NICMOS narrowband infrared photometry of TW Hydrae Association stars

We have obtained 1.64, 1.90, and 2.15 mu m narrowband images of five T Taur i stars in the TW Hya Association (TWA) using the Near-Infrared Camera and Multiobject Spectrometer aboard the Hubble Space Telescope. Most of the T T auri stars in our study show evidence of absorption by H2O vapor in their a tmospheres; in addition, the low-mass brown dwarf candidate, TWA 5B, is bri ghter at 1.9 mu m than predicted by cool star models that include the effec ts of H2O vapor but neglect dust. We conclude that the effect of atmospheri c dust on the opacity is important at 1.9 mu m for TWA 5B, the coolest obje ct in our sample. The available evidence suggests that the TWA is 5-15 Myr old. Comparison of the colors of TWA 5B with theoretical magnitudes as a fu nction of age and mass then confirms previous claims that TWA 5B is substel lar with a mass in the range 0.02-0.03 M.. The accurate single-epoch astrom etry of the relative positions and separation of TWA 5A and TWA 5B reported here should permit the direct measurement of the orbital motion of TWA 5B within only a few years.