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

We present results from a Herschel PACS survey of 8 brown dwarfs in the IC 2391 cluster. Our aim was to determine the brown dwarf disk fraction at ages of ~40-50 Myr. None of the 8 brown dwarfs observed were detected in the PACS 70 or 160mu bands. We have determined the detection limits of our survey using the 1-sigma flux upper limits in the PACS far-infrared and the WISE mid-infrared bands. The sensitivity of our observations would only allow for the detection of debris disks with exceptionally large fractional luminosities (>1%). Considering that only the most extreme and rare debris disks have such high fractional luminosities, it can be hypothesized that Vega-like debris disks, as observed around ~30% of low-mass stars at similar ages, could exist around the targeted IC 2391 brown dwarfs. Most primordial disks 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 disks have transitioned to the debris phase by ~40-50 Myr ages. We also present the sensitivity limits for detecting brown dwarf disks with future SPICA observations.