  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.
