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(Abridged) The water deuterium fractionation (HDO/H$_2$O abundance ratio) hastraditionally been used to infer the amount of water brought to Earth bycomets. Measuring this ratio in deeply-embedded low-mass protostars makes itpossible to probe the critical stage when water is transported from clouds todisks in which icy bodies are formed. We present sub-arcsecond resolutionobservations of HDO in combination with H$_2^{18}$O from the PdBI toward thethree low-mass protostars NGC 1333-IRAS 2A, IRAS 4A-NW, and IRAS 4B. Theresulting HDO/H$_2$O ratio is $7.4\pm2.1\times10^{-4}$ for IRAS 2A,$19.1\pm5.4\times10^{-4}$ for IRAS 4A-NW, and $5.9\pm1.7\times10^{-4}$ for IRAS4B. Derived ratios agree with radiative transfer models within a factor of 2-4depending on the source. Our HDO/H$_2$O ratios for the inner regions (where$T>100$ K) of four young protostars are only a factor of 2 higher than thosefound for pristine, solar system comets. These small differences suggest thatlittle processing of water occurs between the deeply embedded stage and theformation of planetesimals and comets.

The deuterium fractionation of water on solar-system scales in deeply-embedded low-mass protostars