Abstract
Background: Many planetary nebulae retain significant quantities of molecular gas and dust despite their signature hostile radiation environments and energetic shocks. Photoionization and dissociation by extreme UV and (often) X-ray emission from their central stars drive the chemical processing of this material. Their well-defined geometries make planetary nebulae ideal testbeds for modeling the effects of radiation-driven heating and chemistry on molecular gas in photodissociation regions. Methods: We have carried out IRAM 30m/APEX 12m/ALMA radio studies of the Helix Nebula and its molecule-rich globules, exploiting the unique properties of the Helix to follow up our discovery of an anti-correlation between HNC/HCN line intensity ratio and central star UV Luminosity. Results: Analysis of HNC/HCN across the Helix Nebula reveals the line ratio increases with distance from the central star, and thus decreasing incident UV flux, indicative of the utility of the HNC/HCN ratio as a tracer of UV irradiation in photodissociation environments. However, modeling of the observed regions suggests HNC/HCN should decrease with greater distance, contrary to the observed trend. Conclusion: HNC/HCN acts as an effective tracer of UV irradiation of cold molecular gas. Further model studies are required.
Highlights
The molecular chemistries of planetary nebulae (PNe) provide ideal testbeds to explore the role that high-energy irradiation plays in photodissociation regions (PDRs)
Comparison of the HNC/HCN line ratio with the computed UV flux of the Helix Nebula reveals an anticorrelation that is consistent with the results of [2]
Flux from published literature sources further extends the trend of decreasing HNC/HCN into the regime where UV flux is strongest, suggesting that the ratio acts as a powerful tracer of UV irradiation in PDR environments
Summary
The molecular chemistries of planetary nebulae (PNe) provide ideal testbeds to explore the role that high-energy irradiation plays in photodissociation regions (PDRs). The potential value of observations of molecule-rich PNe to our understanding of the processes governing the HNC/HCN ratio has been overlooked in past works, though there is recent interest in the molecular ratio [2,10] Driven by this recent progress, we seek a representative environment to establish whether and how UV emission from the CSPN and gas temperature affect the HNC/HCN ratio. When taken as a whole, the Helix presents a factor ∼20 gradient in UV flux from the inner globules to the edges of the molecular ring at ∼7.50 (Bublitz et al, in prep), and is the best PN in which to establish how HNC/HCN varies across an individual PN In this proceedings paper, we present the current state of our radio molecular line observing campaign targeting the Helix Nebula and its globules, with the primary goal to gain insight into the mechanisms driving the HNC/HCN ratio.
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