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 σ 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, 2σ 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 σ 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.