Observed Deviations from Local Thermodynamical Equilibrium

Observed Deviations from Local Thermodynamical Equilibrium 

There are some particularly interesting phenomena encountered in the observation of interstellar molecular spectra the experimental data show that the rotational distribution is, in most cases, anomalous. There is no common single excitation temperature which can describe the interstellar population distribution over the energy level manifold in terms of a Boltzmann distribution. 

From the preceding discussions it is evident that at least four different temperatures have to be considered which under laboratory conditions are all equal the excitation temperature Tex of the molecule, defined by the relative populations of the levels, the kinetic temperature Tk, corresponding to the Maxwellian velocity distribution of the gas particles, the radiation temperature Traa, assuming a (in some cases diluted) black body radiation distribution, and the grain temperature 7, . With no thermodynamic equilibrium established, as is common in interstellar space, none of these temperatures are equal. These non-equilibium conditions are likely to be caused in part by the delicate balance between the various mechanisms of excitation and de-excitation of molecular energy levels by particle collisions and radiative transitions, and in part by the molecule formation process itself. Table 7 summarizes some of the known distribution anomalies. The non-equilibrium between para- and ortho-ammonia, the very low temperature of formaldehyde, and the interstellar OH and H20 masers are some of the more spectacular examples. 

Note Added in Proof. Recent observations by Solomon et al. 1973 show that the K = 2 line is present at or near its expected intensity. Thus this puzzling result of the first detection has been in error. 

In contrast to the CH3CN situation, the spectra of interstellar ammonia give considerable insight into excitation and de-excitation mechanisms. From the observed intensities of the interstellar ammonia lines it has been derived that the excitation temperature 7 12, determined from the relative intensities of the (1,1) and the (2,2) lines, is notably lower than the excitation temperature r13 determined from the intensities of the (1,1) and (3,3) lines. Thus the (3,3) level shows an excess population over the (1,1), (2,2) levels. In other words, ortho-ammonia is not in equilibrium with para-ammonia. However, a more detailed study of the two para-ammonia levels (1,1) and (2,2) also reveals that their relative populations are not given by a simple Boltzmann factor for each of them. The (1,1) level has population in excess over the Boltzmann distribu- 

---

F. Observed Deviations from Local Thermodynamical Equilibrium... 

---