Probing the Molecular Hearts of Extreme Bipolar Planetary Nebulae with ALMA

TL;DR

This study uses ALMA Band 6 interferometric mapping to probe the cold molecular gas in six extreme bipolar planetary nebulae, revealing that the bulk of the molecular content resides in expanding equatorial tori. By combining CO and multiple trace molecules (CN, HCN, HNC, HCO+, CS, SO) across 12-m and ACA configurations, the authors derive torus geometries, expansion velocities, and dynamical ages that range from about 500 to 11,000 years, generally older than the optical lobes. The results support a jet-lag scenario in which equatorial torus ejection precedes fast polar outflows, implying a significant role for binary interactions and episodic mass loss during post-AGB evolution. The study also infers low $^{12}$C/$^{13}$C isotopic ratios and H$_2$ masses of order 10$^{-3}$–10$^{-1}$ M$_igodot$, suggesting progenitors of relatively high initial mass or interacting binaries, with irradiation effects shaping the chemistry and structure of the molecular gas over ~10 kyr timescales.

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.

Figures (29)

• Figure 1: PNe observed in our ALMA Band 6 program. The fiducial 0.5 pc marker is based on the image field of view given the PN distances adopted here (see § \ref{['sec:Dists']}). The image of PN Hb 5 was obtained from a Dec. 17, 1997 NASA/ESA/Hubble press release; the image of NGC 6302 was obtained from June 18, 2020 NASA/ESA press release; the image of NGC 6445 was obtained from Fang2018; the image of NGC 2440 was obtained from a Feb. 13, 2007 NASA/ESA/STScI press release; the image of NGC 2818 was obtained from a Jan. 28, 2009 NASA/ESA and Hubble Heritage Team (STScI/AURA) press release; the image for NGC 2899 was obtained from a Jan. 15, 2021 ESO/VLT press release through Scientific American.
• Figure 2: Comparison between the integrated fluxes measured with ACA and 12-m data for each molecular line for which observations were made in both ACA and 12-m configurations. Consistent flux measurements would follow along the black lines. The horizontal line in the plot for NGC 6445 indicates results from the C43-1 and C43-4 12-m configuration observations of $^{12}$CO(2--1) and CN hyperfine structure. Error bars were placed for all integrated flux measurements with calibration uncertainties included.
• Figure 3: Color overlay images for all ALMA Band 6 targets. H$\alpha$ (blue) and [N ii] (green) images are overlaid with ALMA Band 6 12-m $^{12}$CO zeroth moment (red) images. White contours are present to show the full extent of $^{12}$CO emission with respect to optical emission lines. H$\alpha$ and [N ii] emission lines were observed with HST/WFPC2 for PN Hb 5, NGC 2440, and NGC 2818. These emission lines were observed with HST/WFC3 for NGC 6302, and NOT for NGC 6445. In the case of NGC 2899, only H$\alpha$ ESO images (blue and green) are displayed.
• Figure 4: Images and spectrum extracted from 12-m $^{12}$CO(2--1) data cube obtained for Hubble 5. Top panel: PN Hb 5 $^{12}$CO(2$-$1) emission in units Jy/beam across multiple velocity channels in increments of 3.2 km s$^{-1}$. Bottom left panel: $^{12}$CO(2$-$1) zeroth moment in units Jy km s$^{-1}$ corresponding to the data cube displayed above. The white dotted oval surrounding the CO(2$-$1) emission represents the region selected for spectral data extraction from the same data cube. Bottom right panel: the resulting spatially integrated spectrum.
• Figure 5: ALMA Band 6 mom0 images of emission lines detected in PN Hb 5 with the ACA array (top 7 panels) and 12-m (bottom 6 panels). Top left panel: archival HST/WFPC2 H$\alpha$ image. Second panel from left: ACA image of the continuum emission at 236 GHz as received from the ALMA pipeline. Following 5 panels: $^{12}$CO, CN (5/2$-$3/2), CN (3/2$-$1/2), HCN, and HCO$^+$ mom0 images, respectively. Following 5 (12-m) panels: 12-m array continuum emission at 236 GHz as received from the ALMA pipeline, $^{12}$CO, CN (5/2$-$3/2), CN (3/2$-$1/2), and H$^{13}$CO$^+$, respectively. All images have a field of view of 40$"$ x 40$"$.