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
Journal article, 2021

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 (Tex > 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.

Methods: data analysis

Radiative transfer

ISM: clouds

ISM: molecules

Methods: statistical

Author

Antoine Roueff

Institut Fresnel

M. Gerin

Paris Observatory

P. Gratier

University of Bordeaux

F. Levrier

Ecole Normale Superieure (ENS)

J. Pety

Institut de Radioastronomie Millimétrique (IRAM)

Paris Observatory

Mathilde Gaudel

Paris Observatory

J.R. Goicoechea

CSIC - Instituto de Fisica Fundamental (IFF)

Jan Orkisz

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics, Galactic Astrophysics

Victor De Souza Magalhaes

Institut de Radioastronomie Millimétrique (IRAM)

Maxime Vono

University of Toulouse

Sébastien Bardeau

Institut de Radioastronomie Millimétrique (IRAM)

E. Bron

Paris Observatory

Jocelyn Chanussot

Université Grenoble Alpes

Pierre Chainais

University of Lille

Viviana Guzman

Pontificia Universidad Catolica de Chile

Annie Hughes

Universite Paul Sabatier Toulouse III

Jouni Kainulainen

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics, Galactic Astrophysics

David Languignon

Paris Observatory

Jacques Le Bourlot

Paris Observatory

Franck Le Petit

Paris Observatory

Harvey Liszt

National Radio Astronomy Observatory

Antoine Marchal

Canadian Institute for Theoretical Astrophysics

Marc Antoine Miville-Deschênes

University Paris-Saclay

Nicolas Peretto

Cardiff University

Evelyne Roueff

Paris Observatory

Albrecht Sievers

Institut de Radioastronomie Millimétrique (IRAM)

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 645 A26

Subject Categories

Astronomy, Astrophysics and Cosmology

Atom and Molecular Physics and Optics

Probability Theory and Statistics

DOI

10.1051/0004-6361/202037776

More information

Latest update

1/21/2021