| Description |
We propose to determine the brown dwarf disc fraction at ages of virgul30-50 Myr. A comparison of the disc frequencies for stars and brown dwarfs shows very different trends with age. For the solar-type stars, optically thick primordial discs are virtually non-existent by an age of virgul10 Myr. At ages >10 Myr, the inner disc material is significantly dissipated, and the discs have made a transition to the debris phase. In comparison, brown dwarf discs at virgul10 Myr exhibit strong excess emission in the Spitzer/IRS 5-35m spectrum, and also show strong excesses at Spitzer 70m. The mid-infrared colors for these virgul10 Myr old discs are found to be similar to the young optically thick discs in the 3 Myr old Orionis cluster, which indicates that these are still in their primordial phase. Thus no clear transition from a primordial to a debris phase is observed for the brown dwarf discs even at 10 Myr ages, unlike the higher mass stars. Furthermore, we find an increase in the brown dwarf disc frequency by a factor of virgul2 between 5 and 10 Myr. We propose to observe with Herschel/PACS a sample of all spectroscopically conrmed brown dwarfs in clusters at ages of 30-50 Myr. At present, PACS is the only instrument with the required high sensitivities to observe brown dwarf discs at these ages. Obtaining data for brown dwarfs at ages older than 10 Myr will be important to map the evolution of brown dwarf discs over a wider age range, and to determine if the brown dwarf disc decay time scale is longer than that observed for the earlier-type stars. |
