2021ApJ...912..116Z

Molecular hydrogen (H₂) is the dominant molecular species in the vast majority of interstellar environments and it plays a crucial role as a radiative coolant. In photodissociation regions (PDRs), it is one of the primary emitters in the near- to mid-infrared, which is due to lines originating from highly excited levels. The sparseness of H₂ collisional data for rotational levels J >= 9, particularly for H₂-H₂ collisions, has limited nonlocal thermal equilibrium (NLTE) studies in ultraviolet-irradiated regions. Utilizing new calculations for para- and ortho-H₂ high rotational collisional rate coefficients colliding with H₂ (up to the maximum value for v = 0: J = 31), existing data for H₂-H and H₂-He collisions, and approximate scaling relations for missing collisional data, we investigate the excitation properties of H₂ in a range of astrophysical environments, with the focus on PDRs (including benchmark PDR models and the Orion Bar). In these NLTE models, H₂ emission is illustrated and shown as a diagnostic for physical conditions, such as temperature and density. Furthermore, we demonstrated the effect of updates in the collisional rates on the modeling results of H₂ excitation. The resulting data sets of H₂ collisional data should find wide application to other molecular environments.