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

Abstract

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 ∼ 110 K and column densities in an 11″ beam of ∼2 × 1014 cm-2 for HDO and ∼2 × 10 17 cm-2 for H2O, with the N(HDO)/N(H 2O) 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 ≈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, H218O 313-220 and SO2 12 0.12-111.11 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. © ESO 2006.