Letter to the Editor

The spatial distribution of substellar objects in IC 348 and the Orion Trapezium cluster

Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal e-mail: [nanda;sschmeja]@astro.up.pt

Received: 14 May 2007
Accepted: 22 June 2007

Abstract

Context.Some theoretical scenarios suggest the formation of brown dwarfs as ejected stellar embryos in star-forming clusters. Such a formation mechanism can result in different spatial distributions of stars and substellar objects.

Aims.We aim to investigate the spatial structure of stellar and substellar objects in two well-sampled and nearby embedded clusters, namely IC 348 and the Orion Trapezium cluster (OTC), to test this hypothesis.

Methods.Deep near-infrared K-band data that are complete enough to sample the substellar population in IC 348 and OTC are obtained from the literature. The spatial distribution of the K-band point sources is analysed using the minimum spanning tree (MST) method. The Q parameter and the spanning trees are evaluated for stellar and substellar objects as a function of cluster core radius R_c.

Results.The stellar population in both IC 348 and OTC display a clustered distribution, whereas the substellar population is distributed homogeneously in space within twice the cluster core radius. Although the substellar objects do not appear to be bound by the cluster potential well, they are still within the limits of the cluster and not significantly displaced from their birth sites.

Conclusions.The spatially homogeneous distribution of substellar objects is best explained by assuming higher initial velocities, distributed in a random manner and going through multiple interactions. The overall spatial coincidence of these objects with the cluster locations can be understood if these objects are nevertheless travelling slowly enough to feel the gravitational influence of the cluster. The observations support the scenario of substellar objects forming as “ejected stellar embryos”. Higher ejection velocities are necessary, but net spatial displacements may not be needed to explain the observational data.