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C18O, 13CO, and 12CO abundances and excitation temperatures in the Orion B molecular cloud: An analysis of the precision achievable when modeling spectral line within the Local Thermodynamic Equilibrium approximation

Antoine Roueff 1 Maryvonne Gerin 2 Pierre Gratier 3 François Levrier 4 Jérôme Pety 5, 6 Mathilde Gaudel 7 Javier Goicoechea 8 Jan Orkisz 9 Victor de Souza Magalhaes 5 Maxime Vono 10 Sébastien Bardeau 5 Emeric Bron 2 Jocelyn Chanussot 11 Pierre Chainais 12 Viviana Guzman 13 Annie Hughes 14 Jouni Kainulainen 15 David Languignon 7 Jacques Le Bourlot 2 Franck Le Petit 2 Harvey Liszt 16 Antoine Marchal 17 Marc-Antoine Miville-Deschênes 18 Nicolas Peretto 19 Evelyne Roueff 2 Albrecht Sievers 20
Abstract : CO isotopologue transitions are routinely observed in molecular clouds to probe the column density of the gas, the elemental ratios of carbon and oxygen, and to trace the kinematics of the environment. We aim at estimating the abundances, excitation temperatures, velocity field and velocity dispersions of the three main CO isotopologues towards a subset of the Orion B molecular cloud. We use the Cramer Rao Bound (CRB) technique to analyze and estimate the precision of the physical parameters in the framework of local-thermodynamic-equilibrium excitation and radiative transfer with an additive white Gaussian noise. We propose a maximum likelihood estimator to infer the physical conditions from the 1-0 and 2-1 transitions of CO isotopologues. Simulations show that this estimator is unbiased and efficient for a common range of excitation temperatures and column densities (Tex > 6 K, N > 1e14 - 1e15 cm-2). Contrary to the general assumptions, the different CO isotopologues have distinct excitation temperatures, and the line intensity ratios between different isotopologues do not accurately reflect the column density ratios. We find mean fractional abundances that are consistent with previous determinations towards other molecular clouds. However, significant local deviations are inferred, not only in regions exposed to UV radiation field but also in shielded regions. These deviations result from the competition between selective photodissociation, chemical fractionation, and depletion on grain surfaces. We observe that the velocity dispersion of the C18O emission is 10% smaller than that of 13CO. The substantial gain resulting from the simultaneous analysis of two different rotational transitions of the same species is rigorously quantified. The CRB technique is a promising avenue for analyzing the estimation of physical parameters from the fit of spectral lines.
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https://hal.archives-ouvertes.fr/hal-02570214
Contributor : Jérôme Pety <>
Submitted on : Wednesday, May 13, 2020 - 1:28:21 PM
Last modification on : Thursday, July 30, 2020 - 3:49:24 AM

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  • HAL Id : hal-02570214, version 1

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Antoine Roueff, Maryvonne Gerin, Pierre Gratier, François Levrier, Jérôme Pety, et al.. C18O, 13CO, and 12CO abundances and excitation temperatures in the Orion B molecular cloud: An analysis of the precision achievable when modeling spectral line within the Local Thermodynamic Equilibrium approximation. 2020. ⟨hal-02570214⟩

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