Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 475, Issue 2, Pages 1508-1520Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stx3263
Keywords
stars: formation; ISM: clouds; ISM: molecules
Categories
Funding
- Science and Technology Facilities Council [ST/N00706/1]
- European Community's Horizon Programme H-COMPET through the StarFormMapper project [687528]
- Deutsche Forschungsge-meinschaft [SFB 881]
- European Research Council under the European Community's Seventh Framework Programme (FP7) via the ERC Advanced Grant STARLIGHT [339177]
- STFC [ST/N000706/1] Funding Source: UKRI
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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 H-2 column density can underestimate the total H-2 molecular gas in GMCs by factors of 2 or 3, depending on the environmental conditions in the clouds.
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