  The spectrometers onboard the Infrared Telescope in Space (IRTS) reveal water
vapor absorption in early M-type stars, as early as M2. Previous observations
detected H_2O vapor absorption only in stars later than M6, with the exception
of the recent detection of H_2O in beta Peg (M2.5 II-III). In our sample of 108
stars, 67 stars have spectral types earlier than M6. The spectral types are
established by means of their near-infrared colors on a statistical basis.
Among the 67 stars of spectral types earlier than M6, we find water vapor
absorption in six stars. The observed absorption features are interpreted using
a local thermodynamic equilibrium model. The features are reasonably fitted by
model spectra with excitation temperatures of 1000-1500 K and water column
densities of 5x10^19 to 1x10^20 cm^-2. These numbers imply that the H_2O
molecules are present in a region of the atmosphere, located above the
photosphere. Furthermore, our analysis shows a good correlation between the
H_2O absorption band strength, and the mid-infrared excess due to the
circumstellar dust. We discuss the relation between the outer atmosphere and
the mass loss.
