2005A&A...434.1021V

The changes in the spectral line profile of the 22GHz H₂O maser are calculated as a function of emerging maser flux. We address not only the narrowing and re-broadening of the maser lines, but also the deviations of Gaussian symmetry as a result of the various hyperfine components of the 22GHz maser line. Non-LTE models of the H₂O maser transition, including hyperfine structure and cross-relaxation, are compared with high spectral resolution Very Long Baseline Interferometry (VLBI) observations of the H₂O masers in the circumstellar envelopes (CSEs) of a sample of late-type stars. This yields estimates on the thermal width in the maser region as well as the emerging maser flux and thus the level of saturation. We also discuss the effect of a velocity gradient along the maser path on the line widths and shapes of the line profile and the effect of the geometry of the maser region. We find that the typical velocity shift along the maser path is of the order of 1.0km/s. The effect of this shift on the shape of the maser spectrum is difficult to distinguish from the effect of the hyperfine components.