2023ApJ...944..197D

We present a survey of the Perseus molecular cloud in the J = 1 → 0 transition of HCN, a widely used tracer of dense molecular gas. The survey was conducted with the CfA 1.2 m telescope, which at 89 GHz has a beamwidth of 11' and a spectral resolution of 0.85 km s^–1. A total of 8.1 deg² was surveyed on a uniform 10' grid to a sensitivity of 14 mK per channel. We compared the survey with similar surveys of CO and dust in order to study and calibrate the HCN line as a dense-gas tracer. We find the HCN emission to extend over a considerable fraction of the cloud. We show that the HCN intensity remains linear with H₂ column density well into the regime where the CO line saturates. We use radiative-transfer modeling to show that this likely results from subthermal excitation of HCN in a cloud where the column and volume densities of H₂ are positively correlated. To match our HCN observations the model requires an exponential decrease in HCN abundance with increasing extinction, consistent with HCN depletion onto grains. The modeling also reveals that the mean volume density of H₂ in the HCN-emitting regions is ∼10⁴ cm^–3, well below the HCN critical density. For the first time, we obtain a direct measurement of the ratio of dense-gas mass to HCN luminosity for an entire nearby molecular cloud: α(HCN) = 92 M_⊙/(K km s^–1 pc²).