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 Atacama Large Millimeter/submillimeter Array. We find (i) two very high-speed knotty, jet-like molecular outflows; (ii) a central high-density (>few x10⁶ 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 x10^-4 M_s yr⁻¹ in the past ∼455 years. 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 M_s) of very large (∼millimeter-sized) grains. The measured expansion ages of the above structural components imply that the torus (age∼160 years) and the younger high-velocity outflow (age∼110 years) 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.