2017ApJ...838...66L

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_/AV_ ≃ 1.0 x 10²¹ cm^–2, N_CO/A_V ≃ 1.5 x 10¹⁷ cm^–2 (1.8 x 10¹⁷ including solid CO), and N_H2_/NCO_ ≃ 6000. The measured N_H2_/NCO_ 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.