Session 6 - HII Regions & Massive Star Formation.
Display session, Wednesday, January 07
Exhibit Hall,

## [6.11] CO and C_2 Absorption Toward W 40

R. Y. Shuping (CASA, Univ. of Colorado), T. P. Snow (CASA, Univ. of Colorado), R. Crutcher (Univ. of Illinois), B. L. Lutz (Northern Arizona Univ.)

The H II region W 40 harbors a small group of young, hot stars behind roughly 10 magnitudes of visual extinction (A_V). These stars are very bright in the near-infrared (IR). Hence, W 40 is one of very few dense cloud sightlines which can be studied with near-IR absorption lines. Gaseous carbon monoxide (CO) and diatomic carbon (C_2) have both been detected in absorption toward the H II region W 40. The 2-0 R0, R1, and R2 lines of ^12CO at 2.3 \mum were measured using the CSHELL on the NASA IR Telescope Facility (with upper limits placed on R3, R4, and R5) yielding an N_CO of roughly 10^18 cm^-2. Boltzmann analysis suggests a rotational temperature for CO of 6.4 \pm 2.0 K which is compared to more sophisticated excitation models. The Phillips system of C_2 transitions near 8775 Å\ was measured using the Kitt Peak 4-m telescope. Ultraviolet (UV) radiative pumping models indicate a total C_2 column density of 10.3 \pm 0.5 \times 10^14 cm^-2 and two excitation temperatures (39 and 126 K). CH was not detected toward W40 in the IR and an upper limit is placed on its abundance. The physics and chemistry of the cloud are discussed in some detail.