Spatial differences between stars and brown dwarfs: a dynamical origin?

Abstract

We use $N$-body simulations to compare the evolution of spatial distributions of stars and brown dwarfs in young star-forming regions. We use three different diagnostics; the ratio of stars to brown dwarfs as a function of distance from the region's centre, ${\mathcal{R}}_{\mathrm{SSR}}$, the local surface density of stars compared to brown dwarfs, ${\Sigma }_{\mathrm{LDR}}$, and we compare the global spatial distributions using the ${\Lambda }_{\mathrm{MSR}}$ method. From a suite of twenty initially statistically identical simulations, 6/20 attain ${\mathcal{R}}_{\mathrm{SSR}}<<1$ $and$ ${\Sigma }_{\mathrm{LDR}}<<1$ $and$ ${\Lambda }_{\mathrm{MSR}}<<1$, indicating that dynamical interactions could be responsible for observed differences in the spatial distributions of stars and brown dwarfs in star-forming regions. However, many simulations also display apparently contradictory results - for example, in some cases the brown dwarfs have much lower local densities than stars (${\Sigma }_{\mathrm{LDR}}<<1$), but their global spatial distributions are indistinguishable (${\Lambda }_{\mathrm{MSR}}=1$) and the relative proportion of stars and brown dwarfs remains constant across the region (${\mathcal{R}}_{\mathrm{SSR}}=1$). Our results suggest that extreme caution should be exercised when interpreting any observed difference in the spatial distribution of stars and brown dwarfs, and that a much larger observational sample of regions/clusters (with complete mass functions) is necessary to investigate whether or not brown dwarfs form through similar mechanisms to stars.

7 pages, 5 figures, accepted for publication in MNRAS

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