Properties Of Highly Rotationally Excited H-2 In Photodissociation Regions

Molecular hydrogen (H-2) 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 H-2 collisional data for rotational levels J >= 9, particularly for H-2-H-2 collisions, has limited nonlocal thermal equilibrium (NLTE) studies in ultraviolet-irradiated regions. Utilizing new calculations for para- and ortho-H-2 high rotational collisional rate coefficients colliding with H-2 (up to the maximum value for v = 0: J = 31), existing data for H-2-H and H-2-He collisions, and approximate scaling relations for missing collisional data, we investigate the excitation properties of H-2 in a range of astrophysical environments, with the focus on PDRs (including benchmark PDR models and the Orion Bar). In these NLTE models, H-2 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 H-2 excitation. The resulting data sets of H-2 collisional data should find wide application to other molecular environments.