Properties of Highly Rotationally Excited H2 in Photodissociation Regions

Abstract

Abstract 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.