SiS in circumstellar shells

The SiS line profiles of the circumstellar molecular shell of the stellar object IRC + 10216 were surveyed in terms of nine transitions from J = 1-0 to J = 16-15. The survey was performed to model the age and physical characteristics of the object and the emission sources. Data were gathered using several radio telescopes at different locations and frequencies from 18.15-290.38 GHz. An abundance ratio of forbidden SiS/forbidden H2 of 24 millionths was detected, with the SiS concentration decreasing with increasing radius. Adsorption by grains is postulated to cause the fast decline in concentration. Some overlap of vibration-rotation lines of SiS with previous IR data at 13.5 microns was noted. Finally, a calculated abundance of CS in IRC + 10216 indicated that Si depletion is the cause of an underabundance of SiS, a condition similar to that seen in planetary nebulae.