CO and C-2 absorption toward W40IRS 1a

The H II region W40 harbors a small group of young, hot stars behind roughl y 9 mag of visual extinction. We have detected gaseous carbon monoxide (CO) and diatomic carbon (C-2) in absorption toward the star W40 IRS 1a. The J = 2-0 R0, R1, and R2 lines of (CO)-C-12 at 2.3 mu m were measured using the CSHELL on the NASA Infrared Telescope Facility (with upper limits placed o n R3, R4, and R5) yielding an N-CO of (1.1 +/- 0.2) x 10(18) cm(-2). Excita tion analysis indicates T-kin > 7 K. The Phillips system of C-2 transitions near 8775 Angstrom was measured using the Kitt Peak 4 m telescope and eche lle spectrometer. Radiative pumping models indicate a total C-2 column dens ity of (7.0 +/- 0.4) x 10(14) cm(-2), two excitation temperatures (39 and 1 26 K), and a total gas density of n similar to 250 cm(-3). The CO ice band at 4.7 mu m was not detected, placing an upper limit on the CO depletion of delta < 1%. We postulate that the sight line has multiple translucent comp onents and is associated with the W40 molecular cloud. Our data for W40 IRS 1a, coupled with other sight lines, shows that the ratio of CO/C-2 increas es from diffuse through translucent environs. Finally, we show that the hyd rogen-to-dust ratio seems to remain constant from diffuse to dense environm ents, whereas the CO-to-dust ratio apparently does not.