water is a dominant gas component in the envelope of oxygen-rich evolved stars. it is an important coolant of the circumstellar gas and can be photodissociated by interstellar uv photons in the periphery of the circumstellar envelope to produce oh. therefore, h2o plays a major role in the thermal balance and in the photo-chemistry inside oxygen-rich circumstellar envelopes. however, the abundance of h2o is not well known. this is because the detection of h2o lines from the ground is hindered by atmospheric water. so far only a few non-thermal radio lines have been detected using ground-based radio telescopes. the determination of the abundance of h2o based on its maser transitions is rather delicate, because the maser intensity is highly variable. our model calculations have shown that h2o infrared lines are thermal. our goal is to observe thermal emission lines of h2o to determine the abundance of this molecule, and to investigate the physico-chemistry in oxygen-rich envelopes, using a model treating radiative transfer coupled with thermal balance and photodissociation. the observed intensities depend mainly on the distance and the mass loss rate of the star and on the abundance of water. we select a sample of nearby (within 350 pc) visible oxygen-rich agb stars to search for 3 infrared h2o transitions with the lws fabry-perot. a bright m supergiant, s per (2300 pc), and a nearby s type star, chi cyg, are also included in this sample to investigate the effects of luminosity and chemical composition on the h2o abundance. the sources exhibit at least one of the maser emission lines of h2o, oh and sio and their mass loss rates are available. the three selected water lines at 179, 108 and 75 microns are expected to be strong and represent the minimum number to provide adequate constraints on the intermediate shell layers (100-300 k). similarly, we estimate 8 targets to be the minimum number needed to adequately sample the range of stellar properties.
Instrument
LWS04 , SWS06
Temporal Coverage
1997-06-02T09:43:02Z/1997-06-02T13:12:37Z
Version
1.0
Mission Description
The Infrared Space Observatory (ISO) was the world's first true orbiting infrared observatory. Equipped with four highly-sophisticated and versatile scientific instruments, it was launched by Ariane in November 1995 and provided astronomers world-wide with a facility of unprecedented sensitivity and capabilities for a detailed exploration of the Universe at infrared wavelengths.
European Space Agency, 1998, Water Abundance In The Envelope Of Optical Oxygen-Rich Agb Stars, 1.0, European Space Agency, https://doi.org/10.5270/esa-ymlxa9j
