Brown-dwarf Disk Surveys

(Riaz, Thompson)

We are conducting WISE based searches for young brown dwarf disks in nearby star-forming regions and associations. Our focus is on the age interval of ~5-10Myr, which is a critical intermediate age when most disks begin to show a transition from a primordial to a debris disk system. We have constructed a WISE SED classification scheme, based on the Ks and WISE bands of 3.4-12mu. 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 disk population. Based on the results obtained so far, the disk frequencies for brown dwarfs are not found to show any clear dependence on the age, stellar density or the BD/star number ratio in a cluster. This is in contrast to the higher mass stars where the inner disk frequency shows an evolutionary decline with age, and the disk fractions are lower in the denser clusters. Also, primordial BD disks are still visible at ages of ~10 Myr, whereas the higher mass stars have all transitioned to the debris stage by this age. We have looked into the various formation mechanisms for BDs, which could lead to different disk properties, and have found that the observed BD disk fraction may only partially be due to an age evolution. The large differences in the fractions between regions may well be due to different BD formation mechanisms, and therefore different initial disk fractions/properties.

Our surveys with the Herschel Space Telescope are focused on brown dwarf disks at relatively older ages of ~10-50 Myr. With these far-infrared observations, we aim to obtain robust disk masses for these older disk sources, and to determine if brown dwarf disk masses show any decline with the age of the system. Also, obtaining a better constraint on the outer disk radius or the spatial extent of the disk will help understand the possible formation mechanism for these brown dwarfs. This work will also highlight new transition disk systems with inner opacity holes, which may have been missed by previous Spitzer surveys. These Herschel surveys will be important to map the evolution of brown dwarf disks over a wider age range, and to determine if the brown dwarf disk decay time scale is longer than that observed for the earlier-type stars.