unsaturated hydrocarbons-such as aromatics, alkynes, and simple aliphatic 

 ketones-are not dominant in interstellar grain mantles along the line of sight 

 toward this object. The 5- to 8-jum spectra of other objects do suggest the 

 presence of such molecules in the grain mantles in those regions. 



t/i 



max 



Figure 3-6. The 5- to 8-micrometer spectrum of the bright, protostellar object 

 W33A obtained with the KAO. 



Many celestial objects show infrared emission features at 3.3, 3.4, 6.2, 7.7, 

 8.6, and 11.3 jum. Some researchers have attributed these features to ultraviolet- 

 pumped infrared fluorescence from a collection of PAHs. In particular, the 3.3- 

 and 6.2-jum features point strongly toward fundamental vibrations in aromatic 

 ring molecules. A particularly striking Raman spectrum from a collection of such 

 molecules (auto exhaust) is compared in figure 3-7 with the 5- to 10-jum spec- 

 trum of the emission features in the Orion bar. The close agreement between the 

 Raman spectrum and the interstellar spectrum is strong circumstantial evidence 

 that they arise from similar groups of species, since Raman-active and infrared- 

 active modes are similar in frequency and number in these molecules. The 

 number of carbon atoms in the emitting molecules, derived from the observa- 

 tions, is about 50. The abundance of these species is estimated to be about 

 2X1 0" 7 of that of hydrogen. If this interpretation of the infrared emission 



68 



