Spatial distribution of stars and brown dwarfs in σ Orionis

Abstract

I have re-visited the spatial distribution of stars and high-mass brown dwarfs in the a Orionis (a Ori) cluster (∼3 Ma, ∼360pc). The input was a catalogue of 340 cluster members and candidates at separations less than 30 arcmin to σ Ori AB. Of them, 70 per cent have features of extreme youth. I fitted the normalized 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 M ⊙ concentrate, however, towards the cluster centre, where there is also an apparent deficit of very low mass objects (M < 0.16 M ⊙ ). ). 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.