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 ≈ 1.0 × 1021 cm−2, ≈ 1.5 × 1017 cm−2 (1.8 × 1017 including solid CO), and ≈ 6000. The measured ratio is consistent with that toward embedded stars in various molecular clouds, but both are less than that derived from millimeter-wave observations of CO and star counts. The difference apparently results from the higher directly measured ratio. [ABSTRACT FROM AUTHOR]