Wide Field Planetary Camera 2 observations of the brown dwarf Gliese 229B: Optical colors and orbital motion

Three-epoch observations of the M1 V-plus-brown dwarf system Gliese 229AB using the Hubble Space Telescope's Wide Field Planetary Camera 2 (WFPC2) are reported. Relative astrometric measurements spanning 1 year confirm the common proper motion of the pair and reveal the first evidence of orbital motion. The radial and azimuthal components of G1 229B's relative motion are -0".087 and +0".049, respectively, indicating an elliptical orbit rather than an inclined circular orbit. The absolute WFPC2 magnitudes of G1 229B are M-1042 = 16.37, M-814 = 20.76, and M-675 = 24.60, assuming a distance of 5.774 pc. The detection of G1 229B through the F675W bandpass is the first reported at R-band wavelengths. The measured flux through F1042M (lambda(c) approximate to 1 mu m) is well matched by the latest models of dust-free brown dwarf photospheres by Tsuji et al. These models severely overestimate the broadband fluxes shortward of 0.8 mu m, however, which indicates that a strong source of optical continuum opacity exists in the photosphere of G1 229B. A search for fainter companions to G1 229A in the Planetary Camera was negative to limiting absolute magnitudes of M-1042 approximate to 19 and M-814 approximate to 24.5 beyond 7" of G1 229A. Nondetection limits for the Wide Field Cameras are 1.5 mag greater than those for the Planetary Camera.