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Chemistry under EUV Irradiation of H2-CO-N2 Gas Mixtures: Implications for Photochemistry in the Outer CSE of Evolved Stars

Abstract : Context. CircumStellar Envelopes (CSEs) of stars are complex chemical objects for which theoretical models encounter difficulties in elaborating a comprehensive overview of the occurring chemical processes. Along with photodissociation, ion-neutral reactions and dissociative recombination might play an important role in controlling molecular growth in outer CSEs. Aims. The aim of this work is to provide experimental insights into pathways of photochemistry-driven molecular growth within outer CSEs to draw a more complete picture of the chemical processes occurring within these molecule-rich environments. A simplified CSE environment was therefore reproduced in the laboratory through gas-phase experiments exposing relevant gas mixtures to an Extreme UltraViolet (EUV) photon source. This photochemical reactor should ultimately allow us to investigate chemical processes and their resulting products occurring under conditions akin to outer CSEs. Methods. We used a recently developed EUV lamp coupled to the APSIS photochemical cell to irradiate CSE relevant gas mixtures of H2, CO and N2, at one wavelength, 73.6 nm. The detection and identification of chemical species in the photochemical reactor was achieved through in-situ mass spectrometry analysis of neutral and cationic molecules. Results. We find that exposing CO-N2-H2 gas mixtures to EUV photons at 73.6 nm induces photochemical reactions that yield the formation of complex, neutral and ionic species. Our work shows that N2H+ can be formed through photochemistry along with highly oxygenated ion molecules like HCO+ in CSE environments. We also observe neutral N-rich organic species including triazole and aromatic molecules. Conclusions. These results confirm the suitability of our experimental setting to investigate photochemical reactions and provide fundamental insights into the mechanisms of molecular growth in the outer CSEs. Especially, this work supports a potential reac- tion mechanism from simple species to monocylic system toward the formation of Nitrogenized Polycyclic aromatic hydrocarbons (NPAHs) that could occur in circumstellar environments but also in the InterStellar Medium (ISM).
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Submitted on : Saturday, January 18, 2020 - 8:16:40 PM
Last modification on : Friday, July 3, 2020 - 4:50:27 PM

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Jérémy Bourgalais, D.V. Bekaert, A. Bacmann, B. Marty, Nathalie Carrasco. Chemistry under EUV Irradiation of H2-CO-N2 Gas Mixtures: Implications for Photochemistry in the Outer CSE of Evolved Stars. Astronomy and Astrophysics - A&A, EDP Sciences, 2020, (in press). ⟨insu-02444689⟩

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