Stellar encounters: a stimulus for disc fragmentation?

Abstract

An interaction between a star-disc system and another star will perturb the disc, possibly resulting in a significant modification of the disc structure and its properties. It is still unclear if such an encounter can trigger fragmentation of the disc to form brown dwarfs or gas giant planets. This paper details high resolution Smoothed Particle Hydrodynamics (SPH) simulations investigating the influence of stellar encounters on disc dynamics. Star-star encounters (where the primary has a self-gravitating, marginally stable protostellar disc, and the secondary has no disc) were simulated with various orbital parameters to investigate the resulting disc structure and dynamics. This work is the first of its kind to incorporate realistic radiative transfer techniques to realistically model the resulting thermodynamics. The results suggest that the effect of stellar encounters is to prohibit fragmentation - compressive and shock heating stabilises the disc, and the radiative cooling is insufficient to trigger gravitational instability. The encounter strips the outer regions of the disc (either through tidal tails or by capture of matter to form a disc around the secondary), which triggers a readjustment of the primary disc to a steeper surface density profile (and a flatter Toomre Q profile). The disc around the secondary plays a role in the potential capture of the secondary to form a binary. However, this applies only to orbits that are parabolic - hyperbolic encounters do not form a secondary disc, and are not captured.