Water Vapor in the Inner 25 AU of a Young Disk around a Low-Mass
  Protostar
  Water is one of the key molecules in the physical and chemical evolution of
star- and planet-forming regions. We here report the first spatially resolved
observation of thermal emission of (an isotopologue of) water with the Plateau
de Bure Interferometer toward the deeply embedded Class 0 protostar NGC
1333-IRAS4B. The observations of the H2-18-O 3_13-2_20 transition at 203.4 GHz
resolve the emission of water toward this source with an extent of about 0.2"
corresponding to the inner 25 AU (radius). The H2-18-O emission reveals a
tentative velocity gradient perpendicular to the extent of the protostellar
outflow/jet probed by observations of CO rotational transitions and water
masers. The line is narrow, about 1 km/s (FWHM), significantly less than what
would be expected for emission from an infalling envelope or accretion shock,
but consistent with emission from a disk seen at a low inclination angle. The
water column density inferred from these data suggests that the water emitting
gas is a thin warm layer containing about 25 M_Earth of material, 0.03% of the
total disk mass traced by continuum observations.
