ALMA observations of water deuteration: a physical diagnostic of the formation of protostars

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 Center for Computational Sciences, University of Tsukuba, Tsukuba, Japan
³ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, 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 Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden

Received: 4 June 2019
Accepted: 18 September 2019

Abstract

Context. How water is delivered to planetary systems is a central question in astrochemistry. The deuterium fractionation of water can serve as a tracer for the chemical and physical evolution of water during star formation and can constrain the origin of water in Solar System bodies.

Aims. The aim is to determine the HDO/H₂O ratio in the inner warm gas toward three low-mass Class 0 protostars selected to be in isolated cores, i.e., not associated with any cloud complexes. Previous sources for which the HDO/H₂O ratio have been established were all part of larger star-forming complexes. Determining the HDO/H₂O ratio toward three isolated protostars allows comparison of the water chemistry in isolated and clustered regions to determine the influence of local cloud environment.

Methods. We present ALMA Band 6 observations of the HDO 3_1,2–2_2,1 and 2_1,1–2_1,2 transitions at 225.897 GHz and 241.562 GHz along with the first ALMA Band 5 observations of the H₂¹⁸O 3_1,3–2_2,0 transition at 203.407 GHz. The high angular resolution observations (0′′.3–1′′.3) allow the study of the inner warm envelope gas. Model-independent estimates for the HDO/H₂O ratios are obtained and compared with previous determinations of the HDO/H₂O ratio in the warm gas toward low-mass protostars.

Results. We successfully detect the targeted water transitions toward the three sources with signal-to-noise ratio (S/N) > 5. We determine the HDO/H₂O ratio toward L483, B335 and BHR71–IRS1 to be (2.2 ± 0.4) × 10⁻³, (1.7 ± 0.3) × 10⁻³, and (1.8 ± 0.4) × 10⁻³, respectively, assuming T_ex = 124 K. The degree of water deuteration of these isolated protostars are a factor of 2–4 higher relative to Class 0 protostars that are members of known nearby clustered star-forming regions.

Conclusions. The results indicate that the water deuterium fractionation is influenced by the local cloud environment. This effect can be explained by variations in either collapse timescales or temperatures, which depends on local cloud dynamics and could provide a new method to decipher the history of young stars.