Red Variables with and without OH Radio Emission

Radio astronomers detected in 1963 hydroxyl molecules (first in absorption), in form of microwave lines at the frequencies 1612, 1665, 1667, and 1720 MHz (1612.231 + 1720.533 (=) 1665.401 + 1667.358), corresponding to a wavelength near 18 cm. Since 1968, some infrared stars have been known to be observable with radio telescopes as sources of such microwave lines, in emission with line widths of 2 kHz or less. In addition to the ground state, a Π state with Λ = 1, the OH molecule has three known electronic excited states, Σ states with Λ = 0, at about 30 000, 60 000, and 90 000 cm−1 above the ground state. These electric-dipole transitions are about 104 times stronger than the magnetic-dipole transitions which produce hydrogen or deuterium lines; the energy of the 2II3/2 state is less than that of the 2II1/2 state what indicates that the observed OH radio emission is subject to far infrared stellar radiation.