Probing the Molecular Hearts of Extreme Bipolar Planetary Nebulae with ALMA

Abstract

We present results from a program of Atacama Large Millimeter Array (ALMA) 1.3 mm (Band 6) molecular line mapping of a sample of nearby, bipolar/pinched-waist, molecule-rich PNe (NGC 6302, Hubble 5, NGC 2440, NGC 6445, NGC 2899, and NGC 2818). Maps of $^{12}$CO(2$-$1) and $^{13}$CO(2$-$1) emission as well as emission lines of HCN, HNC, HCO$^+$, CN, and CS $-$ many of these detected in these PNe for the first time $-$ reveal the molecular mass distributions, compositions, and velocity fields of the equatorial and, in some cases, polar regions of the sample PNe. In each case, the bulk of the molecular gas traces an expanding equatorial torus, with torus expansion velocities ranging from $\sim$15 to $\sim$50 km s$^{-1}$ and molecular masses from $\sim$0.002 to $\sim$0.1 $M_\odot$. The inferred molecular torus dynamical ages, which span the range $\sim$500 yr (Hb 5) to $\sim$11000 yr (NGC 2818), provide support for a model wherein molecular torus ejection precedes bipolar lobe formation. Collectively, these ALMA survey results provide insight into the rapid structural evolution as well as the zones of irradiated molecular gas within bipolar PNe that are descended from relatively massive progenitors, likely residing in interacting binary systems, over $\sim$10 kyr of the post-AGB evolution of such systems.