Infrared molecular spectroscopy toward the Orion IRc2 and IRc7 sources - A new probe of physical conditions and abundances in molecular clouds

Observations of infrared absorption lines of C2H2, HCN, OCS, CO, and NH3 toward the infrared cluster in the Orion molecular cloud are presented. On the basis of the relative line strengths and the detection of several (C-13)CH2 lines, it is contended that the lines are narrow (Gaussian dispersion is about 1 km/s) and saturated. The derived temperature is about 140-150 K, and the column density of C2H2 is 1.4 x 10 exp 17/sq cm toward IRc2, with about half as much C2H2 at a similar temperature toward IRc7. Several lines were observed at 13.5 micron of the nu2 vibrational band of HCN toward IRc2 and IRc7. The observed line strengths toward both sources indicate that HCN is in LTE for rotational levels through J = 14. The temperature is about 150 K, consistent with the temperature derived from C2H2, and the column density of HCN toward IRc2 is about 2.6 x 10 exp 17/sq cm.