Water deuterium fractionation in the high-mass star-forming region
  G34.26+0.15 based on Herschel/HIFI data
  Understanding water deuterium fractionation is important for constraining the
mechanisms of water formation in interstellar clouds. Observations of HDO and
H$_2^{18}$O transitions were carried out towards the high-mass star-forming
region G34.26+0.15 with the HIFI instrument onboard the Herschel Space
Observatory, as well as with ground-based single-dish telescopes. Ten HDO lines
and three H$_2^{18}$O lines covering a broad range of upper energy levels
(22-204 K) were detected. We used a non-LTE 1D analysis to determine the
HDO/H$_2$O ratio as a function of radius in the envelope. Models with different
water abundance distributions were considered in order to reproduce the
observed line profiles. The HDO/H$_2$O ratio is found to be lower in the hot
core ($\sim$3.5 $\times$ 10$^{-4}$ - 7.5 $\times$ 10$^{-4}$) than in the colder
envelope ($\sim$1.0 $\times$ 10$^{-3}$ - 2.2 $\times$ 10$^{-3}$). This is the
first time that a radial variation of the HDO/H$_2$O ratio has been found to
occur in a high-mass source. The chemical evolution of this source was modeled
as a function of its radius and the observations are relatively well
reproduced. The comparison between the chemical model and the observations
leads to an age of $\sim$10$^5$ years after the infrared dark cloud stage.
