Interstellar Molecules

From a chemical viewpoint, an important feature of dark clouds is their relatively high column density in dust, which effectively blocks ambient radiation at both optical and ultraviolet wavelengths. Hence interstellar molecules, which would have short lifetimes against ultraviolet photodissociation in unshielded regions of space, are able to survive and proliferate. To date, over 100 molecules have been identified, ranging from the simplest diatomic species to long chains like the cyanopolyyne HCnN. Dense cores contain many of the more complex species found so far, but the shock-heated regions of Orion and the Galactic center cloud Sagittarius B2 have also been rich sources. Equally important are the distended envelopes of evolved, giant stars, which we have already noted as the birth sites of interstellar grains.

5.1.1 Reaction Energetics

Molecular astrophysics began in the late 1930s, with the discovery of CH, CH+, and CN in diffuse clouds. These simple molecules were detected by their absorption of optical light from background stars. The question of how such species form immediately posed a theoretical challenge, one which deepened with the discovery in the 1960s of OH, NH3, and H2O. The problem is one of energetics. Consider first the collision of two atoms. The particles approach each other with positive total energy. Unless energy can somehow be given to a third body, the atoms will simply rebound after their encounter. The simultaneous collision of a third atom can occur with appreciable frequency at terrestrial densities, but not in the vastly more rarefied interior of a molecular cloud. It is also possible for the energy sink to be a photon, i. e., for

The Formation of Stars. Steven W. Stabler and Francesco Palla Copyright © 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-40559-3

neutral molecule neutral molecule

Figure 5.1 Mechanics of ion-molecule reactions. The charged ion induces a dipole moment in the neutral molecule, creating an electrostatic attraction between the two species.

Figure 5.1 Mechanics of ion-molecule reactions. The charged ion induces a dipole moment in the neutral molecule, creating an electrostatic attraction between the two species.

the two atoms to form an excited molecule which radiatively decays to the ground state before it can dissociate. Again, the probability of such radiative association is generally too low in molecular clouds to be of interest.

In the laboratory, molecules also form through neutral-neutral reactions. Here, the colliding species, whether atoms or molecules, combine temporarily into a configuration known as an activated complex. This then separates into two or more product species that share the total energy. The process involves the making and breaking of chemical bonds, and usually requires a net expenditure of energy. The associated activation barrier has a typical energy, expressed in temperature units, of AE/kB ~ 100 K. A barrier of this magnitude is not insurmountable in shock-heated clouds, but completely suppresses neutral-neutral reactions in the very cold interiors of quiescent clouds.

By the early 1970s, it had become clear that ion-molecule reactions can alleviate the energy difficulty. When a charged ion approaches a neutral molecule (or atom), it induces a dipole moment in the latter, creating an electrostatic attraction between the two. (See Figure 5.1.) The long-range nature of this attraction means that the effective cross section is greatly increased above the geometric value for direct collision. Even at temperatures near 10 K, such reactions can proceed fast enough to account for a large fraction of observed interstellar molecules. Since, however, the fraction of ions available at any time is relatively small, the huge abundance of H2 itself cannot be explained in this manner. In this important special case, to which we shall return later, two neutral atoms can react, but only through the catalytic action of an interstellar grain surface.

In symbolic form, the generic ion-molecule reaction may be written where all species can be atoms or molecules. If C = B and D = A, the reaction is simple charge exchange. Reactions involving negative ions and neutrals are also possible, in which case one of the products is a free electron. If nA+ and nB denote the number densities of the reactants, then we let kim (nA+) (nB) be the reaction rate per unit volume per unit time. For either positive or negative ions, the rate coefficient kim is of order 10~9 cm3 s^1 and is only weakly dependent on temperature.

Positively charged molecules within clouds are also destroyed by ambient free electrons. The electron recombines to create an energetic, unstable neutral molecule. Most of the time, this molecule simply "autoionizes," spitting back the electron. However, if its constituent atoms

separate before autoionization occurs, the molecule falls apart into neutral species:

The rate coefficient for such dissociative recombination is kdr ~ 10"7 cm3 s"1 for a temperature T near 100 K and increases slowly as the temperature falls. Note that, if A represents an atom instead of a molecule, reaction with an electron yields the neutral form of the atom plus a photon. The typical rate for this radiative recombination, krr ~ 10"11 cm3 s"1, is again very low for most circumstances.

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