A Herschel PACS survey of brown dwarfs in IC 2391: limits on primordial and debris disc fractions

Abstract

We present results from a Herschel Photodetector Array Camera and Spectrometer (PACS) survey of eight brown dwarfs in the IC 2391 cluster. Our aim was to determine the brown dwarf disc fraction at ages of ∼40-50 Myr. None of the eight brown dwarfs observed were detected in the PACS 70 or 160 μm bands. We have determined the detection limits of our survey using the 1σ flux upper limits in the PACS far-infrared and the Wide-field Infrared Survey Telescope mid-infrared bands. The sensitivity of our observations would only allow for the detection of debris discs with exceptionally large fractional luminosities (≥1 per cent). Considering that only the most extreme and rare debris discs have such high fractional luminosities, it can be hypothesized that Vega-like debris discs, as observed around ∼30 per cent of low-mass stars at similar ages, could exist around the targeted IC 2391 brown dwarfs. Most primordial discs similar to the ones observed for the younger 1-10 Myr brown dwarfs would be within the detection sensitivities of our survey, and could have been detected. The non-detection for all targets then suggests that brown dwarf discs have transitioned to the debris phase by ∼40-50 Myr ages. We also present the sensitivity limits for detecting brown dwarf discs with future SPICA observations.