Spatial distribution of stars and brown dwarfs in sigma Orionis

Abstract

I have re-visited the spatial distribution of stars and high-mass brown dwarfs in the sigma Orionis cluster (~3 Ma, ~360 pc). The input was a catalogue of 340 cluster members and candidates at separations less than 30 arcmin to sigma Ori AB. Of them, 70 % have features of extreme youth. I fitted the normalised cumulative number of objects counting from the cluster centre to several power-law, exponential and King radial distributions. The cluster seems to have two components: a dense core that extends from the centre to r ~ 20 arcmin and a rarified halo at larger separations. The radial distribution in the core follows a power-law proportional to r^1, which corresponds to a volume density proportional to r^(-2). This is consistent with the collapse of an isothermal spherical molecular cloud. The stars more massive than 3.7 Msol concentrate, however, towards the cluster centre, where there is also an apparent deficit of very low-mass objects (M < 0.16 Msol). Last, I demonstrated through Monte Carlo simulations that the cluster is azimuthally asymmetric, with a filamentary overdensity of objects that runs from the cluster centre to the Horsehead Nebula.