ALMA observations of doubly deuterated water: inheritance of water from the prestellar environment

¹ Niels Bohr Institute & Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K, Denmark
e-mail: sigurd.jensen@nbi.ku.dk
² Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
³ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, UPS-OMP, CNRS, CNES, 9 Av. du Colonel Roche, 31028 Toulouse Cedex 4, France
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ Max-Planck Institute für extraterrestrische Physik (MPE), Giessenbachstrasse, 85748 Garching, Germany
⁶ National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
⁷ Institute of Astronomy and Astrophysics, Academia Sinica, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan, ROC
⁸ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden

Received: 15 February 2021
Accepted: 9 April 2021

Abstract

Context. Establishing the origin of the water D/H ratio in the Solar System is central to our understanding of the chemical trail of water during the star and planet formation process. Recent modeling suggests that comparisons of the D₂O/HDO and HDO/H₂O ratios are a powerful way to trace the chemical evolution of water and, in particular, determine whether the D/H ratio is inherited from the molecular cloud or established locally.

Aims. We seek to determine the D₂O column density and derive the D₂O/HDO ratios in the warm region toward the low-mass Class 0 sources B335 and L483. The results are compared with astrochemical models and previous observations to determine their implications for the chemical evolution of water.

Methods. We present ALMA observations of the D₂O 1_1,0–1_0,1 transition at 316.8 GHz toward B335 and L483 at ≲0.′′5 (≲100 au) resolution, probing the inner warm envelope gas. The column densities of D₂O, HDO, and H₂¹⁸O are determined by synthetic spectrum modeling and direct Gaussian fitting, under the assumption of a single excitation temperature and similar spatial extent for the three water isotopologs.

Results. D₂O is detected toward both sources in the inner warm envelope. The derived D₂O/HDO ratio is (1.0 ± 0.2) × 10⁻² for L483 and (1.4 ± 0.1) × 10⁻² for B335. These values indicate that the D₂O/HDO ratio is higher than the HDO/H₂O ratios by a factor of ≳2 toward both sources.

Conclusions. The high D₂O/HDO ratios are a strong indication of chemical inheritance of water from the prestellar phase down to the inner warm envelope. This implies that the local cloud conditions in the prestellar phase, such as temperatures and timescales, determine the water chemistry at later stages and could provide a source of chemical differentiation in young systems. In addition, the observed D₂O/H₂O ratios support an observed dichotomy in the deuterium fractionation of water toward isolated and clustered protostars, namely, a higher D/H ratio toward isolated sources.