Varshni,Y.P. and Lam,C.S.: 1975, Bull.Amer.Astron.Soc., 7, 551.

Menzel (1970) has given a clear discussion of the possibility of laser action in non-LTE atmospheres. Gudzenko and Shelepin (1963) and Gudzenko et al. (1966) have proposed that if the is sufficiently rapid cooling of the free electrons when a plasma expands, there can be a population inversion in the lower levels of an atom. It is shown that in high-temperature stars in which high speed mass loss is occurring, the rapidly recombining plasma in the stellar envelope can act as an amplifying medium. Model calculations for laser action in He II 4686 , using the collisional-radiative model (Bates et al. 1962), are presented. The results are presented in terms of the population inversion measure P=N2/g2 - N1/g1. (N is atomic population, and g, statistical weight). P is shown as a function of the electron density and electron temperature in the figure. The numbers by the side of contours represent P values in per cubic cm. The bearing of the results on the spectra of Wolf-Rayet stars those of planetary nuclei is discussed.

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