Water in the envelopes and disks around young high-mass stars
Résumé
Single-dish spectra and interferometric maps of (sub-)millimeter lines of H18 2 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 2×1014 cm?2 for HDO and 2×1017 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 H18 2 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 110–111, H18 2 O 313–220 and SO2 120,12–111,11 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?, 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.