Water absorption in Galactic translucent clouds: conditions and history
  of the gas derived from Herschel/HIFI PRISMAS observations
  We present Herschel/HIFI observations of nine transitions of \hho and \hheo
towards six high-mass star-forming regions, obtained as part of the PRISMAS Key
Program. Water vapor in translucent clouds is detected in absorption along
every sightline. We derive the column density of \hho or \hheo for the lower
energy level of each transition observed. The total water column density is
about a few $10^{13} \rm{cm^{-2}}$. We find that the abundance of water
relative to hydrogen nuclei is $1\times10^{-8}$ in agreement with models for
oxygen chemistry with high cosmic ray ionization rates. Relative to \hh, the
abundance of water is remarkably constant at $5\times10^{-8}$. The abundance of
water in excited levels is at most 15%, implying that the excitation
temperature $T_{ex}$ in the ground state transitions is below 10 K. The column
densities derived from the two ortho ground state transitions indicates that
$T_{ex}\simeq5$ K and that the density $n($\hh$)$ in the clouds is $\le10^4
\rm{cm^{-3}}$. For most clouds we derive a water ortho-to-para ratio consistent
with the value of 3 expected in thermodynamic equilibrium in the high
temperature limit. Two clouds with large column densities exhibit a ratio
significantly below 3. This may argue that the history of water molecules
includes a cold phase, either when the molecules were formed on cold grains, or
when they later become at least partially thermalized with the cold gas
($\sim25$ K) in the shielded, low temperature regions of the clouds; evidently,
they have not yet fully thermalized with the warmer ($\sim50$ K) translucent
portions of the clouds.
