On the density regime probed by HCN emission

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