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

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