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.