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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. [ABSTRACT FROM AUTHOR]

To date, about two dozen low-mass embedded protostars exhibit rich spectra with lines of complex organic molecules (COMs). These protostars seem to possess a different enrichment in COMs. However, the statistics of COM abundance in low-mass protostars are limited by the scarcity of observations. This study introduces the Perseus ALMA Chemistry Survey (PEACHES), which aims at unbiasedly characterizing the chemistry of COMs toward the embedded (Class 0/I) protostars in the Perseus molecular cloud. Of the 50 embedded protostars surveyed, 58% of them have emission from COMs. 56%, 32%, and 40% of the protostars have CH3OH, CH3OCHO, and N-bearing COMs, respectively. The detectability of COMs depends neither on the averaged continuum brightness temperature, a proxy of the H2 column density, nor on the bolometric luminosity and the bolometric temperature. For the protostars with detected COMs, CH3OH has a tight correlation with CH3CN, spanning more than two orders of magnitude in column densities normalized by the continuum brightness temperature, suggesting a chemical relation between CH3OH and CH3CN and a large chemical diversity in the PEACHES samples at the same time. A similar trend with more scatter is also found between all identified COMs, which hints at a common chemistry for the sources with COMs. The correlation between COMs is insensitive to the protostellar properties, such as the bolometric luminosity and the bolometric temperature. The abundance of larger COMs (CH3OCHO and CH3OCH3) relative to that of smaller COMs (CH3OH and CH3CN) increases with the inferred gas column density, hinting at an efficient production of complex species in denser envelopes. [ABSTRACT FROM AUTHOR]