Encounters with Protostellar Disks

We have completed a numerical study of encounters between a star and star/disk system. Cross sections and rates for disk tilt, disk disruption, and binary formation are estimated using a large database of encounter simulations. The consequences of these results for star forming regions and our Solar System are discussed. A numerical code is developed which is capable of evolving a mixture of stars and gas in three dimensions. The algorithm is based on the method of smoothed particle hydrodynamics combined with the hierarchical tree method of computing gravitational forces. The code is tested by simulating the collision between two sheets of gas and the radial pulsations of a polytropic gas sphere. A protostellar disk model is developed based on simple assumptions. Test encounters are performed to determine the sensitivity of measured quantities on algorithm parameters, such as the gravitational tolerance and viscosity. This study shows that the Solar System could have had an encounter shortly after its formation of sufficient strength to generate the observed obliquity yet retain enough mass and radial extent to form the planetary system. A circumstellar disk will survive a typical encounter, approximately retaining its initial size and 50% of its mass. Even in dense environments the characteristic lifetime of a disk due to disruptive encounters is many millions of years. An average encounter that penetrates the disk will dissipate an amount of orbital energy equal to approximately 50% of the initial binding energy of the disk. An encounter with m = 1 M_odot, V_0 = 1 km/s, and r_ {per} < 250 AU will result in a capture 27% of the time for a disk mass of 0.1 M_ ☉. For the inner region of the Trapezium cluster we estimate star-disk and disk-disk capture rates of 0.02 and >= 0.04 Myr^{-1}, respectively. All of the estimated rates are lower limits. The number densities and velocity dispersions used are global values, which may not be appropriate if hierarchical structure exists in the clusters or if encounters are a natural outcome of the formation of binaries and higher order systems.