Submillimetre water masers at 437, 439, 471, and 474 GHz towards evolved
  stars. APEX observations and radiative transfer modelling
  Here we aim to characterise submillimetre water masers at 437, 439, 471, and
474 GHz towards a sample of evolved stars.
  We used the Atacama Pathfinder Experiment (APEX) to observe submillimetre
water transitions and the CO (4-3) line towards 11 evolved stars. The sample
included semi-regular and Mira variables, plus a red supergiant star. We
performed radiative transfer modelling for the water masers. We also used the
CO observations to determine mass loss rates for the stars.
  From the sample of 11 evolved stars, 7 display one or more of the masers at
437, 439, 471, and 474 GHz. We therefore find that these masers are common in
evolved star circumstellar envelopes. The fact that the maser lines are
detected near the stellar velocity indicates that they are likely to originate
from the inner circumstellar envelopes of our targets. We tentatively link the
presence of masers to the degree of variability of the target star, that is,
masers are more likely to be present in Mira variables than in semi-regular
variables. We suggest that this indicates the importance of strong shocks in
creating the necessary conditions for the masers. Typically, the 437 GHz line
is the strongest maser line observed among those studied here. We cannot
reproduce the above finding in our radiative transfer models. In general, we
find that maser emission is very sensitive to dust temperature in the lines
studied here. To produce strong maser emission, the dust temperature must be
significantly lower than the gas kinetic temperature. In addition to running
grids of models in order to determine the optimum physical conditions for
strong masers in these lines, we performed smooth wind modelling for which we
cannot reproduce the observed line shapes. This also suggests that the masers
must originate predominantly from the inner envelopes.
