Contents of: VI/111/./abstract/DENGELS_WASSER.abs

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We propose to observe four key transitions of the H2O molecule connected
to the 6(16) and 5(23) levels, which give rise to the prominent and well
 observed 22 GHz (1.3 cm) maser line in circumstellar shells of variable
 stars on the AGB. Most of these transitions emit photons with energies
 in the FIR and part of them are within the LWS range and can be
 observed with a Fabry-Perot line scan.
 Such observations will probe current models for the maser pump. The most
 elaborated model by Cooke and Elitzur (1985) predict pumping by colli-
 sions and on this base an increase of maser luminosity and distance
 from the star with increasing mass loss rate. This is confirmed as
 general trend observationally.
 However,observations tell that individual stars with similar mass loss
 rates can have quite different maser luminosities and line profiles.
 Either the pumping conditions are different in these stars or the
 velocity structure of the outflowing material is different. Observa-
 tions reveal also that the maser is strongly variable in response to
 variations of the luminosity of the central star.
 To probe the influence of the pumping conditions on the observed maser
 properties, we selected stars representing at first extremes in the
 range of mass loss rates and at second extremes in the range of maser
 luminosities. Observations at fixed times of their light curves will
 allow to study the variations of line strength in the FIR. We want to
 probe our suspicion that the extinction of H2O maser emission around
 minimum light in many OH/IR stars is due to a combination of quenching
 and insufficient excitation temperatures.
 We request addition of ISOPHOT photometry in the IRAS 12,25, 60 micron
 bands at each time of an LWS04 observation. With additional NIR data
 the infrared energy distribution obtained by SWS and LWS grating scans
 within the Guar. Time Prg. can be scaled to the date of the LWS04 ob-
 servation. This will allow comparison of dust and gas temperature.

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