CO line ratios in molecular clouds: the impact of environment

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 timedependent 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 ±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.

On the diagnostic power of FIR/sub-mm SED fitting in massive galactic molecular clumps

“…The behavior of dA-Type clumps seem to support the simulations of Peñaloza et al (2018) (hereafter PCGK) that demonstrate the importance of external radiation sources (the local ISRF and cosmic rays) in determining the as density needed for CO to self-shield. Over the range of typi-cal gas number densities calculated for the CHaMP clumps in Paper I, 10 8 n 10 10 m −3 , PCGK show the fraction of the carbon in CO should drop about three orders of magnitude, which covers the full range of [CO/H 2 ] we see, for ISRF fields comparable in strength to the parts of the CNC outside the Tr14 and Tr16 PDRs.…”

“…Oberg et al (2009), Muñoz Caro et al (2010, and Muñoz Caro et al (2016) showed that CO ice desorbs under UV irradiation well below the lowest temperature at which thermal desorption is seen-15 to 23 K-and that both thermal and UV photodesorption rates at a given temperature are anti-correlated with the temperature of the CO at deposition up to about 30 K. Muñoz Caro et al (2016) and Cazaux et al (2017) show the amount of gas-phase CO released via thermal desorption increases by about three orders of magnitude starting between 15 and 23 K out to a peak between 27 and 30 K at typical dense molecular gas densities (10 9 to 10 10 m −3 ). Simulations by Peñaloza et al (2018) show that strong interstellar radiation fields (ISRFs) or cosmic rays can effect the same range of CO abundances over the same range of densities across sub-parsec-scale clumps. Clearly the CO abundance must be mapped, and its environmental dependencies pinned down, to have any hope of knowing a molecular cloud's mass budget to better than back-of-theenvelope precision.…”

Systematic Variations of CO J = 2−1/1–0 Ratio and Their Implications in The Nearby Barred Spiral Galaxy M83

“…( Figure 3(c)), and, presumably, the CRs follow a similar trend spatially and in flux. With numerical simulations, Peñaloza et al (2017Peñaloza et al ( , 2018, and their series of papers, showed that R 21 10 is elevated in a stronger ISRF and/or at higher CR densities.…”

Challenges and Techniques for Simulating Line Emission

“…As mentioned above, CLOUDY and DESPOTIC are restricted to problems in 1D whereas the remaining tools in Table 2 work in 3D. LIME, MOLLIE, and RADMC-3D do exact radiative transfer in 3D and are typically used on smaller scales where non-symmetric features such as filaments and turbulence become important [e.g 61,62]. ART 2 also does radiative transfer in 3D including the continuum from far-UV to radio wavelengths and incorporating the resonant line Lyα [e.g 29,63,64] (an updated version of the code including CO and some prominent far-infrared fine-structure lines such as CII, OI, OIII and NII will be out later this year).…”