Modelling accretion in protobinary systems

A method for following fragmentation simulations further in time usingsmoothed particle hydrodynamics (SPH) is presented. In a normal SPH simulationof the collapse and fragmentation of a molecular cloud, high-density regions ofgas that form protostars are represented by many particles with smallseparations. These high-density regions require small time steps, limiting thetime for which the simulation can be followed. Thus, the end result of thefragmentation can never be definitively ascertained, and comparisons betweencloud fragmentation calculations and the observed characteristics of stellarsystems cannot be made. In this paper, each high-density region is replaced bya single, non-gaseous particle, with appropriate boundary conditions, whichcontains all the mass in the region and accretes any infalling mass. Thisenables the evolution of the cloud and the resulting protostars to be followedfor many orbits or until most of the original cloud mass has been accreted.