New Brown Dwarf Disks in the TW Hydrae Association

TL;DR

This study reports the discovery of two new brown dwarf disks in the TW Hydrae Association using Spitzer data, revealing that brown dwarf disks persist longer than those of higher mass stars at around 10 million years.

Contribution

First detection of transition disks among TWA brown dwarfs, providing insights into disk longevity and evolution in sub-stellar objects.

Findings

Two new brown dwarf disks identified in TWA.

Brown dwarf disk fraction remains high (~60%) at 10 Myr.

Disk decay timescales are longer for brown dwarfs than for higher mass stars.

Abstract

In our analysis of {\it 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 $\micron$. 2M1139 shows no excess emission shortward of $\sim$20 $\micron$, 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 {\it Spitzer}/70 $\micron$ observations, and the presence of a weak {\it absorption} 10 $\micron$ silicate feature for 2M1207. The absorption can be attributed to a close to edge-on disk at a 75$\degr$ inclination. The 10 $\micron$ spectrum for 2M1207 shows crystalline forsterite features, with a peak in absorption near 11.3 $\micron$. 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, {\it all} of the TWA brown dwarfs that have been observed so far with {\it 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 $\sim$10 Myr. A comparison with younger clusters indicates that by the age of the TWA ($\sim$10 Myr), the disk fraction for brown dwarfs has not decreased, whereas it drops by a factor of $\sim$2 for the higher mass stars. This suggests longer disk decay time scales for brown dwarfs compared to higher mass stars.