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 Tauri stars in the TW Hya Association (TWA) using the Near-Infrared Camera and Multiobject Spectrometer aboard the Hubble Space Telescope. Most of the T Tauri stars in our study show evidence of absorption by H2O vapor in their atmospheres; in addition, the low-mass brown dwarf candidate, TWA 5B, is brighter at 1.9 mu m than predicted by cool star models that include the effects of H2O vapor but neglect dust. We conclude that the effect of atmospheric dust on the opacity is important at 1.9 mu m for TWA 5B, the coolest object 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 function of age and mass then confirms previous claims that TWA 5B is substellar with a mass in the range 0.02-0.03 M.. The accurate single-epoch astrometry 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.