2020ApJ...899...23H

We present observations of velocity-resolved [C II] 158 µm emission from both a dense and a more diffuse photodissociation region (PDR) in the Perseus giant molecular cloud using the Heterodyne Instrument for the Far-Infrared on board the Herschel Space Telescope. We detect [C II] emission from 80% of the total positions, with a 95% detection rate from the dense boundary region. The integrated intensity of the [C II] emission remains relatively constant across each boundary, despite the observed range in optical extinction between 1 and 10 mag. This flat profile indicates a constant heating and cooling rate within both regions observed. The integrated intensity of [C II] emission is reasonably well correlated with the neutral hydrogen (H I) column density, as well as the total gas column density. This, in addition to the 80' (7 pc) extent of the [C II] emission from the cloud center, suggests that the H I envelope plays a dominant role in explaining the [C II] emission emanating from Perseus. We compare the [C II] and ¹²CO integrated intensities with predictions from a 1D, two-sided slab PDR model and show that a simple core + envelope, equilibrium model without an additional "CO-dark" H₂ component can reproduce observations well. Additional observations are needed to disentangle how much of the [C II] emission is associated with the "CO-dark" H₂ gas, as well as constrain spatial variations of the dust-to-gas ratio across Perseus.