Non-aromatic carbonaceous species formed in the circumstellar innerlayers of evolved stars

. Evolved stars are a foundry of chemical complexity, gas and dust that contribute to the formation of the building blocks of planets and life. Nucleation of dust takes place first at the photosphere of evolved stars. Despite their importance, these cosmic regions remain hidden to many observations and dust formation processes are still poorly understood. Laboratory astrophysics provides a complementary route to unveil these chemical processes but the majority of experiments are based on combustion or plasma decomposition of molecular precursors in physical conditions far removed from those in space. We have built an unprecedented ultra-high vacuum machine combining atomic gas aggregation with advanced in-situ characterization techniques to reproduce and characterize the bottom-up dust formation process. We show that carbonaceous dust analogues formed from low-pressure gas-phase condensation of C atoms in a hydrogen atmosphere in a C/H 2 ratio similar to that reported for evolved stars leads to the formation of amorphous C nanograins, and C-clusters aliphatic in nature. Aromatic species or fullerenes are not effectively formed under these physical conditions.

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