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
Artikel i vetenskaplig tidskrift, 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
Författare
Antoine Roueff
Institut Fresnel
M. Gerin
Observatoire de Paris
P. Gratier
Université de Bordeaux
F. Levrier
Ecole Normale Superieure (ENS)
J. Pety
Institut de Radioastronomie Millimétrique (IRAM)
Observatoire de Paris
Mathilde Gaudel
Observatoire de Paris
J.R. Goicoechea
CSIC - Instituto de Fisica Fundamental (IFF)
Jan Orkisz
Chalmers, Rymd-, geo- och miljövetenskap, Astronomi och plasmafysik
Victor De Souza Magalhaes
Institut de Radioastronomie Millimétrique (IRAM)
Maxime Vono
Université de Toulouse
Sébastien Bardeau
Institut de Radioastronomie Millimétrique (IRAM)
E. Bron
Observatoire de Paris
Jocelyn Chanussot
Université Grenoble Alpes
Pierre Chainais
Université de Lille
Viviana Guzman
Pontificia Universidad Catolica de Chile
Annie Hughes
Universite Paul Sabatier Toulouse III
Jouni Kainulainen
Chalmers, Rymd-, geo- och miljövetenskap, Astronomi och plasmafysik
David Languignon
Observatoire de Paris
Jacques Le Bourlot
Observatoire de Paris
Franck Le Petit
Observatoire de Paris
Harvey Liszt
National Radio Astronomy Observatory
Antoine Marchal
Institut canadien d'astrophysique théorique
Marc Antoine Miville-Deschênes
Université Paris-Saclay
Nicolas Peretto
Cardiff University
Evelyne Roueff
Observatoire de Paris
Albrecht Sievers
Institut de Radioastronomie Millimétrique (IRAM)
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 645 A26Ämneskategorier
Astronomi, astrofysik och kosmologi
Atom- och molekylfysik och optik
Sannolikhetsteori och statistik
DOI
10.1051/0004-6361/202037776