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

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Abstract

Evolved stars are foundries of chemical complexity, gas and dust that provide the building blocks of planets and life, and dust nucleation first occurs in their photosphere. The circumstellar regions enveloping these stars, despite their importance, remain hidden to many observations, and dust formation processes are therefore 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. To reproduce and characterize the bottom-up dust formation process, we have built an ultra-high vacuum machine combining atomic gas aggregation with advanced in situ characterization techniques. We show that carbonaceous dust analogues that formed from low-pressure gas-phase condensation of carbon atoms in a hydrogen atmosphere, in a ratio of carbon to molecular hydrogen similar to that reported for evolved stars, lead to the formation of amorphous carbon nanograins and aliphatic carbon clusters. Aromatic species and fullerenes do not form effectively under these conditions, raising implications for a revision of the chemical mechanisms taking place in circumstellar envelopes.

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Fig. 1: Production of stardust analogues.
Fig. 2: Production and structure of the stardust analogues.
Fig. 3: Spectroscopic characterization of the molecular species.
Fig. 4: Ex situ LDI-MS.
Fig. 5: Computed evolution of the molecular species formed.
Fig. 6: Representative mass spectrum from a thermal desorption experiment.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request. Address to J.A.M.-G. for all in situ measurments, J.C. for kinetic calculations and C.J. for LDI experiments.

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Acknowledgements

We thank the European Research Council for funding support under Synergy Grant ERC-2013-SyG, G.A. 610256 (NANOCOSMOS). We also acknowledge partial support from the Spanish Research Agency (AEI) through grants MAT2017-85089-c2-1R, FIS2016-77578-R and FIS2016-77726-C3-1-P. Support from the FotoArt-CM Project (P2018/NMT-4367) through the Program of R&D activities between research groups in Technologies 2013, cofinanced by European Structural Funds, is also recognized.

Author information

In situ experiments in Stardust were performed by L.M., G.S., P.M. and M.Accolla; C.J. and H.S. performed LDI experiments; P.M., K.L., R.O. and A.M.-J. performed STM. L.M. performed AFM. R.J.P., V.J.H. and I.T. performed the OES experiments. M.Agúndez made the kinetic calculations. G.J.E. and J.A.M.-G. wrote the first version of the manuscript. J.A.M.-G. supervised in situ experiments, and C.J. and J.C. supervised the astrochemical interpretation. All authors discussed and contributed to the final version of the manuscript.

Correspondence to Christine Joblin or José Cernicharo or José A. Martín-Gago.

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Peer review information Nature Astronomy thanks Farid Salama and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Information

Supplementary text, references, Figs. 1–6 and Tables 1 and 2.

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Martínez, L., Santoro, G., Merino, P. et al. Prevalence of non-aromatic carbonaceous molecules in the inner regions of circumstellar envelopes. Nat Astron (2019) doi:10.1038/s41550-019-0899-4

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