Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt........17g&link_type=abstract
PhD Dissertation, Illinois Univ. Urbana-Champaign, IL United States
Astronomy and Astrophysics
Astronomy
Silicon Carbides, Stellar Envelopes, Astronomical Interferometry, Vapor Phases, Confinement, Millimeter Waves, Abundance, Electron Transitions, Asymmetry, Molecular Interactions
Scientific paper
We have used the Berkeley-Illinois-Maryland Association (BIMA) Array to map emission from the 422-321 and 404-303 transitions of SiC2 in the circumstellar envelope (CSE) surrounding IRC+10216. The NRAO 12-m telescope was used to fill in missing flux from large scale structure not detected by the interferometer. The interferometry and 12-m single-element data were combined to make full synthesis maps of emission from SiC2 toward IRC+10216. We find that full synthesis maps of the 422-321 and 404-303 transitions of SiC2 show a distinctly shell-like structure. The emission is not uniformly distributed. Instead, it shows a clumpy appearance on the plane of the sky. The maps of the 422-321 transition show a distinct bipolar structure with lobes oriented along a roughly north-south axis. The maps of the 404-303 transition show an asymmetric appearance in which the east side of the CSE is approximately 2-3 brighter than the west side. Radiative transfer models of the data suggest that most of the observed SiC2 is confined to a shell with inner radius approximately 2 x 1016 cm and outer radius approximately 6 x 1016 cm. The abundance of SiC2 ((SiC2)/(H2)) within the shell is approximately 10-6. We cannot rule out the possibility that some SiC2 originates in the inner envelope near the photosphere but we can put an upper limit on the abundance of SiC2 in the inner envelope. Our data constrains the fractional abundance of SiC2 in the inner envelope (r approximately less than 2 x 1016 cm) to be no more than approximately 3 x 10-8. The distribution and abundance of SiC2 suggest that ion-molecule reactions involving C2H2 may be responsible for producing much of the SiC2. Our data also places important constraints on the excitation mechanisms for SiC2. Kinetic temperatures in the outer CSE (approximately less than 60 K) where much of the SiC2 resides are not high enough to excite the high excitation temperatures across K-ladders observed by Thaddeus et al. (1984) and Avery et al. (1992). This suggests that radiative excitation through the lowest- lying vibrationally-excited state, the nu3 = 1 antisymmetric mode, may be responsible for the high excitation temperatures across K-ladders. To test this hypothesis we made a sensitive search for rotational transitions of vibrationally excited SiC2 with the NRAO 12-m. Our data suggests that the column density of vibrationally excited SiC2 is at least two orders of magnitude lower than the column density of the ground vibrational state. The upper limit on the column density of vibrationally excited SiC2 is consistent with radiative excitation through the nu3 = 1 antisymmetric mode.
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