Abstract
This chapter discusses the observed interstellar molecules, their structure, and main formation and destruction mechanisms in interstellar conditions. The hydrogen and carbon monoxide molecules are given particular emphasis, and the chapter ends with a discussion of the molecular abundances in dense clouds.
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Duley, W.W., Williams, D.A.: Interstellar Chemistry. Academic Press, London (1984). Excellent monograph, considering all principal aspects involved in the study of interstellar molecules. Table 10.2 is based on this reference
Dyson, J., Williams, D.A.: The Physics of the Interstellar Medium. Institute of Physics Publishing, London (1997). Referred to in Chapter 1. Discusses interstellar molecules, their role as interstellar cloud coolers, and H2 formation on the surface of the grains
Hartquist, T.W., Williams, D.A.: The Chemically Controlled Cosmos. Cambridge University Press, Cambridge (1995). A good elementary introduction to molecular astrophysics. See also the more advanced text by the same authors, The Molecular Astrophysics of Stars and Galaxies. Oxford, Oxford University Press, 1998 and Hartquist, T.W. (ed.). Molecular Astrophysics. Cambridge, Cambridge University Press, 1990. Tables 10.3 and 10.4 are partially based on this last reference
Herzberg, G.: Molecular Spectra and Molecular Structure I. Spectra of Diatomic Molecules. Van Nostrand Reinhold, New York (1950). Classical and advanced book on molecular spectroscopy. See also Herzberg, G. The Spectra and Structures of Simple Free Radicals. Ithaca, Cornell University Press, 1971
Hollenbach, D.J., Thronson, H.A. (eds.): Interstellar Processes. Reidel, Dordrecht (1987). Referred to in Chapter 6. Excellent ensemble of advanced review articles about several aspects of interstellar medium astrophysics, including interstellar molecules
Scheffler, H., Elsässer, H.: Physics of the Galaxy and Interstellar Matter. Springer, Berlin (1988). Referred to in Chapter 1. Includes a general discussion on interstellar molecules, molecular clouds, and maser emission
Spitzer, L.: Physical Processes in the Interstellar Medium. Wiley, New York (1978). Referred to in Chapter 1. Advanced discussion on some of the aspects relative to interstellar molecules, such as their role in cloud cooling and maser emission
Wynn-Williams, G.: The Fullness of Space. Cambridge University Press, Cambridge (1992). Referred to in Chapter 1. Discusses interstellar molecules, observations, and their formation on the surface of the grains
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Exercises
Exercises
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10.1
A rough expression for the potential energy of a bound state of a diatomic molecule in a certain electron configuration is Morse potential, given by
$$ {E_{\mathrm{ p}}}(r)=D{{\left[ {1-{{\mathrm{ e}}^{{-a(r-{r_0})}}}} \right]}^2}, $$where D, a, and r 0 are constants defined for each molecule. (a) Draw a graph of E p(r) as a function of r, taking D = 4.48 eV, a = 2, and r 0 = 0.74 Å. (b) Show that function E p(r) has a minimum for r = r 0 that is interpreted as the equilibrium separation. (c) Show that E p(r) → D for r → ∞. What happens for r → 0?
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10.2
The equilibrium internuclear separation of molecule CS is 1.535 Å. What is the wavelength of the rotational transition corresponding to J = 1 → 0?
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10.3
An H2 molecule in its ground state level absorbs a photon with wavelength λ = 1,000 Å. After excitation, the molecule dissociates emitting a photon with wavelength λ = 1,700 Å. Assuming that the molecule dissociation energy is D = 4.48 eV, what is the mean kinetic energy of each H atom?
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10.4
Assume that molecular abundances may be calculated from chemical equilibrium. In this case, equilibrium between the most abundant molecules, H2 and CO, and molecules CH4 and H2O may be written as
$$ \mathrm{ C}\mathrm{ O} + 3{{\mathrm{ H}}_2}\to \mathrm{ C}{{\mathrm{ H}}_4}+{{\mathrm{ H}}_2}\mathrm{ O} $$Defining the equilibrium constant K, we may write
$$ {n_{{\mathrm{ C}{{\mathrm{ H}}_4}}}}{n_{{{{\mathrm{ H}}_2}\mathrm{ O}}}}=K{n_{\mathrm{ C}\mathrm{ O}}}n_{{{{\mathrm{ H}}_2}}}^3. $$The constant may be obtained from the reaction enthalpy variation, being K ≃ 10−2 cm6 for T ≃ 200 K. Interstellar clouds have T ≲ 100 K, so that this value must be considered as a lower limit. Consider a cloud with \( {n_{{{{\mathrm{ H}}_2}}}}\sim{} 1{0^4}\mathrm{ c}{{\mathrm{ m}}^{-3 }} \) and \( {n_{\mathrm{ CO}}}/{n_{{{{\mathrm{ H}}_2}}}}\sim{} 1{0^{-6 }} \). Assuming that \( {n_{{\mathrm{ C}{{\mathrm{ H}}_4}}}}\sim{} {n_{{{{\mathrm{ H}}_2}\mathrm{ O}}}} \), what is the methane equilibrium abundance? Compare the result with H2O abundance. What conclusion can you draw regarding the cloud’s chemical equilibrium?
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10.5
Show that, in orders of magnitude, (10.42) may be written in the form of (10.38).
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Maciel, W.J. (2013). Interstellar Molecules. In: Astrophysics of the Interstellar Medium. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-3767-3_10
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DOI: https://doi.org/10.1007/978-1-4614-3767-3_10
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