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

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 HO 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  cm^-2 for HDO and  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 HO 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 , HO  and SO₂  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 , 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.