On the density regime probed by HCN emission

Author(s): Gerwyn H. Jones, Paul C. Clark, Simon C. O. Glover, Alvaro Hacar
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.
Organisation(s): Department of Astrophysics
Journal: Monthly Notices of the Royal Astronomical Society
Volume: 520
Pages: 1005-1021
No. of pages: 17
ISSN: 0035-8711
DOI: https://doi.org/10.48550/arXiv.2112.05543
Publication date: 03-2023
Peer reviewed: Yes
Austrian Fields of Science 2012: 103004 Astrophysics, 103003 Astronomy
Keywords
Portal url: https://ucrisportal.univie.ac.at/en/publications/d0f618f4-073f-4925-8250-56302e8732c9

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