Photo-processing of astro-PAHs

C Joblin,G Mulas,D Toublanc,A Bonnamy,L Nahon,A Simon,S Rodriguez Castillo,M Ji,G Wenzel,H Sabbah,A Giuliani

doi:10.1088/1742-6596/1412/6/062002

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are key species in astrophysical environments in which vacuum ultraviolet (VUV) photons are present, such as star-forming regions. The interaction with these VUV photons governs the physical and chemical evolution of PAHs. Models show that only large species can survive. However, the actual molecular properties of large PAHs are poorly characterized and the ones included in models are only an extrapolation of the properties of small and medium-sized species. We discuss here experiments performed on trapped ions including some at the SOLEIL VUV beam line DESIRS. We focus on the case of the large dicoronylene cation, , and compare its behavior under VUV processing with that of smaller species. We suggest that C2 H2 is not a relevant channel in the fragmentation of large PAHs. Ionization is found to largely dominate fragmentation. In addition, we report evidence for a hydrogen dissociation channel through excited electronic states. Although this channel is minor, it is already effective below 13.6 eV and can significantly influence the stability of astro-PAHs. We emphasize that the competition between ionization and dissociation in large PAHs should be further evaluated for their use in astrophysical models.

Highlights

Polycyclic aromatic hydrocarbons (PAHs) are present in photodissociation regions (PDRs) associated with massive star-forming regions, such as the prototypical Orion Bar region [1]
The critical size is estimated from the ability of the molecule to lose C2H2 since in PDRs the absorption of UV photons is faster than the chemistry that could rebuild the carbon skeleton
We discuss here some results obtained in ion traps for species containing up to ∼ 50 carbon atoms and came to the following conclusions

Summary

Introduction

Polycyclic aromatic hydrocarbons (PAHs) are present in photodissociation regions (PDRs) associated with massive star-forming regions, such as the prototypical Orion Bar region [1] Their interaction with vacuum ultraviolet (VUV) photons can trigger various molecular processes: -(i)- ionization resulting in gas heating by thermalisation of the emitted electrons [2], -(ii)photodissociation limiting the survival of PAHs and producing molecules such as H2 and C2H2 in PDRs [3,4,5,6,7], and -(iii)- radiative cooling leading to the well-known aromatic infrared (IR) emission bands between 3 and 15 μm, which constitute the only direct diagnosis we have so far for the presence of these large molecules in astrophysical environments as proposed in the initial PAH model [8, 9]. In this case the competition with rehydrogenation by reactivity with the abundant H and H2 species needs to be considered (see for instance [5])

Methods

Results

Conclusion