Examinando por Autor "Sabbah, H."

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Prevalence of non-aromatic carbonaceous molecules in the inner regions of circumstellar envelopes
(Nature, 2019-10-21) Martínez, Lidia; Santoro, G.; Merino, P.; Accolla, M.; Lauwaet, K.; Sobrado, J. M.; Sabbah, H.; Peláez, R. J.; Herrero, V. J.; Tanarro, I.; Agúndez, Marcelino; Martín Jiménez, Alberto; Otero, Roberto; Ellis, G. J.; Joblin, C.; Cernicharo, J.; Martín Gago, J. A.; Instituto Nacional de Técnica Aeroespacial (INTA); European Commission (EC); Agencia Estatal de Investigación (AEI)
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.
Acceso Abierto
The Chemistry of Cosmic Dust Analogs from C, C2, and C2H2 in C-rich Circumstellar Envelopes
(The Institute of Physics (IOP), 2020-06-02) Santoro, G.; Martínez, Lidia; Lauwaet, K.; Accolla, M.; Tajuelo Castilla, G.; Merino, Pablo; Sobrado, J. M.; Peláez, R. J.; Herrero, V. J.; Tanarro, I.; Mayoral, Á.; Agúndez, Marcelino; Sabbah, H.; Joblin, C.; Cernicharo, J.; Martín Gago, J. A.; European Commission (EC); Comunidad de Madrid; Ministerio de Economía y Competitividad (MINECO); Santorio, G. [0000-0003-4751-2209]; Accolla, M. [0000-0002-9509-5967]; Agúndez, M. [0000-0003-3248-3564]; Sabbah, H. [0000-0001-5722-4388]; Joblin, C. [0000-0003-1561-6118]; Cernicharo, J. [0000-0002-3518-2524]; Martín Gago, J. M. [0000-0003-2663-491X]; Unidad de Excelencia Científica María de Maeztu Centro de Astrobiología del Instituto Nacional de Técnica Aeroespacial y CSIC, MDM-2017-0737
Interstellar carbonaceous dust is mainly formed in the innermost regions of circumstellar envelopes around carbon-rich asymptotic giant branch stars (AGBs). In these highly chemically stratified regions, atomic and diatomic carbon, along with acetylene, are the most abundant species after H and CO. In a previous study, we addressed the chemistry of carbon (C and C) with H showing that acetylene and aliphatic species form efficiently in the dust formation region of carbon-rich AGBs whereas aromatics do not. Still, acetylene is known to be a key ingredient in the formation of linear polyacetylenic chains, benzene, and polycyclic aromatic hydrocarbons (PAHs), as shown by previous experiments. However, these experiments have not considered the chemistry of carbon (C and C) with CH. In this work, by employing a sufficient amount of acetylene, we investigate its gas-phase interaction with atomic and diatomic carbon. We show that the chemistry involved produces linear polyacetylenic chains, benzene, and other PAHs, which are observed with high abundances in the early evolutionary phase of planetary nebulae. More importantly, we have found a nonnegligible 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. The incorporation of alkyl substituents into aromatics can be rationalized by a mechanism involving hydrogen abstraction followed by methyl addition. All the species detected in the gas phase are incorporated into nanometric-sized dust analogs, which consist of a complex mixture of sp, sp, and sp hydrocarbons with amorphous morphology.