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

¹ Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands
² Jodrell Bank Observatory, University of Manchester, Macclesfield, Cheshire SK11 9DL, UK
³ Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands

Corresponding author: W. H. T. Vlemmings, vlemming@strw.leidenuniv.nl

Received: 28 May 2002
Accepted: 6 August 2002

Abstract

We present circular polarization measurements of circumstellar H₂O masers. The magnetic fields in circumstellar envelopes are generally examined by polarization observations of SiO and OH masers. SiO masers probe the high temperature and density regime close to the central star. OH masers are found at much lower densities and temperatures, 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 temperature and density regime. The magnetic fields are derived using a general, LTE Zeeman analysis 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 saturation 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 analysis. The H₂O masers are shown to be unsaturated, 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 density maser clumps at the inner edge of the H₂O maser region. This allows us to estimate the density 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 estimate of the magnetic field on the surface of the stars. In particular we discuss the differences between Supergiants and Mira variable stars.