Observations of water with Herschel/HIFI toward the high-mass protostar
  AFGL 2591
  Water is a sensitive tracer of physical conditions in star-forming regions
because of its large abundance variations between hot and cold regions. We use
spectrally resolved observations of rotational lines of H$_2$O and its
isotopologs with Herschel/HIFI to constrain the physical conditions of the
water emitting region toward the high-mass protostar AFGL2591. We use
analytical estimates and rotation diagrams to estimate T$_{ex}$ and column
densities of H$_2$O of the envelope, the outflow, and a foreground cloud.
Furthermore, we use the non-LTE radiative transfer code to estimate the
temperature and volume density of the H$_2$O emitting gas. Assuming LTE, we
estimate an T$_{ex}$ of 42 K and a column density of 2$\times$10$^{14}$
cm$^{-2}$ for the envelope and 45 K and 4$\times$10$^{13}$ cm$^{-2}$ for the
outflow, in beams of 4" and 30", respectively. Non-LTE models indicate a
kinetic temperature of 60-230 K and a volume density of 7$\times$10$^6$-10$^8$
cm$^{-3}$ for the envelope, and a kinetic temperature of 70-90 K and a gas
density of 10$^7$-10$^8$ cm$^{-3}$ for the outflow. The o/p ratio of the
foreground absorption is 1.9$\pm$0.4, suggesting a low temperature. In
contrast, the o/p ratio seen in absorption by the outflow is 3.5$\pm$1.0, as
expected for warm gas. The water abundance in the envelope is 10$^{-9}$,
similar to the low values found for other high- and low-mass protostars,
suggesting that this abundance is constant during the embedded phase of
high-mass star formation. The water abundance in the outflow is 10$^{-10}$,
which is 10$\times$ lower than in the envelope and in the outflows of high- and
low-mass protostars. Since beam size effects can only increase this estimate by
a factor of 2, we suggest that the water in the outflow is affected by
dissociating UV radiation due to the low extinction in the outflow lobe.
