Destruction of protoplanetary discs in the Orion Nebula Cluster

Abstract

We use numerical N-body simulations of the Orion Nebula Cluster (ONC) to investigate the destruction of protoplanetary discs by close stellar encounters and UV radiation from massive stars. The simulations model a cluster of 4000 stars and we consider separately cases in which the discs have fixed radii of 100 and 10 au. In the former case, depending on a star's position and orbit in the cluster over 107 yr, UV photoevaporation removes at least 0.01 M⊙from its disc, and can remove up to 1 M⊙. We find no dynamical models of the ONC consistent with the suggestion of Störzer and Hollenbach that the observed distribution and abundance of proplyds could be explained by a population of stars on radial orbits that spend relatively little time near θ1C On (the most massive star in the ONC). Instead, the observations require either massive discs (e.g. a typical initial disc mass of 0.4 M⊙) or a very recent birth for θ1C Ori. When we consider the photoevaporation of the inner 10 au of discs in the ONC, we find that planet formation would be hardly affected. Outside that region, planets would be prevented from forming in about half the systems, unless either the initial disc masses were very high (e.g. 0.4M⊙) or they formed quickly (in less than ∼2 Myr) and θ1C Ori has only very recently appeared. We also present statistics on the distribution of minimum stellar encounter separations. This peaks at 1000 au, with only about 4 per cent of stars having had an encounter closer than 100 au at the cluster's present age, and less than 10 per cent after 107 yr. We conclude that stellar encounters are unlikely to play a significant role in destroying protoplanetary discs. In the absence of any disruption mechanism other than those considered here, we would thus predict planetary systems like our own to be common amongst stars forming in ONC-like environments. Also, although almost all stars will have experienced an encounter at the radius of the Oort cloud in our own system, this only places a firm constraint on the possible birthplace of the Sun if the Oort cloud formed in situ, rather than through the secular ejection of matter from the planetary zone.