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

We present new Kand L-band imaging of a representative sample of members of the young 3−5Myr old σOrionis cluster. We identified objects with (K − L) excess by analysing colour-colour diagrams and comparing the observations with empirical mainsequence 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-σ 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 σOri cluster is very important: a cluster age of 3Myr would support the overall disk lifetime of 6Myr proposed in the literature, while an age > 4Myr would point to a slower disk destruction rate.