Water in the envelopes and disks around young high-mass stars

Single-dish spectra and interferometric maps of (sub-)millimeter lines of H$_2^{18}$O and HDO are used to study the chemistry of water in eight regions of high-mass star formation. The spectra indicate HDO excitation temperatures of ~110 K and column densities in an 11'' beam of ${\sim}2\times10^{14}$ cm-2 for HDO and ${\sim}2\times10^{17}$ cm-2 for H2O, with the N(HDO)/N(H2O) ratio increasing with decreasing temperature. Simultaneous observations of CH3OH and SO2 indicate that 20-50% of the single-dish line flux arises in the molecular outflows of these objects. The outflow contribution to the H$_2^{18}$O and HDO emission is estimated to be 10-20%. Radiative transfer models indicate that the water abundance is low (~10-6) outside a critical radius corresponding to a temperature in the protostellar envelope of ≈100 K, and “jumps” to H2O/H2 ~ 10-4 inside this radius. This value corresponds to the observed abundance of solid water and together with the derived HDO/H2O abundance ratios of ~10-3 suggests that the origin of the observed water is evaporation of grain mantles. This idea is confirmed in the case of AFGL 2591 by interferometer observations of the HDO $1_{10}{-}1_{11}$, H$_2^{18}$O $3_{13}{-}2_{20}$ and SO2 $12_{0,12}{-}11_{1,11}$ lines, which reveal compact (Ø ~ 800 AU) emission with a systematic velocity gradient. This size is similar to that of the 1.3 mm continuum towards AFGL 2591, from which we estimate a mass of ≈0.8 $M_{\odot}$, or ~5% of the mass of the central star. We speculate that we may be observing a circumstellar disk in an almost face-on orientation.