ALMA Observations Of Doubly Deuterated Water: Inheritance Of Water From The Prestellar Environment


Schematics of the predicted ice structure in the prestellar core stage (left) and the observed water deuterium fractionation toward embedded low-mass protostars (right). The outer layer of the ice mantle, which has high water D/H ratio, may stem from nonthermal desorption in the cold envelope or cold gas-phase formation. Meanwhile, the whole mantle is sublimated in the hot corino, lowering the HDO/H2O ratio. The schematical structure is proposed by Furuya et al. (2016). The figure is adopted from van Dishoeck et al. (2021).

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. 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.

We present ALMA observations of the D2O 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 H182O 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. D2O is detected toward both sources in the inner warm envelope. The derived D2O/HDO ratios is (1.0±0.2)×10−2 for L483 and (1.4±0.1)×10−2 for B335. 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

S. S. Jensen, J. K. Jørgensen, L. E. Kristensen, A. Coutens, E. F. van Dishoeck, K. Furuya, D. Harsono, M. V. Persson

Comments: Accepted for publication in A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA); Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2104.13411 [astro-ph.GA] (or arXiv:2104.13411v1 [astro-ph.GA] for this version)
Submission history
From: Sigurd Sigersen Jensen
[v1] Tue, 27 Apr 2021 18:11:44 UTC (11,039 KB)
https://arxiv.org/abs/2104.13411
Astrochemistry, Astrobiology

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