On the density regime probed by HCN emission

HCN $J = 1\,\!-\!0$ emission is commonly used as a dense gas tracer, thought to mainly arise from gas with densities ~10⁴-10⁵ cm^-3. This has made it a popular tracer in star formation studies. However, there is increasing evidence from observational surveys of 'resolved' molecular clouds that HCN can trace more diffuse gas. We investigate the relationship between gas density and HCN emission through post-processing of high-resolution magnetohydrodynamical simulations of cloud-cloud collisions. We find that HCN emission traces gas with a mean volumetric density of ~3 × 10³ cm^-3 and a median visual extinction of ~5 mag. We therefore predict a characteristic density that is an order of magnitude less than the 'standard' characteristic density of n ~3 × 10⁴ cm^-3. Indeed, we find in some cases that there is clear HCN emission from the cloud even though there is no gas denser than this standard critical density. We derive independent luminosity-to-mass conversion factors for the amount of gas at A_V > 8 or at densities n > 2.85 × 10³ cm^-3 or n > 3 × 10⁴ cm^-3, finding values of α_HCN = 6.79, 8.62, and $27.98\,{\rm M_{\odot}} ({\rm K\, km \, s^{-1} \, pc^{2}})$, respectively.