Heavy water stratification in a low-mass protostar^⋆

¹ Université de Toulouse, UPS–OMP, IRAP, Toulouse France
e-mail: acoutens@nbi.dk, cvastel@irap.omp.eu
² CNRS, Institut de Recherche en Astrophysique et Planétologie, 9 Av. Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
³ Kapteyn Astronomical Institute, PO Box 800, 9700AV Groningen, The Netherlands
⁴ Institut de Planétologie et d’Astrophysique de Grenoble, UMR 5274, UJF-Grenoble 1/CNRS, 38041 Grenoble, France
⁵ CNRS and Université de Bordeaux, Observatoire Aquitain des Sciences de l’Univers, 2 rue de l’Observatoire, BP 89, 33271 Floirac, France

Received: 20 December 2012
Accepted: 18 March 2013

Abstract

Context. Despite the low elemental deuterium abundance in the Galaxy, enhanced molecular deuterium fractionation has been found in the environments of low-mass star-forming regions and, in particular, the Class 0 protostar IRAS 16293-2422.

Aims. The key program Chemical HErschel Surveys of Star forming regions (CHESS) aims at studying the molecular complexity of the interstellar medium. The high sensitivity and spectral resolution of the Herschel/HIFI (Heterodyne Instrument for Far-Infrared) instrument provide a unique opportunity to observe the fundamental 1_1,1–0_0,0 transition of ortho–D₂O at 607 GHz and the higher energy 2_1,2–1_0,1 transition of para–D₂O at 898 GHz, both of which are inaccessible from the ground.

Methods. The ortho–D₂O transition at 607 GHz was previously detected. We present in this paper the first tentative detection for the para–D₂O transition at 898 GHz. The spherical Monte Carlo radiative transfer code RATRAN was used to reproduce the observed line profiles of D₂O with the same method that was used to reproduce the HDO and H₂¹⁸O line profiles in IRAS 16293-2422.

Results. As for HDO, the absorption component seen on the D₂O lines can only be reproduced by adding an external absorbing layer, possibly created by the photodesorption of the ices at the edges of the molecular cloud. The D₂O column density is found to be about 2.5  ×  10¹² cm^-2 in this added layer, leading to a D₂O/H₂O ratio of about 0.5%. At a 3σ uncertainty, upper limits of 0.03% and 0.2% are obtained for this ratio in the hot corino and the colder envelope of IRAS 16293-2422, respectively.

Conclusions. The deuterium fractionation derived in our study suggests that the ices present in IRAS 16293-2422 formed on warm dust grains (~15–20 K) in dense (~10⁴–5  ×  10⁴ cm^-3) translucent clouds. These results allow us to address the earliest phases of star formation and the conditions in which ices form.