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Published online by Cambridge University Press: 09 June 2023
We examine the physical conditions required for the formation of H2 in the solar neighborhood by comparing H i emission and absorption spectra toward 58 lines of sight at b < −5∘ to CO(1–0) and dust data. Our analysis of CO-associated cold and warm neutral medium (CNM and WNM) shows that the formation of CO-traced molecular gas is favored in regions with high column densities where the CNM becomes colder and more abundant. In addition, our comparison to the one-dimensional steady-state H i-to-H2 transition model of Bialy et al. (2016) suggests that only a small fraction of the clumpy CNM participates in the formation of CO-traced molecular gas. Another possible interpretation would be that missing physical and chemical processes in the model could play an important role in H2 formation.