Dicke's Superradiance in Astrophysics. II -- The OH 1612 MHz Line

TL;DR

This paper explores the application of Dicke's superradiance concept to the OH 1612 MHz spectral line in astrophysics, proposing it as an explanation for observed intensity flares in certain stellar objects.

Contribution

It demonstrates that superradiance can occur in astrophysical environments and explains specific observed flares in evolved stars and nebulae.

Findings

Superradiance explains intensity flares in OH masers.

High coherence and population inversion are necessary conditions.

Superradiance occurs over seconds to years depending on conditions.

Abstract

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.