Prevalence of non-aromatic carbonaceous molecules in the inner regions of circumstellar envelopes

Evolved stars are a foundry of chemical complexity, gas and dust that provides the building blocks of planets and life, and dust nucleation first occurs in their photosphere. Despite their importance, the circumstellar regions enveloping these stars remain hidden to many observations, thus dust formation processes are still poorly understood. Laboratory astrophysics provides complementary routes to unveil these chemical processes, but most experiments rely on combustion or plasma decomposition of molecular precursors under physical conditions far removed from those in space. We have built an 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/H₂ ratio similar to that reported for evolved stars, leads to the formation of amorphous C nanograins and aliphatic C-clusters. Aromatic species or fullerenes do not form effectively under these conditions, raising implications for the revision of the chemical mechanisms taking place in circumstellar envelopes.