Hidden Molecular Bullets in Compact Young Planetary Nebulae

Abstract For the three planetary nebulae M 1-11, Vy 2-2, and Hen 2-459, we report the detection of kinematically distinct structures that exhibit different states and mechanisms of excitation in molecular hydrogen (H 2 ). The observations were obtained with the Immersion Grating INfrared Spectrometer (or IGRINS), a high-resolution ( R ∼ 45,000) near-IR H - and K- band (1.45−2.45 μ m) spectrometer. All three nebulae exhibit the following features: a central photoionized zone traced by H and He recombination lines, and collisionally excited lines from heavier species; a low-velocity expanding shell of UV-excited H 2 , seen in numerous H 2 emission lines from highly excited rovibrational levels; and two “bullets” (compact, fast-moving clumps) of hot H 2 that are symmetrical and equidistant from the nebular center in position and velocity. In Vy 2-2, a bipolar structure traced by [Fe ii ] and [Fe iii ] emission extends through the ionized region and terminates in bright, high-velocity (∼75 km s −1 ) redshifted and blueshifted bullets of thermal H 2 emission, consistent with shock heating, with temperatures of ∼1200 K. M 1-11 hosts similar H 2 bullets but, unlike Vy 2-2, the [Fe ii ] and [Fe iii ] lines do not appear to trace the axis of a bipolar structure. Hen 2-459 shows a morphology similar to Vy 2-2, with high-velocity H 2 bullets and [Fe ii ] and [Fe iii ] emission, but the evidence for a bipolar structure is less clear in this case. The structures observed in these nebulae might arise from collimated bipolar outflows or from converging conical shocks.