2006A&A...447.1011V

Single-dish spectra and interferometric maps of (sub-) millimeter lines of H₂¹⁸O 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 ∼110K and column densities in an 11'' beam of ∼2x10¹⁴cm^–2 for HDO and ∼2x10¹⁷cm^–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 H₂¹⁸O 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 ≃100K, 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 1₁₀-1₁₁, H₂¹⁸O 3₁₃-2₂₀ and SO₂ 12_0,12-11_1,11 lines, which reveal compact (O∼800AU) emission with a systematic velocity gradient. This size is similar to that of the 1.3mm continuum towards AFGL 2591, from which we estimate a mass of ≃0.8M_☉, 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.