2012MNRAS.420.2497R

We have investigated the brown dwarf (BD) and stellar disc fractions in the Upper Scorpius (USco) OB association and compared them with several other young regions. We have compiled the most complete sample of all spectroscopically confirmed BDs in USco and have made use of the Wide-field Infrared Survey Explorer (WISE) catalogue to identify the disc candidates. We report on the discovery of 12 new BD discs in USco, with spectral types between M6 and M8.5. The WISE colours for the new discs are similar to the primordial (transition) discs earlier detected in USco. Combining with previous surveys, we find the lowest inner disc fractions ( ∼ 20–25 per cent) for a wide range in stellar masses ( ∼ 0.01–4.0 M_☉) in the USco association. The low disc fractions for high-mass stars in USco (and the other clusters) are consistent with an evolutionary decline in inner disc frequency with age. However, BD disc fractions are higher than those for the stars in 1–3 Myr clusters, but very low in the ∼ 5 Myr old USco. Also, primordial BD discs are still visible in the ∼ 10 Myr old TW Hydrae association, whereas the higher mass stars have all transitioned to the debris stage by this age. The disc frequencies for BDs and low-mass stars do not show any dependence on the stellar density or the BD/star number ratio in a cluster. We also find no convincing evidence that any of the well-known disc dispersal mechanisms for stars are active in BD discs. We suggest that the large differences in the observed BD disc fractions between regions may well be due to different BD formation mechanisms and therefore different initial disc fractions/properties.

We also present a WISE spectral energy distribution classification scheme, based on the K_s and WISE bands of 3.4–12 µm. We have determined certain thresholds in the WISE spectral slope versus spectral type diagrams to distinguish between the red population of Class I/II systems and the Class III sequence. We have found the WISE [3.4] - [12] colour to provide the best distinction between the photospheric and the disc population. Our work includes a comparison of the sensitivities of WISE and Spitzer disc surveys. We estimate that WISE can be incomplete for discs at spectral type later than M8 in distant clusters such as SOri. WISE should be able to recover the M8–M9 discs in the nearby young clusters.