Circular polarization of water masers in the circumstellar envelopes of late type stars

We present circular polarization measurements of circumstellar H2O masers. The magnetic fields in circumstellar envelopes are generally examd. by polarization observations of SiO and OH masers. SiO masers C probe the high temp. and d. regime close to the central star. OH masers are found at much lower densities and temps., generally much further out in the circumstellar envelope. The circular polarization detected in the (616-523) rotational transition of the H2O maser can be attributed to Zeeman splitting in the intermediate temp. and d. regime. The magnetic fields are derived using a general, LTE Zeeman anal. as well as a full radiative transfer method (non-LTE), which includes a treatment of all hyperfine components simultaneously as well as the effects of satn. and unequal populations of the magnetic substates. The differences and relevances of these interpretations are discussed extensively. We also address a non-Zeeman interpretation as the cause for the circular polarization, but this is found to be unlikely. We favor the non-LTE anal. The H2O masers are shown to be unsatd., on the basis of their line widths and the lack of linear polarization. The field strengths are compared with previous detections of the magnetic field on the SiO and OH masers. Assuming a r-2 dependence of the magnetic field on the distance to the star, similar to a solar-type magnetic field, our results seem to indicate that we are probing the highest d. maser clumps at the inner edge of the H2O maser region. This allows us to est. the d. of the clumps, and the extent of the H2O maser region. We show that the magnetic pressure dominates the thermal pressure by a factor of 20 or more. We also give an order of magnitude est. of the magnetic field on the surface of the stars. In particular we discuss the differences between supergiants and Mira variable stars. [on SciFinder (R)]