High Resolution Studies of Circumstellar Masers

Doktorsavhandling, 1996

This thesis describes in detail the observational procedures that we have followed and developed to analyze several research projects organized at Onsala since 1990 to study the regions of SiO maser emission at 43 GHz in the circumstellar envelopes of late-type stars with very high angular resolution. The compact nature of these SiO maser regions has been demonstrated in this work. In fact, we find that they are an order of magnitude smaller than previously reported. As a consequence, they can be studied with the available VLBI instruments. We have used the EVN and VLBA telescopes to produce images of the distribution of the SiO maser features in several sources, in particular R Cassiopeiae and VX Sagittarii. Through this work, we discuss in detail the procedures involved in all the project steps, from the scheduling of the observations to the calibration, imaging, and modelling of the source structure. We compare our results with those of other instruments at different frequencies for the same objects. This thesis also describes the analysis of observations of water masers at 22 GHz in several late-type stars using the VLA. The limited resolution of these observations made us develop a computer program, "FG3", that would search for the maser sources (blended spatially and in frequency) and extract them via 3D least squares fitting to Gaussian functions. This is, to our knowledge, a new approach in this kind of investigations. This program provides information of the source structure distribution and kinematics, and also measures parameters that are directly related with the physical conditions in the masing regions. In particular, we conclude by the study of the fitted maser linewidths that water masers in late-type stars are unsaturated. We are not able to reach a clear conclusion for the SiO masers in the supergiant star VX Sgr, but there are indications that they might be saturated.