H{sub 2}, CO, and Dust Absorption through Cold Molecular Clouds

The abundance of H{sub 2} 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 H{sub 2}, 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{sub H{sub 2}}/A{sub V} ≈ 1.0 × 10{sup 21} cm{sup −2}, N{sub CO}/A{sub V} ≈ 1.5 × 10{sup 17} cm{sup −2} (1.8 × 10{sup 17} including solid CO), and N{sub H{sub 2}}/N{sub CO} ≈ 6000. The measured N{sub H{sub 2}}/N{sub CO} 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 N{sub CO}/A{sub V} ratio.