An L'-band survey for circumstellar disks around low-mass stars in the young sigma Orionis cluster

We present new K- and L'-band imaging of a representative sample of members of the young 3-5 Myr old sigma Orionis cluster. We identified objects with (K-L') excess by analysing colour-colour diagrams and comparing the observations with empirical main-sequence colours. The derived disk frequency depends on the method used: (54 +/- 15)% if measured directly from the JHKL' colour-colour diagram; or (46 +/-14)% if excesses are computed with respect to predicted photospheric colours (according to the objects spectral types, 2-sigma excess detections). We compare the (K-L') excess with other indicators and show that this is a robust and reliable disk indicator. We also compare the derived disk frequency with similarly aged clusters and discuss possible implications for disk lifetimes. The computed age of the sigma Ori cluster is very important: a cluster age of 3 Myr would support the overall disk lifetime of 6 Myr proposed in the literature, while an age > 4 Myr would point to a slower disk destruction rate.