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  1. OA@INAF
  2. PRODOTTI RICERCA INAF
  3. 1 CONTRIBUTI IN RIVISTE (Journal articles)
  4. 1.01 Articoli in rivista
Please use this identifier to cite or link to this item: http://hdl.handle.net/20.500.12386/26847
Title: Nucleation of Small Silicon Carbide Dust Clusters in AGB Stars
Authors: Gobrecht, David
CRISTALLO, Sergio 
PIERSANTI, Luciano 
Bromley, Stefan T.
Issue Date: 2017
Journal: THE ASTROPHYSICAL JOURNAL 
Number: 840
Issue: 2
First Page: 117
Abstract: Silicon carbide (SiC) grains are a major dust component in carbon-rich asymptotic giant branch stars. However, the formation pathways of these grains are not fully understood. We calculate ground states and energetically low-lying structures of (SiC)<SUB> n </SUB>, n = 1, 16 clusters by means of simulated annealing and Monte Carlo simulations of seed structures and subsequent quantum-mechanical calculations on the density functional level of theory. We derive the infrared (IR) spectra of these clusters and compare the IR signatures to observational and laboratory data. According to energetic considerations, we evaluate the viability of SiC cluster growth at several densities and temperatures, characterizing various locations and evolutionary states in circumstellar envelopes. We discover new, energetically low-lying structures for Si<SUB>4</SUB>C<SUB>4</SUB>, Si<SUB>5</SUB>C<SUB>5</SUB>, Si<SUB>15</SUB>C<SUB>15</SUB>, and Si<SUB>16</SUB>C<SUB>16</SUB> and new ground states for Si<SUB>10</SUB>C<SUB>10</SUB> and Si<SUB>15</SUB>C<SUB>15</SUB>. The clusters with carbon-segregated substructures tend to be more stable by 4-9 eV than their bulk-like isomers with alternating Si-C bonds. However, we find ground states with cage geometries resembling buckminsterfullerens (“bucky-like”) for Si12C12 and Si16C16 and low-lying stable cage structures for n ≥ 12. The latter findings thus indicate a regime of cluster sizes that differ from small clusters as well as from large-scale crystals. Thus—and owing to their stability and geometry—the latter clusters may mark a transition from a quantum-confined cluster regime to a crystalline, solid bulk-material. The calculated vibrational IR spectra of the ground-state SiC clusters show significant emission. They include the 10-13 μm wavelength range and the 11.3 μm feature inferred from laboratory measurements and observations, respectively, although the overall intensities are rather low.
URI: http://hdl.handle.net/20.500.12386/26847
URL: https://iopscience.iop.org/article/10.3847/1538-4357/aa6db0
ISSN: 0004-637X
DOI: 10.3847/1538-4357/aa6db0
Bibcode ADS: 2017ApJ...840..117G
Fulltext: open
Appears in Collections:1.01 Articoli in rivista

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