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Snyder, Lewis E. Gensheimer, Paul David 2011-05-07T12:03:24Z 2011-05-07T12:03:24Z 10000-01-01 1994 We have used the Berkeley-Illinois-Maryland Association (BIMA) Array to map emission from the 4$\sb{22}$-$3\sb{21}$ and 4$\sb{04}$-$3\sb{03}$ transitions of SiC$\sb2$ 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 SiC$\sb2$ toward IRC+10216. We find that full synthesis maps of the 4$\sb{22}$-$3\sb{21}$ and 4$\sb{04}$-$3\sb{03}$ transitions of SiC$\sb2$ 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 4$\sb{22}$-$3\sb{21}$ transition show a distinct bipolar structure with lobes oriented along a roughly north-south axis. The maps of the 4$\sb{04}$-$3\sb{03}$ transition show an asymmetric appearance in which the east side of the CSE is $\sim$2-3 brighter than the west side. Radiative transfer models of the data suggest that most of the observed SiC$\sb2$ is confined to a shell with inner radius $\sim$2 $\times$ 10$\sp{16}$ cm and outer radius $\sim$6 $\times$ 10$\sp{16}$ cm. The abundance of SiC$\sb2$ ( (SiC$\sb2$) / (H$\sb2$)) within the shell is $\sim$10$\sp{-6}$. We cannot rule out the possibility that some SiC$\sb2$ originates in the inner envelope near the photosphere but we can put an upper limit on the abundance of SiC$\sb2$ in the inner envelope. Our data constrains the fractional abundance of SiC$\sb2$ in the inner envelope (r $\sbsp{\sim}{<}$ 2 $\times$ 10$\sp{16}$ cm) to be no more than $\sim$3 $\times$ 10$\sp{-8}$. The distribution and abundance of SiC$\sb2$ suggest that ion-molecule reactions involving C$\sb2$H$\sb2$ may be responsible for producing much of the SiC$\sb2$. Our data also places important constraints on the excitation mechanisms for SiC$\sb2$. Kinetic temperatures in the outer CSE ($\sbsp{\sim}{<}$60 K) where much of the SiC$\sb2$ 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 $\nu\sb3$ = 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 SiC$\sb2$ with the NRAO 12-m. Our data suggests that the column density of vibrationally excited SiC$\sb2$ 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 SiC$\sb2$ is consistent with radiative excitation through the $\nu\sb3$ = 1 antisymmetric mode. Made available in DSpace on 2011-05-07T12:03:24Z (GMT). No. of bitstreams: 2 license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5) 9512370.pdf: 8774793 bytes, checksum: 2b3a9f9f6a90139904475d413cca4b88 (MD5) Previous issue date: 1994 Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding (hding2@illinois.edu) on 2011-05-07T14:36:04Z Item is restricted indefinitely. Restriction data tranferred 2014-07-01T11:14:27-05:00 Original Data Group with Access UIUC Users [automated] Release Date: none Reason: ETDs are only available to UIUC Users without author permission ETDs are only available to UIUC Users without author permission U of I Only AAI9512370 (UMI)AAI9512370 http://hdl.handle.net/2142/19307 eng Copyright 1994 Gensheimer, Paul David Physics, Astronomy and Astrophysics A millimeter-wave interferometric study of gas-phase silicon dicarbide in the circumstellar envelope surrounding IRC+10216 text Astronomy Astronomy University of Illinois at Urbana-Champaign Dissertation Ph.D.