New Brown Dwarf Disks in the TW Hydrae Association
Abstract
In our analysis of Spitzer/IRS archival data on the stellar and sub-stellar 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 . 2M1139 shows no excess emission shortward of 20 , but flares up at longer wavelengths, and is the first transition disk detected among the sub-stellar members of TWA. We also report on the Spitzer/70 observations, and the presence of a weak absorption 10 silicate feature for 2M1207. The absorption can be attributed to a close to edge-on disk at a 75 inclination. The 10 spectrum for 2M1207 shows crystalline forsterite features, with a peak in absorption near 11.3 . No silicate absorption/emission is observed towards SSSPM 1102. While only 6 out 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 a relatively older age of 10 Myr. A comparison with younger clusters indicates that by the age of the TWA (10 Myr), the disk fraction for brown dwarfs has not decreased, whereas it drops by a factor of 2 for the higher mass stars. This suggests longer disk decay time scales for brown dwarfs compared to higher mass stars.