2022ApJ...941...62H

We present new observations of [C II] ²P_3/2 → ²P_1/2 fine structure line emission from an isolated molecular cloud using the upGREAT instrument on board SOFIA. These data are analyzed together with archival CO J=1-0 and H I 21 cm emission spectra to investigate the role of converging atomic gas flows in the formation of molecular clouds. Bright [C II] emission is detected throughout the mapped area that likely originates from photodissociation regions excited by UV radiation fields produced by newborn stars within the cloud. Upon spatial averaging of the [C II] spectra, we identify weak [C II] emission within velocity intervals where the H I 21 cm line is brightest; these are blueshifted relative to velocities of the CO and bright [C II] emission by 4 km s^–1. The brightness temperatures, velocity dispersions, and alignment with H I 21 cm velocities connect this [C II] emission component to the cold, neutral atomic gas of the interstellar medium, known as the cold, neutral medium (CNM). We propose that this CNM feature is an accretion flow onto the farside of the existing molecular cloud. The mass infall rate is 3.2 × 10^–4M_⊙ yr^–1. There is no direct evidence of a comparable redshifted component in the [C II] or H I 21 cm spectral lines that would indicate the presence of a converging flow.