2014A&A...563A..74P

The chemical evolution of water through the star formation process directly affects the initial conditions of planet formation. The water deuterium fractionation (HDO/H₂O abundance ratio) has traditionally been used to infer the amount of water brought to Earth by comets. Measuring this ratio in deeply-embedded low-mass protostars makes it possible to probe the critical stage when water is transported from clouds to disks in which icy bodies are formed. We aim to determine the HDO/H₂O abundance ratio in the warm gas in the inner 150AU for three deeply-embedded low-mass protostars NGC 1333-IRAS 2A, IRAS 4A-NW, and IRAS 4B through high-resolution interferometric observations of isotopologues of water. We present sub-arcsecond resolution observations of the 3_1,2-2_2,1 transition of HDO at 225.89672GHz in combination with previous observations of the 3_1,3-2_2,0 transition of H₂¹⁸O at 203.40752GHz from the Plateau de Bure Interferometer toward three low-mass protostars. The observations have similar angular resolution (0.7"-1.3"), probing scales R≲150AU. In addition, observations of the 2_1,1-2_1,2 transition of HDO at 241.561GHz toward IRAS 2A are presented to constrain the excitation temperature. A direct and model independent HDO/H₂O abundance ratio is determined for each source and compared with HDO/H₂O ratios derived from spherically symmetric full radiative transfer models for two sources. From the two HDO lines observed toward IRAS 2A, the excitation temperature is found to be T_ex=124±60K. Assuming a similar excitation temperature for H₂¹⁸O and all sources, the HDO/H₂O ratio is 7.4±2.1x10^–4 for IRAS 2A, 19.1±5.4x10^–4 for IRAS 4A-NW, and 5.9±1.7x10^–4 for IRAS 4B. The abundance ratios show only a weak dependence on the adopted excitation temperature. The abundances derived from the radiative transfer models agree with the direct determination of the HDO/H₂O abundance ratio for IRAS 16293-2422 within a factor of 2-3, and for IRAS 2A within a factor of 4; the difference is mainly due to optical depth effects in the HDO line. Our HDO/H₂O ratios for the inner regions (where T>100 K) of four young protostars are only a factor of 2 higher than those found for pristine, solar system comets. These small differences suggest that little processing of water occurs between the deeply embedded stage and the formation of planetesimals and comets.