Asymmetric circumstellar OH maser emission: effect of density fluctuations

Asymmetries of the 1612-MHz OH maser line profiles are investigated in terms of density fluctuations in the circumstellar envelope. The integrated spectrum and spatial distribution of maser emission are calculated for a model of the circumstellar envelope with density condensations. It is suggested that the ratio of the front and back peak fluxes in the 1612-MHz spectrum deviates significantly from unity when the mean diameter of discrete spherical blobs is comparable with the thickness of the envelope. The profile asymmetry in a large sample of OH/IR stars with the IRAS fluxes available is analyzed. The youngest sources with OH emission tend to be more asymmetrical than the older sources. The envelopes around the older sources tend to be greater. The ratio of the 1612-MHz line peaks is correlated with the stage of evolution of related objects and may be partly explained by density fluctuations in the circumstellar envelopes.