Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138
A variety of theoretical studies over the past two decades have indicated
that gaseous water should be a powerful coolant of warm, dense gas in
the interstellar medium. These predictions were motivated by three
assumptions: (1) chemical models suggest that above gas temperatures
of about 400 K water
is produced rapidly and in great abundance; (2) owing to water's large
dipole moment, the cooling rate is minimally quenched by collisional
de-excitations over a broad range of densities; and (3) water's
many transitions allow it to radiate efficiently over a broad
range of gas temperatures. Unfortunately, before ISO, the role of water
in the ISM went untested due to the difficulty of making such
measurements from within the atmosphere. We report the findings
of both SWS and LWS observations of gaseous water towards both
star forming and circumstellar regions. These observations confirm
the above assumptions and demonstrate that water, along with
H2 and CO, is a major gas coolant. In particular, observations
toward Orion-KL indicate that the post-shock chemical processing into
water of all gas-phase oxygen not bound in CO results in a high
water abundance, i.e. H2O/H2 on the order of
.
Similarly, spectra obtained towards the oxygen-rich late-type stars
W Hydrae and VYCMa exhibit strong water emission in a large number
of lines and are consistent with the assumption of abundant water
vapor in their envelopes and with water being a major coolant of
these circumstellar regions.