H-atom laser without inversion (LWI) in space: a possible explanation for the intense, narrow-band, H(alpha) emission frequently observed in reddened early-type stars

Abstract

A model is suggested to explain the frequently observed presence of intense, narrow-band, H(alpha) emission lines in the optical spectra of reddened, early-type stars (e.g. HD 44179, IRAS 18179-1346, IRAS 20298+4011). It is proposed that hydrogen atoms surrounding compact H II regions enveloping such stars become coherently phased via a nonlinear photonic mechanism that leads to `electromagnetically induced transparency (EIT)'. EIT is a powerful technique that can be used to make a material system transparent to resonant laser radiation, while still allowing large nonlinear resonant processes to occur with high probability. In EIT terminology, a `Lambda' configuration, involving H-atom levels (1s, 3p, and 2s), is here assumed to be operative. The EIT `coupling beam' is the narrow-band H(alpha) radiation predicted to be coherently generated via a standard `laser without inversion (LWI)' scenario when coherently phased atoms are excited to the 3p level by means of a separate nonlinear excitation process known as resonant hyper-Raman scattering (HRS). In the unit HRS pumping process, a pair of far-ultraviolet (FUV) photons, with frequencies lying very close to Ly- beta but offset from it by equal amounts to high and low energies, are absorbed from the star's blackbody continuum, a photon at Ly-beta line center is emitted, and an atom is excited to the 3p level - with all events in this energy conserving process occurring simultaneously. The EIT `probe beam' is the light predicted to be coherently generated at Ly-beta line center, which - as a result of the complete linear transparency afforded at this frequency by the coherently phased H atoms - can propagate completely unattenuated through the optically thick H-atom cloud surrounding the star.