ALMA Observations of the Water Fountain Pre-Planetary Nebula IRAS
  16342-3814: High-velocity bipolar jets and an Expanding Torus
  We have mapped 12CO J=3-2 and other molecular lines from the "water-fountain"
bipolar pre-planetary nebula (PPN) IRAS 16342-3814 with ~0."35 resolution using
ALMA. We find (i) two very high-speed knotty, jet-like molecular outflows, (ii)
a central high-density (> few x 10^6 cm^{-3}), expanding torus of diameter 1300
AU, and (iii) the circumstellar envelope of the progenitor AGB, generated by a
sudden, very large increase in the mass-loss rate to >3.5 x 10^{-4} Msun/yr in
the past ~455 yr. Strong continuum emission at 0.89 mm from a central source
(690 mJy), if due to thermally-emitting dust, implies a substantial mass (0.017
Msun) of very large (~mm-sized) grains. The measured expansion ages of the
above structural components imply that the torus (age~160 yr) and the younger
high-velocity outflow (age~110 yr) were formed soon after the sharp increase in
the AGB mass-loss rate. Assuming a binary model for the jets in IRAS 16342, the
high momentum rate for the dominant jet-outflow in IRAS 16342 implies a high
minimum accretion rate, ruling out standard Bondi-Hoyle-Lyttleton wind
accretion and wind Roche lobe overflow (RLOF) models with white-dwarf or
main-sequence companions. Most likely, enhanced RLOF from the primary or
accretion modes operating within common envelope evolution are needed.
