Circumstellar discs in the young σ Orionis cluster

We present new K-and L ′-band imaging observations for members of the young (3−5 Myr) σ Orionis cluster, obtained with UIST at UKIRT. We determine (K − L ′) colour excesses with respect to the photospheres, finding evidence for warm cir-cumstellar dust around 27 out of 83 cluster members that have masses between 0.04 M ⊙ and 1.0 M ⊙. This indicates a circumstellar disc frequency of at least (33 ± 6)% for this cluster, consistent with previous determinations from smaller samples (Oliveira, Jeffries & van Loon 2004) and also consistent with the 3 Myr disc half-life suggested by Haisch, Lada & Lada (2001b). There is marginal evidence that the disc frequency declines towards lower masses, but the data are also consistent with no mass-dependence at all. There is no evidence for spatial segregation of objects with and without circumstellar discs. Key words: circumstellar matter – infrared: stars – stars: pre-main-sequence – stars: low-mass, brown dwarfs – open clusters and associations: individual (σ Orionis)