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Spectral Lines
Published in Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr, Fundamentals of Radio Astronomy, 2019
Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr
Carbon monoxide is unique in having a low critical density and large abundance; therefore we can usually infer the gas kinetic temperature because the line center is likely to be optically thick and its brightness temperature will equal the gas kinetic temperature. However, for many molecular lines, low optical depth can be assumed. For example, since the rarer isotopes of carbon and oxygen are far less abundant, the lines from isotopologues of CO, such as 13CO, are usually optically thin. IsotopologueMolecules!CO!isotopologues Emission lines from these isotopologues are useful in determining the column density of that molecule. Starting with Equation 4.12 and converting intensity and frequency to brightness temperature and velocity, we have ∫TBdv=hc38πkν2NuAul.Column density!moleculesMolecules!column densityEquation 4.37, though, yields a measure only of the column density of the molecules in the upper state of that transition. If, for example, one measured the J = 1 − 0 transition of 13CO, the integrated intensity would directly give the column density of 13CO in the J = 1 rotational level.
Spectroscopy of astrophysically relevant ions in traps
Published in Molecular Physics, 2020
Absolute fragmentation cross-sections were obtained by irradiation of as a function of laser fluence, Φ, and typical data are shown in Figure 4. The unity quantum yield for dissociation leads to identical values for absorption, and the result inferred for is at [39]. This enabled the determination of the column density in interstellar environments without the need to rely on theoretical oscillator strengths. is related to the absorption cross-section through the simple expression, . In observations toward , an intensity ratio of 0.91 at was measured [77]. Taking this value, together with the experimentally determined absorption cross-section, leads to . To put this result in context, it is instructive to compare with other known species in these interstellar environments. It turns out that this value is similar to the column densities of diatomics such as observed along the same line-of-sight [83].