  During their asymptotic giant branch, evolution low-mass stars lose a
significant fraction of their mass through an intense wind, enriching the
interstellar medium with products of nucleosynthesis. We observed the nearby
oxygen-rich asymptotic giant branch star IK Tau using the high-resolution HIFI
spectrometer onboard Herschel. We report on the first detection of H_2^{16}O
and the rarer isotopologues H_2^{17}O and H_2^{18}O in both the ortho and para
states. We deduce a total water content (relative to molecular hydrogen) of
6.6x10^{-5}, and an ortho-to-para ratio of 3:1. These results are consistent
with the formation of H_2O in thermodynamical chemical equilibrium at
photospheric temperatures, and does not require pulsationally induced
non-equilibrium chemistry, vaporization of icy bodies or grain surface
reactions. High-excitation lines of 12CO, 13CO, 28SiO, 29SiO, 30SiO, HCN, and
SO have also been detected. From the observed line widths, the acceleration
region in the inner wind zone can be characterized, and we show that the wind
acceleration is slower than hitherto anticipated.
