CO and C₂ absorption toward W40 IRS 1a

The H II region W40 harbors a small group of young, hot stars behind roughly 9 mag of visual extinction. We have detected gaseous carbon monoxide (CO) and diatomic carbon (C₂) in absorption toward the star W40 IRS 1a. The J = 2-0 R0, R1, and R2 lines of ¹²CO at 2.3 μm were measured using the CSHELL on the NASA Infrared Telescope Facility (with upper limits placed on R3, R4, and R5) yielding an N_CO of (1.1 ± 0.2) × 10¹⁸ cm^-2. Excitation analysis indicates T_kin > 7 K. The Phillips system of C₂ transitions near 8775 Å was measured using the Kitt Peak 4 m telescope and echelle spectrometer. Radiative pumping models indicate a total C₂ column density of (7.0 ± 0.4) × 10¹⁴ cm^-2, two excitation temperatures (39 and 126 K), and a total gas density of n ∼ 250 cm^-3. The CO ice band at 4.7 μm was not detected, placing an upper limit on the CO depletion of δ < 1%. We postulate that the sight line has multiple translucent components 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/C₂ increases from diffuse through translucent environs. Finally, we show that the hydrogen-to-dust ratio seems to remain constant from diffuse to dense environments, whereas the CO-to-dust ratio apparently does not.