  High angular resolution H2CO 218 GHz line observations have been carried out
toward the low-mass protostars IRAS 16293-2422 and L1448-C using the Owens
Valley Millimeter Array at ~2" resolution. Simultaneous 1.37 mm continuum data
reveal extended emission which is compared with that predicted by model
envelopes constrained from single-dish data. For L1448-C the model density
structure works well down to the 400 AU scale to which the interferometer is
sensitive. For IRAS 16293-2422, a known proto-binary object, the interferometer
observations indicate that the binary has cleared much of the material in the
inner part of the envelope, out to the binary separation of ~800 AU. The
morphology and velocity structure of the H2CO array data have been used to
investigate whether the abundance enhancements inferred from single-dish
modelling are due to thermal evaporation of ices or due to liberation of the
ice mantles by shocks in the inner envelope. A scenario in which the H2CO
abundance drops in the cold dense part of the envelope where CO is frozen out
but is undepleted in the outermost region provides good fits to the single-dish
and interferometer data on short baselines for both sources. Emission on the
longer baselines is best reproduced if the H2CO abundance is increased by about
an order of magnitude from ~1E-10 to ~1E-9 in the inner parts of the envelope
due to thermal evaporation when the temperature exceeds ~50K. Other scenarios,
including weak outflow-envelope interactions and photon heating of the
envelope, are discussed and predictions for future generation interferometers
are presented, illustrating their potential in distinguishing these competing
scenarios.
