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 TW Hydrae association (TWA), we have discovered two new brown dwarf disks: a flat, optically thick disk around SSSPM J1102 - 3431 (SSSPM 1102) and a transition disk around 2MASS J1139511 - 315921 (2M1139). The disk structure for SSSPM 1102 is found to be very similar to the known brown dwarf disk 2MASSW J1207334 - 393254 (2M1207), with excess emission observed at wavelengths as short as 5 mu m. No excess emission shortward of similar to 20 mu m is seen from 2M1139, but it flares up at longer wavelengths and is the first transition disk detected among the substellar members of the TWA. We also report on Spitzer 70 mu m observations and the presence of a 10 mu m silicate absorption feature for 2M1207. The absorption can be attributed to a nearly edge-on disk, at 75 degrees inclination. The 10 mu m spectrum for 2M1207 shows crystalline forsterite features, with a peak in absorption near 11.3 mu m. No silicate absorption or emission is observed toward SSSPM 1102. While only six of 25 stellar members show excess emission at these mid-infrared wavelengths, all of the TWA brown dwarfs that have been observed so far with Spitzer show signs of disks around them, resulting in a disk fraction of at least 60%. This is a considerable fraction at the relatively old age of similar to 10 Myr. A comparison with younger clusters indicates that by the age of the TWA (similar to 10 Myr), the disk fraction for brown dwarfs has not decreased, whereas it drops by a factor of similar to 2 for the higher mass stars. This suggests longer disk decay timescales for brown dwarfs as compared with higher mass stars.