ALMA Observations of Molecular Line Emission from High-excitation Bipolar Planetary Nebulae

TL;DR

This work uses ALMA Band 6 molecular-line mapping to study a sample of nearby, molecule-rich, high-excitation bipolar planetary nebulae, focusing on the spatial distribution and kinematics of molecular gas and its response to UV and X-ray irradiation from hot central stars. By mapping CO isotopologues and high-energy tracers across six PNe, the authors reveal central equatorial molecular tori, with younger nebulae showing more coherent, massive structures and richer isotopologue/S-bearing chemistry, while older PNe exhibit clumpier, more fragmented gas. The study proposes an evolutionary sequence for the equatorial molecular gas in bipolar PNe, shaped by intense radiation and winds, and demonstrates ALMA’s power to constrain AGB/post-AGB mass-loss histories and binary-influenced PN shaping. These results provide concrete spatio-kinematic and chemical constraints that improve our understanding of PN evolution and the fate of molecular material in harsh PN environments.

Abstract

We present early results from our program of ALMA Band 6 (1.3mm) molecular line mapping of a sample of nearby, well-studied examples of high-excitation, bipolar/pinched-waist and molecule-rich planetary nebulae (Hubble 5 and NGC 2440, 2818, 2899, 6302, and 6445). We have mapped these planetary nebulae (PNe) in isotopologues of CO as well as various molecular line tracers of high-energy irradiation, such as HCN, CN, HNC, and HCO+, with the complementary goals of establishing nebular kinematics as well as the zones of UV-heated and X-ray-ionized molecular gas within each nebula. The resulting high-resolution ALMA molecular emission-line maps reveal the regions of high-excitation bipolar PNe in which molecular gas, presumably ejected during asymptotic giant branch stages of the PN progenitor stars, survives and evolves chemically. We present a summary of molecular species detected to date in the sample nebulae, and we use example results for one PN (NGC 6455) to demonstrate the power of the ALMA data in revealing the structures, kinematics, and compositions of the equatorial molecular tori that are a common feature of the sample objects.

Figures (4)

• Figure 1: Comparison of optical and ALMA $^{12}$CO images of (from L to R) PNe Hb 5, NGC 6302, NGC 2440, NGC 6445, NGC 2818, and NGC 2899. Top row: color composites of optical images of the six PNe. Archival HST images are presented for Hb 5, NGC 6302, NGC 2440, and NGC 6445; archival ESO and NOT images are presented for NGC 2899 and NGC 6445, respectively. Middle and bottom rows: Band 6 $^{12}$CO (2--1) velocity-integrated images obtained with ALMA's ACA and 12-m arrays, respectively.
• Figure 2: Below: Montages of ALMA continuum and velocity-integrated $^{12}$CO, CN and CS images of NGC 6445 as obtained with ACA ( top panels) and the 12-m array ( bottom panels). Each ALMA montage is presented alongside the NOT H$\alpha$ image Fang2018, for reference.
• Figure 3: Velocity channel maps extracted from the NGC 6445 $^{12}$CO (2--1) ACA data cube. From left to right and top to bottom, the maps are displayed from $\sim$46 km s$^{-1}$ to $\sim-$13 km s$^{-1}$ in increments of 1.9 km s$^{-1}$.
• Figure 4: Contours of velocity-resolved $^{12}$CO (2--1) emission extracted from the NGC 6445 ACA data cube, overlaid on a color composite of NOT archival optical images (H$\alpha$, green; [N ii], red; [O iii], blue). The contours are color-coded as indicated in the legend (LSR velocities), with red, orange, cyan, and blue indicating a range from more highly redshifted to more highly blueshifted emission with respect to the systemic velocity of NGC 6445 ($V_{LSR} \sim +15$ km s$^{-1}$).