¹R.A.Arneson et al. (>10)*
The Astrophysical Journal 843, 51 Link to Article [https://doi.org/10.3847/1538-4357/aa75cf]
¹Minnesota Institute for Astrophysics, School of Physics and Astronomy, University of Minnesota, 106 Pleasant Street S.E., Minneapolis, MN 55455, USA
*Find the extensive, full author and affiliation list on the publishers website

We present a SOFIA FORCAST grism spectroscopic survey to examine the mineralogy of the circumstellar dust in a sample of post-asymptotic giant branch (post-AGB) yellow supergiants that are believed to be the precursors of planetary nebulae. Our mineralogical model of each star indicates the presence of both carbon-rich and oxygen-rich dust species—contrary to simple dredge-up models—with a majority of the dust in the form of amorphous carbon and graphite. The oxygen-rich dust is primarily in the form of amorphous silicates. The spectra do not exhibit any prominent crystalline silicate emission features. For most of the systems, our analysis suggests that the grains are relatively large and have undergone significant processing, supporting the hypothesis that the dust is confined to a Keplerian disk and that we are viewing the heavily processed, central regions of the disk from a nearly face-on orientation. These results help to determine the physical properties of the post-AGB circumstellar environment and to constrain models of post-AGB mass loss and planetary nebula formation.

^1,4David Gobrecht, ¹Sergio Cristallo, ¹Luciano Piersanti, ^2,3Stefan T. Bromley
The Astrophysical Journal 840, 2 Link to Article [https://doi.org/10.3847/1538-4357/aa6db0]
¹Osservatorio Astronomico di Teramo, INAF, I-64100 Teramo, Italy
²Departament de Cincia de Materials i Química Fisica and Institut de Química Terica i Computacional (IQTCUB),Universitat de Barcelona, E-08028 Barcelona, Spain
³Institucio Catalana de Recerca i Estudis Avancats (ICREA), E-08010 Barcelona, Spain
⁴Instituut voor Sterrenkunde, Celestijnenlaan 200 D, B-3001 Heverlee (Leuven), Belgium

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) 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 Si4C4, Si5C5, Si15C15, and Si16C16 and new ground states for Si10C10 and Si15C15. 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.