Water abundance variations around high-mass protostars: HIFI observations of the DR21 region

Abstract

<i>Context. <i/>Water is a key molecule in the star formation process, but its spatial distribution in star-forming regions is not well known.<i>Aims. <i/>We study the distribution of dust continuum and H<sub>2<sub/>O and <sup>13<sup/>CO line emission in DR21, a luminous star-forming region with a powerful outflow and a compact H ii region.<i>Methods. <i/><i>Herschel<i/>-HIFI spectra near 1100 GHz show narrow <sup>13<sup/>CO 10–9 emission and H<sub>2<sub/>O 1<sub>11<sub/>–0<sub>00<sub/> absorption from the dense core and broad emission from the outflow in both lines. The H<sub>2<sub/>O line also shows absorption by a foreground cloud known from ground-based observations of low-<i>J<i/> CO lines.<i>Results. <i/>The dust continuum emission is extended over 36” FWHM, while the <sup>13<sup/>CO and H<sub>2<sub/>O lines are confined to <i>≈<i/>24” or less. The foreground absorption appears to peak further North than the other components. Radiative transfer models indicate very low abundances of ~2×10<sup>-10<sup/> for H<sub>2<sub/>O and ~8×10<sup>-7<sup/> for <sup>13<sup/>CO in the dense core, and higher H<sub>2<sub/>O abundances of ~4×10<sup>-9<sup/> in the foreground cloud and ~7×10<sup>-7<sup/> in the outflow.<i>Conclusions. <i/>The high H<sub>2<sub/>O abundance in the warm outflow is probably due to the evaporation of water-rich icy grain mantles, while the H<sub>2<sub/>O abundance is kept down by freeze-out in the dense core and by photodissociation in the foreground cloud.