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F. Freund (Seti Institute/NASA Ames), M.M. Freund (NASA/GSFC), A. Staple (Stanford Univ.), J. Scoville (U. of Kentucky)
Dust in the ISM carries an ``organic" signature in form of a distinct group of CH bands, both in emission and absorption, around 3.4\micron (2800-3000 cm-1). These bands agree with the symmetrical and asymmetrical C-H stretching vibrations of aliphatic CH2 entities and are thought to be associated with organic molecules on the surface of dust grains. We show that this interpretation is inconsistent with laboratory experiments. Synthetic MgO and natural olivine single crystals, grown from a CO/CO2/H2O-saturated melt, exhibit the same CH bands, but those bands are clearly associated with C-H entities inside the dense mineral matrix. The multitude of CH bands indicates that the C-H bonds arise from polyatomic Cn entities. We heated the MgO and olivine crystals to temperatures between 550-1000K to pyrolyze the C-H bonds and to cause the CH bands to disappear. Upon annealing at moderate temperatures between 300-390K the CH bands reappear within a few days to weeks. The CH intensity increases linearly with the square root of time. Thus the pyrolysis broke the C-H bonds and caused the H to disperse in the surrounding mineral matrix, the H atoms (or H2 molecules) are sufficiently mobile to return during annealing and reestablish the same C-H bonds. Dust grains that condense in a gas-laden environment (outflow of late-stage stars or in dense molecular clouds) probably incorporate the same type of Cn-H entities. Imbedded in and in part bonded to the surrounding mineral matrix, the Cn-H entities display CH bands in the 3.4\micron region, but their lower frequency librational modes are so strongly coupled to the lattice modes that they broaden excessively and thus become inobservable.
The author(s) of this abstract have provided an email address for comments about the abstract: mino@stars.gsfc.nasa.gov