Low-Mass Stars and Brown Dwarfs Around σ Orionis

We present optical spectroscopy of 71 photometric candidate low-mass members of the cluster associated with σ Orionis. Thirty-five of these are found to pass the lithium test and hence are confirmed as true cluster members, covering a mass range of ≤ 0.055-0.3M⊙, assuming a mean cluster age of ≤ 5Myr. We find evidence for an age spread on the (I, I−J) colour magnitude diagram, members appearing to lie in the range 1-7Myr. There are, however, a significant fraction of candidates that are non-members, including some previously identified as members based on photometry alone. We see some evidence that the ratio of spectroscopically confirmed members to photometric candidates decreases with brightness and mass. This highlights the importance of spectroscopy in determining the true initial mass-function.