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}}}_{{{H}}₂}/{A}_{{V}} ≈ 1.0 × 10²¹ cm^-2, {{{N}}}_{CO}/{A}_{{V}} ≈ 1.5 × 10¹⁷ cm^-2 (1.8 × 10¹⁷ including solid CO), and {{{N}}}_{{{H}}₂}/{{{N}}}_{CO} ≈ 6000. The measured {{{N}}}_{{{H}}₂}/{{{N}}}_{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}}}_{CO}/{A}_{{V}} ratio.