Irradiation Investigation: Exploring the Molecular Gas in NGC 7293

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.

The First Mid-infrared Detection of HNC in the Interstellar Medium: Probing the Extreme Environment toward the Orion Hot Core

“…The HNC column densities in Table 5 are consistently lower than HCN by about two orders of magnitude, evidence for a highly irradiated environment. Indeed, studies of planetary nebulae and the Helix Nebula find that HCN/HNC correlates with increasing UV radiation (Bublitz et al 2019(Bublitz et al , 2020. HNC is less photostable than HCN (Chenel et al 2016;Aguado et al 2017) and is destroyed in increasingly advanced and hotter stages of stellar evolution (Jin et al 2015).…”

Valence-shell ionization of acetyl cyanide: simulation of the photoelectron and infra-red spectra

“…Only for o 0 10;9;8;7;6;5;4 central vibrational modes in the final electronic state ranging between 500 cm À1 and 1550 cm À1 significant rotations occur. However, in spite of rotation effects as described using eqn (8), the changes of the intensities in the energy region between 500 and 1500 cm À1 are very small and the bijective approach assuming a one-to-one correspondence between each initial and final vibrational mode is here not so different to the surjective model application.…”

“…This assumption consists of reducing the (3N À 6) sum of (3N À 6) products in eqn (8) to only one product of 3N À 6 terms for which a i,k is maximum (for each (i) 3N À 6 normal modes in the initial state a k mode is selected (i 2 k) for which a i,k is maximum) and the residual overlap of interest is…”

“…In principle this approach takes into account full normalization conditions between the normal modes in the initial state and in the final state. However, the price to pay is that this approach is undeniably more greedy in computing time since it requires to consider numerous terms as described in eqn (8). Temperature effect was here fully neglected.…”

“…[3][4][5][6][7] Most of these atomic or molecular identifications can be accomplished by the comparison of recorded spectra on earth to the observations of the interstellar medium. Consequently, the determination of the physical/chemical properties as well as the understanding of the various reaction paths of these atomic/chemical species in space 8,9 require laboratory measurements assisted by numerical models. Within this framework, infra-red and photoemission spectroscopies are precisely two of the well-known techniques to explore the structure of matter.…”