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SCIENTIFIC ABSTRACT Earth's atmosphere is so rich in water vapor, that radiation having wavelengths anywhere near a low-lying water vapor transition is completely absorbed in relatively thin layers of air. With ISO well above these atmospheric layers, we propose to search for low-lying transitions of water vapor in interstellar clouds, particularly in shocked regions, and in the extended atmospheres of oxygen-rich stars. Such regions are characterized by densities and temperatures sufficiently high to (1) chemically drive a large fraction of the available oxygen into water, and (2) excite high-lying transitions. Detecting a high abundance of water in interstellar molecular clouds could account for the surprisingly low oxygen-to-carbon ratio observed in other chemical species in those clouds, and thereby resolve a major puzzle of cosmic chemical abundance ratios. For M42 and VY CMa on a spring launch, or alternatively W49 and W Hya, for a launch in the fall, we will also seek to measure the abundance of molecular oxygen. OBSERVATION SUMMARY We will make use of Fabry-Perot observations, at short wavelengths, as well as long, making use of AOTs LWS04 and SWS07. With LWS, we will establish a continuum base by observing four elements each side of the line, and make observations at two steps per resolution element. We will aim for a signal to noise ratio of about 10 on individual spectral lines, but will set a maximum duration on the observation of individual scans of about 18000 sec. For stellar observations, line strength variability may be expected and the observations on any given star will need to be concatenated, so as to fall within a given 24 hour orbit. Since pointing at precisely the same location may be a problem on successive orbits, and shocked regions are complex, we will also concatenate the line observations for any given shocked region. In no case, however, will the duration of a concatenated observation exceed a period of the order of two hours. For stellar spectra, we have selected four water vapor LWS lines, respectively at 95.627, 99.493, 100.983 and 113.538 microns. For SWS we have chosen two lines, again respectively, at 22.639 and 23.194 microns. Where observations seem dominated primarily by overheads, we have set a minimum observing time for 15 sec for each LWS Fabry-Perot position. Generally, that increases the observing time by about a factor of two, but assures us flexibility in case the source is somewhat weaker than anticipated. For the SWS07 observations, we will be simultaneously observing the near- infrared continuum in the grating mode, with grating position fixed for the duration of each of the F-P wavelength coverage. For the duration of the 22.639 micron observations the grating will remain fixed at a position at 5.1 microns. For the 23.194 micron observations the wavelength falling on the detectors will be shifted one spectral resolution element longward. For H_2 shocks, we will be searching for a whole series of lines. The ortho (O) and para (P) lines to be observed, in all cases are the 179.527(O), 174.624(O), 138.527(P), 125.356(P), 108.073(O), 99.493(O), 95.627(P), 89.989(P), 83.283(P), 82.032(P), 63.458(P), 56.816(O), a combination of 51.445(O) and 51.461(P) centered on 51.453 microns, as well as the 47.973(O), 46.608(O) and 45.111 micron lines. These will all be observed with the LWS Fabry-Perot instrument. The first seven will always be included in the first AOT listed, while the other nine will be in the second AOT on the lists given below. Expected flux levels are highly uncertain. However, for shocked regions the projected observing times for water vapor lines are based on the best shock models available, and the most detailed transition probability and level cascade studies. Even if those were to prove faulty, significant astrophysical information would still be derived through the proposed observations, since the sensitivities to be achieved for the listed lines would be of the order of or below the line strengths expected in far-infrared OH transitions, which are well observed and documented in Orion. If no water vapor cooling were observed at our expected sensitivities, we would conclude water to be a less effective coolant than OH for shocked regions, at least as judged from present data on shocked OH/H_2 line ratios in Orion. For two of the stars and two of the shocked regions, we have undertaken to observe two of the molecular oxygen lines, each, and to spend of the order of an hour for each line observation. The stars are VY CMa and WHya, respectively for the spring and autumn launches. The shocked regions are the Orion BN/KL region and W 49, again respectively for the spring and autumn launches. These observations are indicated by the designation "O_2" on the source lists below. As half of these sources fall into the unobservable holes, we spend two hours on a shocked region and two on a star, in either launch window. We felt it reasonable to devote four hours of observing time to a search for molecular oxygen, since O_2 could constitute a