H2, CO, and Dust Absorption through Cold Molecular Clouds

The abundance of H2 in molecular clouds, relative to the commonly used tracer CO, has only been measured toward a few embedded stars, which may be surrounded by atypical gas. We present observations of near-infrared absorption by H2, CO, and dust toward stars behind molecular clouds, providing a representative sample of these molecules in cold molecular gas, primarily in the Taurus Molecular Cloud. We find N_H2/A_V ~ 1.0x10^21 cm^-2, N_CO/A_V ~ 1.5x10^17 cm^-2 (1.8x10^17 including solid CO), and N_H2/N_CO ~ 6000. The measured N_H2/N_CO ratio is consistent with that toward embedded stars in various molecular clouds, but both are less than that derived from mm-wave observations of CO and star counts. The difference apparently results from the higher directly measured N_CO/A_V ratio.