On the density regime probed by HCN emission

TL;DR

This study uses high-resolution simulations to show that HCN emission traces less dense gas than traditionally assumed, challenging its use as a dense gas tracer in star formation research.

Contribution

It demonstrates that HCN emission predominantly traces gas with lower densities and visual extinctions than previously thought, providing revised conversion factors and insights into molecular cloud diagnostics.

Findings

HCN traces gas with mean density ~3x10^3 cm^-3

Characteristic density for HCN emission is an order of magnitude lower than standard

Luminosity-to-mass conversion factors vary with density thresholds

Abstract

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.