ENCOUNTERS WITH PROTOSTELLAR DISKS

Abstract

ABSTRACT A numerical study of encounters between stars with circumstellar disks has bee completed. Cross sections and rates for disk tilt, disk disruption, and binary formation are estimated using a large data base 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 heirarchical 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. It is shown 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. 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 enoucnter 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% are obtained, respectively. Encounters between equal-mass stars with periastra of 0.5, 1.0, 1.5, and 2.0 disk radii will retain on average about 15%, 40%, 55%, and 75% of the disk mass, respectively. For encounters that do not penetrate the disk a minimum of 15% of the mass is retained. Even in dense environments the characteristic lifetime of a disk due to disruptive encounters can be many millions of years. On average, an encounter that penetrates the disk will dissipate an amount of orbital energy equal to approximately 50% of the initial binding energy of the disk. For the central region of NGC 2024 a star-disk capture rate is estimated at Gammac=0.03 and 0.07 Myr-1 for disk masses of 0.1 and 0.5 solar mass, respectively. The equivalent disk-disk capture rates are ≥0.05 and 0.08 Myr-1. The most favorable rate implies that during a 1-Myr period of time 16% of the stars will be captured into a binary. These rates are lower limits because of factors such as higher stellar densities in the past due to cluster expansion, the presence of substructure, and the existence of binary and higher-order systems. In particular, encounters may be a natural outcome of the star-formation process, at a time when protostars are surrounded by large massive disks and extended envelopes of material.