THE IRC + 10216 CIRCUMSTELLAR ENVELOPE .3. INFRARED MOLECULAR LINE-PROFILES

The 10 mum spectrum (780-1240 cm-1) has been surveyed with a resolution of 0.009 cm-1. Numerous molecular vibration-rotation transitions were detected in absorption by the circumstellar shell. Analysis of the line profiles by means of spectral synthesis from models revealed radial abundance ratio variations in some species. SiO is approximately uniformly mixed through the inner shell. SiH4 and NH3 are apparently produced in the region 10 to 40 R*, possibly by formation on grains. CS is very strongly depleted in the region 100-1000 R*, perhaps onto grains. From nondetection of H-2 the current mass-loss rate is constrained to be less than 4 x 10(-5) M. yr-1. Gathering evidence for a recent decline in the mass-loss rate is strengthened by analysis of the low-excitation heterodyne NH3 line observation, which indicates that the mass-loss rate may have been 2-3 x larger 1000 yr ago (material currently near 700 R*).