  We report high angular resolution observations of the HCN (3-2) line emission
in the circumstellar envelope of the O-rich star W Hya with the Submillimeter
Array. The proximity of this star allows us to image its molecular envelope
with a spatial resolution of just ~40 AU, corresponding to about 10 times the
stellar diameter. We resolve the HCN (3-2) emission and find that it is
centrally peaked and has a roughly spherically symmetrical distribution. This
shows that HCN is formed in the innermost region of the envelope (within ~10
stellar radii), which is consistent with predictions from pulsation-driven
shock chemistry models, and rules out the scenario in which HCN forms through
photochemical reactions in the outer envelope. Our model suggests that the
envelope decreases steeply in temperature and increases smoothly in velocity
with radius, inconsistent with the standard model for mass-loss driven by
radiative pressure on dust grains. We detect a velocity gradient of ~5 km/s in
the NW--SE direction over the central 40 AU. This velocity gradient is
reminescent of that seen in OH maser lines, and could be caused by the rotation
of the envelope or by a weak bipolar outflow.
