Stellar interactions in dense and sparse star clusters
  Stellar encounters potentially affect the evolution of the protoplanetary
discs in the Orion Nebula Cluster (ONC). However, the role of encounters in
other cluster environments is less known. We investigate the effect of the
encounter-induced disc-mass loss in different cluster environments. Starting
from an ONC-like cluster we vary the cluster size and density to determine the
correlation of collision time scale and disc-mass loss. We use the NBODY6++
code to model the dynamics of these clusters and analyze the effect of
star-disc encounters. We find that the disc-mass loss depends strongly on the
cluster density but remains rather unaffected by the size of the stellar
population. The essential outcome of the simulations are: i) Even in clusters
four times sparser than the ONC the effect of encounters is still apparent. ii)
The density of the ONC itself marks a threshold: in less dense and less massive
clusters it is the massive stars that dominate the encounter-induced disc-mass
loss whereas in denser and more massive clusters the low-mass stars play the
major role for the disc mass removal. It seems that in the central regions of
young dense star clusters -- the common sites of star formation -- stellar
encounters do affect the evolution of the protoplanetary discs. With higher
cluster density low-mass stars become more heavily involved in this process.
This finding allows for the extrapolation towards extreme stellar systems: in
case of the Arches cluster one would expect stellar encounters to destroy the
discs of most of the low- and high-mass stars in several hundred thousand
years, whereas intermediate mass stars are able to retain to some extant their
discs even under these harsh environmental conditions.
