Dicke's Superradiance in Astrophysics. II -- The OH 1612 MHz Line
  We apply the concept of superradiance introduced by Dicke in 1954 to the OH
molecule 1612 MHz spectral line often used for the detection of masers in
circumstellar envelopes of evolved stars. As the detection of 1612 MHz OH
masers in the outer shells of envelopes of these stars implies the existence of
a population inversion and a high level of velocity coherence, and that these
are two necessary requirements for superradiance, we investigate whether
superradiance can also happen in these regions. Superradiance is characterized
by high intensity, spatially compact, burst-like features taking place over
time-scales on the order of seconds to years, depending on the size and
physical conditions present in the regions harboring such sources of radiation.
Our analysis suggests that superradiance provides a valid explanation for
previous observations of intensity flares detected in that spectral line for
the U Orionis Mira star and the IRAS18276-1431 pre-planetary nebula.
