2025A&A...704A.345S

Context. Polycyclic aromatic hydrocarbons (PAHs) are considered ubiquitous in the interstellar medium. There is now solid evidence for their presence. The mechanisms that lead to their formation and their destruction, remain debated, however. Of the processes that drive their evolution, the shock-induced alteration of PAHs has received little attention.
Aims. Our objective is to explore the gaseous volatiles and solid residues generated by shock-processing of coronene C₂₄H₁₂, which is a prototypical compact PAH with seven aromatic rings.
Methods. A pressure-driven shock tube was employed to sublimate and heat coronene up to 4000 K. The time evolution of the shock products was probed in situ by optical emission spectroscopy on a microsecond timescale. Solid residues were collected and analyzed ex situ by a variety of methods, including infrared microspectroscopy, Raman spectroscopy, X-ray diffraction, transmission electron microscopy, and laser desorption laser ionization mass spectrometry. The experiments were supported by molecular dynamics (MD) simulations.
Results. The experiments revealed a dominant dehydrogenation pathway for coronene under shock conditions. In situ spectroscopy confirmed the presence of C₂ radicals and a broad continuum emission attributed to large carbon clusters (C_n) and small, weakly hydro-genated hydrocarbons (C_nH_x). The ex situ analysis of solid residues indicates the formation of graphitic and graphenic nanostructures, including carbon nano-onions, nanotubes, and nanoribbons in the cooling phase. Laser desorption laser ionization mass spectrometry analysis validates the carbonization of the shock products, while MD simulations support the dehydrogenation and fragmentation processes of the rapid-heating phase.
Conclusions. Shock waves drive the transformation of PAHs into small hydrocarbons and carbon clusters that recombine in the cooling phase into graphene-like structures. This affects the carbon life cycle of the interstellar medium. The identification of C_nH_x species as potential carriers of the broad green emission seen in the laboratory suggests a possible link to the λ5450 diffuse interstellar band. This study underscores the value of shock tubes as a tool for simulating astrophysical environments and investigating the chemical evolution of large molecules and small particles in space.