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 $\mathrm{\sim 10^4\ -\ 10^5\
cm^{-3}}$. 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
$\mathrm{\sim 3 \times 10^3\ cm^{-3}}$ and a median visual extinction of
$\mathrm{\sim 5\ mag}$. We therefore predict a characteristic density that is
an order of magnitude less than the "standard" characteristic density of
$\mathrm{n \sim 3 \times 10^4\ cm^{-3}}$. 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 luminosity-to-mass conversion
factors for the amount of gas at $A_{\rm V} > 8$ or at densities $n > 2.85
\times 10^{3} \: {\rm cm^{-3}}$ or $n > 3 \times 10^{4} \: {\rm cm^{-3}}$,
finding values of $\alpha_{\rm HCN} = 6.79, 8.62$ and $27.98 \: {\rm M_{\odot}}
({\rm K \, km \, s^{-1} \, pc^{2}})$, respectively. In some cases, the
luminosity to mass conversion factor predicted mass in regions where in
actuality there contains no mass.
