  We have obtained 1.64, 1.90 and 2.15 micron narrow-band 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 microns 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 microns for TWA 5B, the
coolest object in our sample. The available evidence suggests that the TWA is
5-15 MY 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 solar masses. 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.
