New Brown Dwarf Disks in the TW Hydrae Association

Abstract

In our analysis of Spitzer IRS archival data on the stellar and substellar members of the TWHydrae association (TWA), we have discovered two new brown dwarf disks: a flat, optically thick disk around SSSPM J1102?3431 (SSSPM1102) and a transition disk around2MASS J1139511?315921 (2M1139).The disk structure for SSSPM1102 is found to be very similar to the known brown dwarf disk2MASSWJ1207334?393254 (2M1207),with excess emis- sion observed at wavelengths as short as 5 ?m. No excess emission shortward of ?20 ?mis seen from2M1139, but it flares up at longerwavelengths and is the first transition disk detected among the substellar members of the TWA.We also report on Spitzer 70 ?mobservations and the presence of a 10 ?msilicate absorption feature for2M1207. The ab- sorption can be attributed to a nearly edge-on disk, at 75? inclination. The 10 ?m spectrum for 2M1207 shows crys- talline forsterite features, with a peak in absorption near 11.3 mu m.Nosilicate absorption oremission is observed toward SSSPM1102.While only six of 25 stellarmembers showexcessemission at thesemid-infraredwavelengths, all of the TWAbrowndwarfs that have been observed so farwith Spitzer showsigns of disks around them, resulting in a disk frac- tion of at least 60%. This is a considerable fraction at the relatively old age of ~ 10 Myr. A comparison with younger clusters indicates that by the age of the TWA(~10Myr), the disk fraction for brown dwarfs has not decreased,whereas it drops by a factor of ?2 for the highermass stars.This suggests longer disk decay timescales for browndwarfs as com- pared with higher mass stars.