Wide spectrum H₂O sources - astrophysical Raman masers

Abstract

The extraordinarily wide 22 GHz emission from W49A and two other galactic HII regions is discussed, and arguments presented why the entire width of these spectra cannot be attributed to the 6₁₆ → 5₂₃ transition of H₂O. It is suggested that most of the weak emission with large frequency shifts is due to stimulated Raman scattering of the strong central features appearing at the expected frequency of the water vapour line. It has not been possible to identify the scattering agent, but it is predicted that the spectra of millimetre-wave maser sources should also show such frequency-shifted features.