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J. D. Monnier (SAO), W. C. Danchi, D. S. Hale (SSL, UC, Berkeley), P. G. Tuthill (Chatterton Astronomy Dept, U. Sydney), C. H. Townes (SSL, UC, Berkeley)
The observed abundances of ammonia and silane around evolved stars are orders of magnitude greater than predicted from chemical equilibrium 'freeze-out' models. It has been suggested that chemical reactions on dust grain surfaces enhance the formation of these polyatomic molecules, which have rovibrational transitions in the mid-IR. A novel, fast-switching filterbank system has been incorporated into the Infrared Spatial Interferometer (ISI) to allow high spectral resolution observations of mid-IR spectral lines, while precisely calibrating atmospheric fluctuations. The high spectral resolving power (R~450000) and spatial resolution (~1/10 arcsecond) allow the molecular stratification to be directly probed.
Surprisingly, ammonia and silane were observed around carbon star IRC +10216 to form beyond both the dust formation and gas acceleration zones, indicating that the mere presence of dust grains is not sufficient to catalyze the production of these molecules. The ammonia lines show evidence for decaying gas turbulence in the molecular formation region (beyond 20 stellar radii), while silane appears to form even further out in a relatively smooth gas flow (beyond 80 stellar radii). For red supergiant VY CMa, ammonia appears to form near the termination of the gas acceleration phase, also in a region of high turbulence (about 40 stellar radii). It is not clear what process is setting the formation time scales, perhaps the slow absorption of SiS onto dust grains as fuel for the silane-producing reactions.
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The author(s) of this abstract have provided an email address for comments about the abstract: jmonnier@cfa.harvard.edu