CO line ratios in molecular clouds: the impact of environment

TL;DR

This study models CO emission from simulated giant molecular clouds to understand how environmental factors like radiation and cosmic rays influence line ratios and the accuracy of H2 mass estimates.

Contribution

It systematically investigates the impact of various environmental parameters on CO line ratios using SPH simulations with chemistry and radiative transfer.

Findings

Line ratios vary by up to ±0.3 dex with environmental changes.

ISRF and CRIR significantly affect CO abundance and temperature.

Standard CO-to-H2 conversion methods can underestimate molecular gas mass by factors of 2-3.

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 (SPH) 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 ionisation rate (CRIR) -- were systematically varied. The simulations were then post-processed using radiative transfer to produce synthetic emission maps in the 3 transitions quoted above. We find that the cloud-averaged values of the line ratios can vary by up to $\pm 0.3$ 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.