE NONZERO SOLAR OBLIQUITY

A numerical study has been completed which examines the ability of a s tellar encounter to tilt a circumstellar disk with respect to the rota tion axis of the central star. A numerical code has been developed and tested which is capable of evolving a mixture of stars and gas in thr ee dimensions. Disk tilt cross sections and rates are estimated from a large data base of encounter simulations for a variety of environment s. It is shown that the nonzero obliquity of the solar system could be the result of an encounter shortly after its formation. For the Orion B clusters as a whole, it is estimated that during a one million year period of time a few percent of the stars will experience an encounte r that results in a disk tilt of 7-degrees or greater. For the central regions of NGC 2024 and the Trapezium cluster values of 24% and 39% a re obtained, respectively. These estimates are lower limits and when f actors such as subclustering, cluster expansion, and disk sizes are co nsidered, substantially greater tilt rates are possible with the resul t that the generation of disk tilts by encounters may in fact be commo n.