An L′-band survey for circumstellar discs around low-mass stars in the young σ 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 disc frequency depends on the method used: (54 +/- 15) per cent if measured directly from the JHKL' colour-colour diagram, or (46 +/- 14) per cent if excesses are computed with respect to predicted photospheric colours (according to the object spectral types, 2 excess detections). We compare the (K-L') excess with other indicators and show that this is a robust and reliable disc indicator. We also compare the derived disc frequency with similarly aged clusters and discuss possible implications for disc lifetimes. The computed age of the Ori cluster is very important; a cluster age of 3 Myr would support the overall disc lifetime of 6 Myr proposed in the literature, while an age > 4 Myr would point to a slower disc destruction rate.