Paper: astro-ph/0510640
Date: Fri, 21 Oct 2005 13:25:19 GMT (356kb)

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

Authors: Floris van der Tak, Malcolm Walmsley, Fabrice Herpin, Cecilia
Ceccarelli

Comments: Accepted by A&A; 16 A4 pages; 12 colour figures
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Single-dish spectra and interferometric maps of (sub)mm lines of H2O-18 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 ~2x10^14 cm^-2 for HDO and ~2x10^17 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 H2O-18 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 ~1/1000 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 HDO, H2O-18 and SO2 lines,
which reveal compact (~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 M0, 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.

\\ ( http://arXiv.org/abs/astro-ph/0510640 , 356kb)