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We investigate the use of 183 GHz H2O masers for characterization of thephysical conditions and mass loss process in the circumstellar envelopes ofevolved stars. We used APEX SEPIA Band 5 to observe the 183 GHz H2O linetowards 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 detectedthe 183 GHz H2O line towards all the stars with peak flux densities greaterthan 100 Jy, including a new detection from VY CMa. Towards all 5 targets, thewater line had indications of being due to maser emission and had higher peakflux densities than for the SiO lines. The SiO lines appear to originate fromboth thermal and maser processes. Comparison with simulations and modelsindicate that 183 GHz maser emission is likely to extend to greater radii inthe circumstellar envelopes than SiO maser emission and to similar or greaterradii than water masers at 22, 321 and 325 GHz. We speculate that a prominentblue-shifted feature in the W Hya 183 GHz spectrum is amplifying the stellarcontinuum, and is located at a similar distance from the star as mainline OHmaser emission. From a comparison of the individual polarizations, we find thatthe SiO maser linear polarization fraction of several features exceeds themaximum fraction allowed under standard maser assumptions and requires stronganisotropic pumping of the maser transition and strongly saturated maseremission. The low polarization fraction of the H2O maser however, fits with theexpectation for a non-saturated maser. 183 GHz H2O masers can provide strongprobes of the mass loss process of evolved stars. Higher angular resolutionobservations of this line using ALMA Band 5 will enable detailed investigationof the emission location in circumstellar envelopes and can also provideinformation on magnetic field strength and structure.

Simultaneous 183 GHz H2O Maser and SiO Observations Towards Evolved Stars Using APEX SEPIA Band 5