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Title: Nucleation of Small Silicon Carbide Dust Clusters in AGB Stars

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}, 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}C{sub 4}, Si{sub 5}C{sub 5}, Si{sub 15}C{sub 15}, and Si{sub 16}C{sub 16} and new ground states for Si{sub 10}C{sub 10} and Si{sub 15}C{sub 15}. 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 Si{sub 12}C{sub 12} and Si{sub 16}C{sub 16} and low-lying stable cage structures for n ≥ 12. The latter findings thus indicate a regime of clustermore » 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 μ m wavelength range and the 11.3 μm feature inferred from laboratory measurements and observations, respectively, although the overall intensities are rather low.« less

Authors:
; ;  [1];  [2]
  1. Osservatorio Astronomico di Teramo, INAF, I-64100 Teramo (Italy)
  2. Departament de Cincia de Materials i Química Fisica and Institut de Química Terica i Computacional (IQTCUB),Universitat de Barcelona, E-08028 Barcelona (Spain)
Publication Date:
OSTI Identifier:
22663604
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 840; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; CARBON; COMPUTERIZED SIMULATION; CRYSTALS; DENSITY; DENSITY FUNCTIONAL METHOD; DUSTS; EMISSION; GROUND STATES; INFRARED SPECTRA; MOLECULES; MONTE CARLO METHOD; QUANTUM MECHANICS; SILICON; SILICON CARBIDES; SOLIDS; STAR CLUSTERS; STARS; WAVELENGTHS

Citation Formats

Gobrecht, David, Cristallo, Sergio, Piersanti, Luciano, and Bromley, Stefan T. Nucleation of Small Silicon Carbide Dust Clusters in AGB Stars. United States: N. p., 2017. Web. doi:10.3847/1538-4357/AA6DB0.
Gobrecht, David, Cristallo, Sergio, Piersanti, Luciano, & Bromley, Stefan T. Nucleation of Small Silicon Carbide Dust Clusters in AGB Stars. United States. doi:10.3847/1538-4357/AA6DB0.
Gobrecht, David, Cristallo, Sergio, Piersanti, Luciano, and Bromley, Stefan T. Wed . "Nucleation of Small Silicon Carbide Dust Clusters in AGB Stars". United States. doi:10.3847/1538-4357/AA6DB0.
@article{osti_22663604,
title = {Nucleation of Small Silicon Carbide Dust Clusters in AGB Stars},
author = {Gobrecht, David and Cristallo, Sergio and Piersanti, Luciano and Bromley, Stefan T.},
abstractNote = {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}, 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}C{sub 4}, Si{sub 5}C{sub 5}, Si{sub 15}C{sub 15}, and Si{sub 16}C{sub 16} and new ground states for Si{sub 10}C{sub 10} and Si{sub 15}C{sub 15}. 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 Si{sub 12}C{sub 12} and Si{sub 16}C{sub 16} 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 μ m wavelength range and the 11.3 μm feature inferred from laboratory measurements and observations, respectively, although the overall intensities are rather low.},
doi = {10.3847/1538-4357/AA6DB0},
journal = {Astrophysical Journal},
number = 2,
volume = 840,
place = {United States},
year = {Wed May 10 00:00:00 EDT 2017},
month = {Wed May 10 00:00:00 EDT 2017}
}