2003A&A...406...23B

The microturbulent approximation of turbulent motions is widely used in radiative transfer calculations. Mainly motivated by its simple computational application it is probably in many cases an oversimplified treatment of the dynamical processes involved. This aspect is in particular important in the analysis of maser lines, since the strong amplification of radiation leads to a sensitive dependence of the radiation field on the overall velocity structure. To demonstrate the influence of large scale motions on the formation of maser lines we present a simple stochastic model which takes velocity correlations into account. For a quantitative analysis of correlation effects, we generate in a Monte Carlo simulation individual realizations of a turbulent velocity field along a line of sight. Depending on the size of the velocity correlation length we find huge deviations between the resulting random profiles in respect of line shape, intensity and position of single spectral components. Finally, we simulate the emission of extended maser sources. A qualitative comparison with observed masers associated with star forming regions shows that our model can reproduce the observed general spectral characteristics. We also investigate briefly, how the spectra are affected when a systematic velocity field (simulating expansion) is superposed on the fluctuations. Our results convincingly demonstrate that hydrodynamical motions are of great importance for the understanding of cosmic masers.