[Show abstract][Hide abstract] ABSTRACT: Observations of star formation in the Galaxy support the conclusion that most stars – including our own – form in an environment like the Orion nebula cluster (ONC). We construct a range of dynamical models of the ONC, using Aarseth's nbody6 code, and explore their consequences for the origins of the cluster and its subsequent evolution. We find that the most acceptable fits to the cluster density profile are obtained in models where the cluster is set up in virial equilibrium and where the cluster extends well outside the limits of existing photometric surveys. However, current estimates of the virial ratio suggest the cluster is already unbound. We show that the size and age of the ONC in this case imply either that it became unbound only very recently, or else that it has expanded quasi-statically. In the latter case, its initial central density may have exceeded its current value by 1–2 orders of magnitude. We stress the importance of future proper motion experiments to distinguish between these possibilities.
Preview · Article · Mar 2005 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: We use numerical N-body simulations of the Orion Nebula Cluster (ONC) to investigate the possibility of substructure in its formation. There is no substructure apparent in the ONC today. However, unless there was a remarkable degree of homogeneity in the molecular cloud from which it formed, it seems unlikely that this would have been true of the cluster in its earliest phase. More plausibly, the early structure of the cluster would have consisted of groups or clumps of subclusters, following the structure of the cloud itself. We have explored the extent to which such subclusters could subsequently have merged, and find that the age of the cluster is a critical factor. The most inhomogeneous initial conditions, comprising a small number of subclusters with many members, are ruled out by an age of 2 Myr or less. There is a considerable amount of freedom in the other direction, however, which suggests that fragmentation in the original cloud is more likely to have been on the scale of small clumps, each producing fewer than 100 stars. These initial subclusters could have been very dense – perhaps two or three orders of magnitude more dense than the core of the ONC today.
Preview · Article · Jun 2002 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: We present the results of multiple simulations of open clusters, modelling the dynamics of a population of brown dwarf members. We consider the effects of a large range of primordial binary populations, including the possibilities of having brown dwarf members contained within a binary system. We also examine the effects of various cluster diameters and masses. Our examination of a population of wide binary systems containing brown dwarfs, reveals evidence for exchange reactions whereby the brown dwarf is ejected from the system and replaced by a heavier main-sequence star. We find that there exists the possibility of hiding a large fraction of the brown dwarfs contained within the primordial binary population. We conclude that it is probable that the majority of brown dwarfs are contained within primordial binary systems which then hides a large proportion of them from detection. Comment: 16 pages, 8 figures; to appear in MNRAS
Preview · Article · Feb 2002 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] 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 Ori (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. 0.4 M⊙) 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.
Preview · Article · Jul 2001 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: Using proper motion data for 894 stars in the Orion nebula cluster (ONC) compiled by Jones & Walker in 1988, we search for binaries with apparent separations in the range 1000–5000 au, and find an upper limit of three. Using a Monte
Carlo method, we test the consistency of this result with two hypotheses: (i) that the cluster contains a binary population
identical to that found in the solar neighbourhood, and (ii) that the cluster contains no binaries at all in this separation
range. We obtain results strongly favouring the latter hypothesis. Star formation in the Galaxy is seen to occur in a variety
of different environments, but it has been proposed that most stars may be formed in dense regions similar to the ONC, rather
than in less dense groupings like that found in Taurus-Auriga. As roughly 15 per cent of Galactic field stars are known to
be in binaries with separations greater than 1000 au, the apparent absence of such binaries in the ONC places an upper limit
on the contribution that dense clusters can make to galactic star formation.
Preview · Article · Feb 1999 · Monthly Notices of the Royal Astronomical Society