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 (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 of13CO. 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.
Highlights
Spectroscopic measurements are commonly used to probe astrophysical objects
With a wide range of physical conditions, from bright far-UV illuminated regions to cold and shielded regions through diffuse and translucent gas irradiated by a moderate radiation field, the Orion B molecular cloud is an ideal place for probing the extent to which fractionation and selective photodissociation can modify the elemental abundance ratio
This paper presents an analysis of the precision of the estimation of physical parameters (∆V, σV, N, Tex) when trying to fit spectra of low J transitions for the most common CO isotopologues using the local thermodynamic equilibrium (LTE) radiative transfer model
Summary
Spectroscopic measurements are commonly used to probe astrophysical objects. In the interstellar medium, the moderate temperatures and densities of diffuse and molecular clouds (Tkin ∼ 10−100 K, and n ∼ 102−105 cm−3, Draine 2011) are wellsuited for the emission in the low-energy rotational lines of molecules such as carbon monoxide, which are accessible at millimeter wavelengths. With a wide range of physical conditions, from bright far-UV illuminated regions to cold and shielded regions through diffuse and translucent gas irradiated by a moderate radiation field, the Orion B molecular cloud is an ideal place for probing the extent to which fractionation and selective photodissociation can modify the elemental abundance ratio. It is a good region for probing the differences in excitation between isotopologues as the simple hypothesis of equal excitation temperatures for 12CO, 13CO, and C18O may not be valid, as discussed in Bron et al (2018). We only used the spectra denoised by ROHSA to provide a spatially coherent estimation of the number of components and some initial estimation of their associated central velocities for each pixel
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