  Asymptotic giant branch (AGB) stars lose their envelopes by means of a
stellar wind whose driving mechanism is not understood well. Characterizing the
composition and thermal and dynamical structure of the outflow provides
constraints that are essential for understanding AGB evolution, including the
rate of mass loss and isotopic ratios.
  We modeled the CO emission from the wind of the low mass-loss rate
oxygen-rich AGB star W Hya using data obtained by the HIFI, PACS, and SPIRE
instruments onboard the Herschel Space Observatory and ground-based telescopes.
$^{12}$CO and $^{13}$CO lines are used to constrain the intrinsic
$^{12}$C/$^{13}$C ratio from resolved HIFI lines.
  The acceleration of the outflow up to about 5.5 km/s is quite slow and can be
represented by a beta-type velocity law with index 5. Beyond this point,
acceleration up the terminal velocity of 7 km/s is faster. Using the J=10-9,
9-8, and 6-5 transitions, we find an intrinsic $^{12}$C/$^{13}$C ratio of
$18\pm10$ for W Hya, where the error bar is mostly due to uncertainties in the
$^{12}$CO abundance and the stellar flux around 4.6 $\mu$m. To match the
low-excitation CO lines, these molecules need to be photo-dissociated at about
500 stellar radii. The radial dust emission intensity profile measured by PACS
images at 70 $\mu$m shows substantially stronger emission than our model
predicts beyond 20 arcsec.
  The initial slow acceleration of the wind implies inefficient wind driving in
the lower part of the envelope. The final injection of momentum in the wind
might be the result of an increase in the opacity thanks to the late
condensation of dust species. The derived intrinsic isotopologue ratio for W
Hya is consistent with values set by the first dredge-up and suggestive of an
initial mass of 2 M$_\odot$ or more. However, the uncertainty in the
main-sequence mass derived based on this isotopologic ratio is large.
