C¹⁸O, ¹³CO, and ¹²CO abundances and excitation temperatures in the Orion B molecular cloud

Analysis of the achievable precision in modeling spectral lines within the approximation of the local thermodynamic equilibrium

¹ Aix Marseille Univ., CNRS, Centrale Marseille, Institut Fresnel, Marseille, France
e-mail: antoine.roueff@fresnel.fr
² LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 75014 Paris, France
³ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, Allee Geoffroy Saint-Hilaire, 33615 Pessac, France
⁴ Laboratoire de Physique de l’Ecole normale supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université de Paris, Sorbonne Paris Cité, Paris, France
⁵ IRAM, 300 rue de la Piscine, 38406 Saint Martin d’Hères, France
⁶ Instituto de Física Fundamental (CSIC). Calle Serrano 121, 28006 Madrid, Spain
⁷ Chalmers University of Technology, Department of Space, Earth and Environment, 412 93 Gothenburg, Sweden
⁸ University of Toulouse, IRIT/INP-ENSEEIHT, CNRS, 2 rue Charles Camichel, BP 7122, 31071 Toulouse Cedex 7, France
⁹ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 92190 Meudon, France
¹⁰ Univ. Grenoble Alpes, Inria, CNRS, Grenoble INP, GIPSA-Lab, 38000 Grenoble, France
¹¹ Univ. Lille, CNRS, Centrale Lille, UMR 9189 – CRIStAL, 59651 Villeneuve d’Ascq, France
¹³ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹³ Institut de Recherche en Astrophysique et Planétologie (IRAP), Université Paul Sabatier, Toulouse Cedex 4, France
¹⁴ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹⁵ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 Saint George Street, 14th floor, Toronto, ON, M5S 3H8, Canada
¹⁶ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
¹⁷ School of Physics and Astronomy, Cardiff University, Queen’s buildings, Cardiff CF24 3AA, UK

Received: 19 February 2020
Accepted: 28 April 2020

Abstract

Context. CO isotopologue transitions are routinely observed in molecular clouds for the purpose of probing the column density of the gas and the elemental ratios of carbon and oxygen, in addition to tracing the kinematics of the environment.

Aims. Our study is aimed 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, which includes IC 434, NGC 2023, and the Horsehead pillar.

Methods. We used the Cramer Rao bound (CRB) technique to analyze and estimate the precision of the physical parameters in the framework of local-thermodynamic-equilibrium (LTE) excitation and radiative transfer with added 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 proves efficient for a common range of excitation temperatures and column densities (T_ex > 6 K, N > 10¹⁴−10¹⁵  cm⁻²).

Results. Contrary to general assumptions, the various 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 the 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.

Conclusions. The CRB technique is a promising avenue for analyzing the estimation of physical parameters from the fit of spectral lines. Future works will generalize its application to non-LTE excitation and radiative transfer methods.