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Understanding the role of HCN in star formation studies

Jones, Gerwyn 2023. Understanding the role of HCN in star formation studies. PhD Thesis, Cardiff University.
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Star formation studies currently rely on the HCN J = 1 - 0 emission since it is thought to trace the same dense gas regions in which star formation occurs (at densities above 104 cm-3). However, new studies show that HCN J = 1 - 0 emission trace densities below 104 cm-3 at densities of around 103 cm-3. In this work, we focus on modelling the HCN emission from simulated molecular clouds and investigate the density regime probed by HCN emission. We model the molecular cloud environments through cloud-cloud collisions using a Voronoi moving-mesh hydrodynamical code called Arepo. These molecular clouds are then post-processed by a radiative transfer code called Radmc-3D that produces synthetic images of HCN emission. 10 simulations in total were made; 4 simulations were produced that varied the initial collisional velocity of the clouds. 6 simulations were produced that varied the interstellar radiation field strength (ISRF). For the variations in the initial collisional cloud velocity, we find HCN emission traces gas with a volumetric density of 2.85 x 103 cm-3 and a visual extinction of 5.05 mag. For the work on the variation of the ISRF strength, we find that HCN traces gas with a density of 4.3 x 103 cm-3 and the visual extinction of 7 mag. These characteristic densities of 2.85 x 103 cm-3 and 4.3 x 103 cm-3 are more consistent with the observational studies that suggest that HCN emission traces more diffuse densities. This finding has implications in observational studies of HCN emission where HCN emission no longer provides evidence of "slow" star formation. Our work on HCN emission has shown that it is not as simple to say that HCN emission traces a certain density or that the effective density of HCN emission can be described by one particular observational property such as the optical depth. We show that the effective density of HCN emission has some correlation with the local ISRF strength of the molecular cloud environment. We suggest that those observing HCN emission in the local Milky-Way or an environment similar to it should use an effective density of 3 x 103 cm-3 with an upper limit of 1 x 104 cm-3 for those observing regions of high star formation

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Physics and Astronomy
Subjects: Q Science > QC Physics
Uncontrolled Keywords: ISM: Molecular clouds: Star formation: HCN
Funders: STFC
Date of First Compliant Deposit: 21 November 2023
Last Modified: 22 Nov 2023 10:32

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