Gravitational Instabilities induced by Cluster Environment? - The   encounter-induced angular momentum transfer in discs
  AIM:The aim of this work is to understand to what extend gravitational interactions between the stars in high-density young stellar clusters, like the Orion Nebula Cluster (ONC), change the angular momentum in their protoplanetary discs. METHOD:Two types of simulations were combined -- N-body simulations of the dynamics of the stars in the ONC, and angular momentum loss results from simulations of star-disc encounters. RESULTS:It is shown that in a star-disc encounter the angular momentum loss is usually larger than the mass loss, so that the disc remnant has a lower specific angular momentum. Assuming an age of 1-2 Myr for the ONC, the disc angular momentum in the higher density region of the Trapezium is reduced by 15-20% on average. Encounters therefore play an important part in the angular momentum transport in these central regions but are not the dominant process. More importantly, even in the outer cluster regions the angular momentum loss is on average 3-5%. Here it is shown that a 3-5% loss in angular momentum might be enough to trigger gravitational instabilities even in low-mass discs - a possible prerequisite for the formation of planetary systems. 
S. Pfalzner, C. Olczak