Simultaneous 183 GHz H2O Maser and SiO Observations Towards Evolved
  Stars Using APEX SEPIA Band 5
  We investigate the use of 183 GHz H2O masers for characterization of the
physical conditions and mass loss process in the circumstellar envelopes of
evolved stars. We used APEX SEPIA Band 5 to observe the 183 GHz H2O line
towards 2 Red Supergiant and 3 Asymptotic Giant Branch stars. Simultaneously,
we observed lines in 28SiO v0, 1, 2 and 3, and for 29SiO v0 and 1. We detected
the 183 GHz H2O line towards all the stars with peak flux densities greater
than 100 Jy, including a new detection from VY CMa. Towards all 5 targets, the
water line had indications of being due to maser emission and had higher peak
flux densities than for the SiO lines. The SiO lines appear to originate from
both thermal and maser processes. Comparison with simulations and models
indicate that 183 GHz maser emission is likely to extend to greater radii in
the circumstellar envelopes than SiO maser emission and to similar or greater
radii than water masers at 22, 321 and 325 GHz. We speculate that a prominent
blue-shifted feature in the W Hya 183 GHz spectrum is amplifying the stellar
continuum, and is located at a similar distance from the star as mainline OH
maser emission. From a comparison of the individual polarizations, we find that
the SiO maser linear polarization fraction of several features exceeds the
maximum fraction allowed under standard maser assumptions and requires strong
anisotropic pumping of the maser transition and strongly saturated maser
emission. The low polarization fraction of the H2O maser however, fits with the
expectation for a non-saturated maser. 183 GHz H2O masers can provide strong
probes of the mass loss process of evolved stars. Higher angular resolution
observations of this line using ALMA Band 5 will enable detailed investigation
of the emission location in circumstellar envelopes and can also provide
information on magnetic field strength and structure.
