  Water is present during all stages of star formation: as ice in the cold
outer parts of protostellar envelopes and dense inner regions of circumstellar
disks, and as gas in the envelopes close to the protostars, in the upper layers
of circumstellar disks and in regions of powerful outflows and shocks. In this
paper we probe the mechanism regulating the warm gas-phase water abundance in
the innermost hundred AU of deeply embedded (Class~0) low-mass protostars, and
investigate its chemical relationship to other molecular species during these
stages. Millimeter wavelength thermal emission from the para-H2-18O
3(1,3)-2(2,0) (Eu=203.7 K) line is imaged at high angular resolution (0.75";
190 AU) with the IRAM Plateau de Bure Interferometer towards the deeply
embedded low-mass protostars NGC 1333-IRAS2A and NGC 1333-IRAS4A. Compact
H2-18O emission is detected towards IRAS2A and one of the components in the
IRAS4A binary; in addition CH3OCH3, C2H5CN, and SO2 are detected. Extended
water emission is seen towards IRAS2A, possibly associated with the outflow.
The detections in all systems suggests that the presence of water on <100 AU
scales is a common phenomenon in embedded protostars. We present a scenario in
which the origin of the emission from warm water is in a flattened disk-like
structure dominated by inward motions rather than rotation. The gas-phase water
abundance varies between the sources, but is generally much lower than a
canonical abundance of 10^-4, suggesting that most water (>96 %) is frozen out
on dust grains at these scales. The derived abundances of CH3OCH3 and SO2
relative to H2-18O are comparable for all sources pointing towards similar
chemical processes at work. In contrast, the C2H5CN abundance relative to
H2-18O is significantly lower in IRAS2A, which could be due to different
chemistry in the sources.
