The Transition from Diuse Molecular Gas to Molecular Cloud Material in Taurus

We study four lines of sight that probe the transition from diuse molecular gas tomolecular cloud material in Taurus. Measurements of atomic and molecular absorption are used to infer the distribution of species and the physical conditions in the material toward stars behind the Taurus Molecular Cloud. New high-resolution spectra at visible and near infrared wavelengths of interstellar Ca II, Ca I, K I, CH, CH+, C2, CN, and CO toward HD 28975 and HD 29647 are combined with data at visible wavelengths and published results on CO from ultraviolet measurements for HD 27778 and HD 30122. Gas densities and temperatures are inferred from analyses of C2, CN, and CO excitation and of the chemistry leading to CN. Our results for HD 29647 are noteworthy in that the CO column density is 1018 cm􀀀2 and that analysis of C2 and CO excitation reveals a temperature of 10 K and density about 1000 cm􀀀3, more like conditions found in dark molecular clouds. Similar results arise from our chemical analysis leading to CN through reactions involving the observed species CH, C2, and NH. Enhanced potassiumdepletion and a much reduced CH/H2 column density ratio also suggest the presence of a dark cloud. The directions toward HD 27778 and HD 30122 probe material typical of molecule-rich diuse clouds, which can be considered CO-dark gas, while the sight line toward HD 28975 represents an intermediate case. Maps of dust temperature help refine the description of the material along the four sight lines and provide an estimate of the distance between HD 29647 and a core in the Taurus Molecular Cloud. An Appendix provides results for the sight line toward HD 26571 based on available data; this star also probes diuse molecular gas.