interstellar molecules, including H2, tells us that the latter explanation is correct.
The discovery of optical absorption lines of CH, CH+ and CN raised the possibility that molecules might be an important constituent of the interstellar gas. However, it was thought that the densities were too low for chemistry to proceed very far. In fact, the existence of these three unstable species supported that general notion. If chemistry had proceeded very far, these species would have been incorporated into more complex molecules.
To see how these arguments worked, we can estimate the rate at which molecules form in a cloud. Let's take the example of C and O coming together to form the simple molecule CO. The rate of formation of CO per unit volume is given by
where nC and nO are the C and O densities, respectively, v is the relative speed of the atoms, and a is the cross section for a collision. We take a to be the geometric cross section (the approximate size of an atom, 10-16 cm-2) and v to be the average thermal speed at a temperature of 100 K (about 105 cm/s). Finally, since both C and O have cosmic abundances of about 10~3 that of H, we take each of their densities to be 10~3 nH. This will give us a factor of nH in the rate, so the density is very important. If nH = 10 cm~3 the rate becomes 10-15 cm~3/s.
We have to compare this with the rate at which CO is destroyed. One destruction mechanism is photodissociation. An ultraviolet photon strikes the molecule with a sufficient energy to break it apart. An unprotected CO molecule can live an average of 103 years (3 X 1010 s) in the interstellar radiation field. The dissociation rate per molecule is the inverse of the lifetime. The dissociation rate per unit volume is the density of CO molecules divided by the lifetime:
If we equate the formation and destruction rates, we can solve for the equilibrium CO abundance:
Since nH = 10 cm~3 the fractional abundance of CO, nCO/nH, is about 3 X 10~6. This is low enough that it did not raise the hopes of finding very complex molecules. We have even been very optimistic by assuming that every collision between C and O leads to a CO molecule.
However, radio searches for small molecules were carried out, with some of the initial candidates being chosen by the availability of convenient radio transitions. In the 1960s three simple molecules were found, OH (at a wavelength of 18 cm), H2O (at a wavelength of 1 cm), and NH3 (at a wavelength of 1 cm). The abundances of these molecules were surprisingly high, and astronomers were encouraged to carry out searches for other molecules. In 1969 one of the most important molecular discoveries took place. CO was found at a wavelength of 2.6 mm, by a group at Bell Laboratories, led by Arno Penzias and Robert Wilson (who shared the Nobel Prize in physics for their earlier discovery of the cosmic background radiation, to be discussed in Chapter 21). They used the NRAO millimeter telescope shown in Fig. 4.28(a). This was the first molecule to be found at millimeter wavelengths. Remember, at shorter wavelengths we can produce good angular resolution with modest sized telescopes. (Of course, the telescope surfaces require greater precision and must be placed at dry sites.) The abundance of CO is also very high, with CO densities of about 1 cm~3, much higher than our previous estimate. As we will see, the 2.6 mm line of CO has taken its place alongside the 21 cm line as one of the important tools in studying the cool interstellar gas.
Following these initial discoveries, a large number of interstellar molecules were found. Over 100 have been discovered to date. They are listed in Table 14.1. There are many familiar molecules, such as formaldehyde (H2CO), methyl alcohol (CH3OH), and ethyl alcohol (CH2CH3OH). There are some unfamiliar molecules. Some of these are charged species, such as HCO+, and
Table 14.1. Interstellar molecules, arranged by number of atoms.
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