Encounters in the ONC – observing imprints of star-disc interactions

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str.77, 50937 Köln, Germany e-mail: olczak@ph1.uni-koeln.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 18 March 2008
Accepted: 3 June 2008

Abstract

Context. The external destruction of protoplanetary discs in a clustered environment acts mainly due to two mechanisms: gravitational drag by stellar encounters and evaporation by strong stellar winds and radiation. It is a fundamental question whether either of these mechanisms is important for the stellar evolution and planet formation process in young star clusters.

Aims. We focus on the effect of stellar encounters in young dense clusters and investigate whether there are any observables that could trace this mechanism and its impact on disc evolution. If encounters play a role in disc destruction, one would expect that stars devoid of disc material would show unexpectedly high velocities as an outcome of close interactions. We want to quantify this effect by numerical simulations and compare it to observations.

Methods. As a model cluster we chose the Orion Nebula Cluster (ONC). We reanalyzed observational data of the ONC to find encounter signatures in the velocity distribution and a possible correlation with signatures of circumstellar discs. We use the ++ code to model the dynamics of an ONC-like cluster and analyze the velocity distribution and the disc-mass loss due to encounters.

Results. We found from the observational data that 8 to 18 stars leave the ONC with velocities several times the velocity dispersion. The majority of these high-velocity stars are young low-mass stars (t ≲ 10⁵ yr, m ≈ 0.2–0.3 ), among them several lacking infrared excess emission. Interestingly, the high-velocity stars are found only in two separate regions of the ONC – i) close to the cluster centre and ii) in the outer cluster region. Our simulations give an explanation for the location of the high-velocity stars and provide evidence for a strong correlation between location and disc destruction.

Conclusions. The high-velocity stars can be explained as the outcome of close three-body encounters; the partial lack of disc signatures is attributed to gravitational interaction. The spatial distribution of the high-velocity stars reflects the initial structure and dynamics of the ONC. Our approach can be generalized to study the evolution of other young dense star clusters, like the Arches cluster, back in time.