Encounters with protostellar disks. I. Disk tilt and the nonzero solar obliquity

A numerical study has been completed which examines the ability of a stellar encounter to tilt a circumstellar disk with respect to the rotation 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 three dimensions. Disk tilt cross sections and rates are estimated from a large data base of encounter simulations for a variety of environments. 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 encounter that results in a disk tilt of 7° or greater. For the central regions of NGC 2024 and the Trapezium cluster values of 24% and 39% are obtained, respectively. These estimates are lower limits and when factors such as subclustering, cluster expansion, and disk sizes are considered, substantially greater tilt rates are possible with the result that the generation of disk tilts by encounters may in fact be common.