Suggested Mechanism for the Anomalous Excitation of OH Microwave Emissions from H-II Regions

Abstract

STRONG microwave emission lines at 18 cm have recently been reported from a number of sources^1–3, the frequencies being close to those of the hyperfine components of the Λ-doublet of the ground state (X ²Π_3/2, J = 3/2) as measured in the laboratory⁴. Λ-doubling is due to coupling between the rotation of the nuclei and the orbital rotation of the electrons, which has a quantum number Λ = 1 in the Π state. The energies differ slightly according to the relative orientation of the two angular momenta. Coupling with the nuclear spin of the proton leads to further hyperfine splitting (Fig. 1). Galactic absorption lines at these frequencies, detected in the past 3 years, have rather broad profiles similar to those of the 14-cm H-emission line; and, except for OH near the galactic centre, the intensities of the hyperfine components are compatible with populations of levels in thermodynamic equilibrium⁵. The emission lines, by contrast, show a number of remarkable features.