Spitzer observations of two TW Hydrae association brown dwarfs

Abstract

We present Spitzer Space Telescope observations of two TW Hydrae association brown dwarfs, 2MASSW J1207334-393254 and 2MASSW J1139511-315921, in the IRAC and MIPS 24 mm bands. On the basis of their IRAC colors, we have classified them as classical and weak-line T Tauri stars, respectively. For 2MASSW J1207334-393254, we have found that a flat-disk model fits the data very well. This brown dwarf shows the presence of warm (T >= 100 K) circumstellar dust close (R <= 0.2 AU) to it and does not display any signs of cleansing of dust within several AU of the star. In comparison with other TWA members that show excess in IR, we suggest that there exists a different disk evolution/dust processing mechanism for stellar and substellar objects. The star 2MASSW J1139511-315921 does not show any significant excess in any of the IRAC bands but a small one at 24 mm, which is not significant enough to suggest the presence of warm dust around this star. It shows signs of dust cleansing in the inner several AU, similar to most of the other TWA members.