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A&A 447, 1011-1025 (2006)
DOI: 10.1051/0004-6361:20053937

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

F. F. S. van der Tak1, C. M. Walmsley2, F. Herpin3 and C. Ceccarelli4

1  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
    e-mail: vdtak@mpifr-bonn.mpg.de
2  Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
3  Observatoire de Bordeaux, L3AB, UMR 5804, BP 89, 33270 Floirac, France
4  Laboratoire Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble, France

(Received 28 July 2005 / Accepted 21 October 2005 )

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 ${\sim}2\times10^$ cm-2 for HDO and ${\sim}2\times10^$ 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 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 $\approx$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 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 $\approx$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.


Key words: ISM: molecules -- molecular processes -- stars: formation -- astrochemistry

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