Water vapour in circumstellar envelopes

We present observations of the 313 →220 line of p-H2O at 183 GHz in the direction of the circumstellar envelopes of 23 O–rich evolved stars. The 183 GHz data are complemented with observations of CO J=2 →1, p-H2O 515 →422 at 325 GHz, o-H2O 616 →523 at 22 GHz and SiO v=1 J=2 →1. The line profiles of these transitions are compared and discussed in terms of the regions where they are formed. We have also made time variability studies of the 183 GHz water line. The changes of the line profile and flux from one epoch to another are small in comparison with those of the o-H2O line at 22 GHz and the SiO v=1 J=2 →1 line as reported in the literature. The main characteristics of the 183 GHz emission can be summarized as follows: a) the 183 GHz line is of maser nature; b) in stars with low mass loss rates (Ṁ ) the emission peaks at a velocity similar to that of the star, whilst in stars with high Ṁ the emission peaks at velocities close to the terminal velocity of the envelope; c) in stars with high Ṁ the blue–shifted emission –relative to the stellar velocity– is often more intense than the red–shifted emission; d) the full width at zero power of the 183 GHz line relative to the terminal velocity of the envelope increases with Ṁ ; e) the power emitted in the line and Ṁ are also linearly correlated. The latter empirical relationship is compatible with the emission of a saturated maser line and with small variations of H2O abundance from object to object.