2014/03/31 by László Szücs, László Szűcs, Simon C. O. Glover +1
Earth and Planetary Sciences · Physics and Astronomy · #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Atmospheric Ozone and Climate #Atomic and Molecular Physics #Mechanics #Molecular cloud #Physics #Stars #Turbulence #astro-ph.GA
paper · pdf · doi:10.1093/mnras/stu2013
published as Monthly Notices of the Royal Astronomical Society, Volume 445, Issue 4, 2014, p.4055-4072 · 20 pages, 16 figures. Accepted for publication in MNRAS. Treatment of CO shielding was improved after referee feedback. No qualitative changes in results and conclusions, but the numerical values of the fitting formulae were updated
arxiv created 2014/09/24 · openalex publication_date 2014/11/03 · arxiv updated 2016/04/25 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
The 13CO molecule is often used as a column density tracer in regions where the 12CO emission saturates. The 13CO column density is then related to that of 12CO by a uniform isotopic ratio. A similar approximation is frequently used when deriving 13CO emission maps from numerical simulations of molecular clouds. To test this assumption, we calculate the 12CO/13CO ratio self-consistently, taking the isotope-selective photodissociation and the chemical fractionation of CO into account. We model the coupled chemical, thermal and dynamical evolution and the emergent 13CO emission of isolated, starless molecular clouds in various environments. Selective photodissociation has a minimal effect on the ratio, while the chemical fractionation causes a factor of 2–3 decrease at intermediate cloud depths. The variation correlates with both the 12CO and the 13CO column densities. Neglecting the depth dependence results in ≤60 per cent error in 12CO column densities derived from 13CO. The same assumption causes ≤50 per cent disparity in the 13CO emission derived from simulated clouds. We show that the discrepancies can be corrected by a fitting formula. The formula is consistent with millimetre-wavelength isotopic ratio measurements of dense molecular clouds, but underestimates the ratios from the ultraviolet absorption of diffuse regions.