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CO line ratios in molecular clouds: the impact of environment

Penaloza Cabrera, Camilo, Clark, Paul, Glover, Simon C O and Klessen, Ralf S 2018. CO line ratios in molecular clouds: the impact of environment. Monthly Notices of the Royal Astronomical Society 475 (2) , pp. 1508-1520. 10.1093/mnras/stx3263

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Abstract

Line emission is strongly dependent on the local environmental conditions in which the emitting tracers reside. In this work, we focus on modelling the CO emission from simulated giant molecular clouds (GMCs), and study the variations in the resulting line ratios arising from the emission from the J = 1–0, J = 2–1, and J = 3–2 transitions. We perform a set of smoothed particle hydrodynamics simulations with time-dependent chemistry, in which environmental conditions – including total cloud mass, density, size, velocity dispersion, metallicity, interstellar radiation field (ISRF), and the cosmic ray ionization rate (CRIR) – were systematically varied. The simulations were then post-processed using radiative transfer to produce synthetic emission maps in the three transitions quoted above. We find that the cloud-averaged values of the line ratios can vary by up to ±0.3 dex, triggered by changes in the environmental conditions. Changes in the ISRF and/or in the CRIR have the largest impact on line ratios since they directly affect the abundance, temperature, and distribution of CO-rich gas within the clouds. We show that the standard methods used to convert CO emission to H2 column density can underestimate the total H2 molecular gas in GMCs by factors of 2 or 3, depending on the environmental conditions in the clouds.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Physics and Astronomy
Subjects: Q Science > QB Astronomy
Publisher: Oxford University Press
ISSN: 0035-8711
Date of First Compliant Deposit: 15 February 2018
Date of Acceptance: 12 December 2017
Last Modified: 15 Feb 2018 10:36
URI: http://orca.cf.ac.uk/id/eprint/109135

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