| #1 – BREAKTHROUGHS in CARBON CHEMISTRY in CIRCUMSTELLAR ENVELOPES (CSEs) |
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| Theoretical goals Breakthroughs in our current understanding on the formation of carbonaceous dust and complex molecules in the circumstellar envelopes (CSEs) of AGB stars and the interstellar medium (ISM). |
| Experimental goals Production and analysis of carbon dust seeds in conditions resembling those in the dust formation zones of CSEs contrasting to all previous experiments in the field. |
| Innovative experimental methodology 1 – Production of carbon dust seeds at the Stardust Machine using exclusively gas-phase carbon atoms and molecular hydrogen in a ratio close to that in the atmospheres of AGB stars under ultra-high vacuum (UHV) conditions. 2 – Expansion of the previous study to investigate the interaction of atomic carbon and diatomic carbon with acetylene. 3 – First use of the sputtering gas aggregation source (SGAS) in Laboratory Astrophysics to generate small clusters of nanometre-sized particles by gas-phase aggregation of individual atoms in a weakly ionized environment, thus resembling what happens in the dust formation zones of CSEs. 4 – Full experimental analysis: atomic force microscopy (AFM), scanning tunnel microscopy (STM), transmission electron microscopy (TEM), optical emission spectroscopy (OES), infrared spectroscopy in transmission geometry and quadrupole mass spectrometry (QMS) at the Stardust Machine. Ex-situ laser desorption ionization/mass spectrometry (LDI-MS) in the AROMA machine that resembles the reactions on the surfaces of dust grains. |
| Results 1) From experiment 1. Efficient production of carbonaceous nanometre-sized grains, nanometer-sized small amorphous carbon clusters, acetylene (C2H2), along with fragments of ethylene (C2H4), ethane (C2H6) and larger aliphatic molecules, saturated aliphatic species and marginal detection of aromatic species (benzene, small PAHs like naphthalene) and no fullerenes. We reproduce the abundances of the acetylene and ethylene found in CSEs around AGB stars. 2) From experiment 2. Production of a non-negligible amount of pure and hydrogenated carbon clusters as well as aromatics with aliphatic substitutions, both being a direct consequence of the addition of atomic carbon |
| Conclusions Our experiments, that closely resemble the chemistry involved in the CSEs, do not favour the formation of aromatic species (PAHs and fullerenes), which can account for up to 18% of the total carbon species in the interstellar medium. We also show that aromatics with aliphatic substitutions as well as pure and hydrogenated carbon clusters can be produced as a direct consequence of the addition of atomic carbon. |
| Breakthroughs 1) SGAS, a technique not previously used in laboratory astrophysics, can be a very valuable tool to gain information on the chemistry operating in CSEs and the interstellar medium. 2) PAHs might not be efficiently formed during gas-phase growth in CSEs. 3) New theoretical plausible scenario: Thermal processing of aliphatic species deposited on dust grains in CSEs could lead to the formation of larger molecules or aromatic species. Such a temperature rise happens in later stages of stellar evolution (protoplanetary nebula PPNe) when the star emits UV radiation that leads to photo-processing of the carbon dust. Indeed, aromatic infrared bands, the signature for PAHs, are not convincingly detected in AGBs, but are observed at these later stages. 4) Unveiling of chemical routes: these results could unveil chemical routes leading to the formation of acetylene-based molecular species in the external layers of AGB stars and in PPNe, and to foster the search for alkyl-substituted aromatics in these environments. |
| Outstanding Publications – Prevalence of non-aromatic carbonaceous molecules in the inner regions of circumstellar envelopes (L. Martínez et al., Nature Astronomy, volume 4, pages 97–105, 2020), DOI link. EPMC link. – A new take on circumstellar carbon chemistry (M. Gatchell, News and Views, Nature Astronomy, volume 4, pages 21 – 22, 2020), share link). – The Chemistry of Cosmic Dust Analogs from C, C2, and C2H2 in C-rich Circumstellar Envelopes (G. Santoro et al., The Astrophysical Journal, volume 895, number 2, 2020). DOI link. EPMC link. |
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