B. Riaz, N. Lodieu, S. Goodwin, D. Stamatellos, M. Thompson
We have investigated the brown dwarf (BD) and stellar disc fractions in the
Upper Scorpius OB Association (USco) and compared them with several other young
regions. We have compiled the most complete sample of of all spectroscopically
confirmed BDs in USco, and have made use of the WISE catalog to identify the
disc candidates. We report on the discovery of 12 new BD discs in USco, with
spectral type (SpT) between M6 and M8.5. The WISE colors 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%) for a wide range in stellar masses (~0.01-4.0 Msun) 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 TW Hydrae association, whereas the higher mass
stars have all transitioned to the debris stage by this age. The disc
frequencies for BDs do not show any dependence on the stellar density or the
BD/star number ratio in a cluster. 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 SED classification scheme, based on the Ks and WISE
bands of 3.4-12micron, to distinguish between the Class I/II and the Class III
sequence. Our work includes a comparison of the sensitivities of WISE and
Spitzer disc surveys. We estimate that WISE can be incomplete for discs at SpT
later than M8 in distant clusters such as SOri. WISE should be able to recover
the M8-M9 discs in the nearby young clusters.
View original:
http://arxiv.org/abs/1111.3929
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