2013A&A...549L...3P

Measuring the water deuterium fractionation in the inner warm regions of low-mass protostars has so far been hampered by poor angular resolution obtainable with single-dish ground- and space-based telescopes. Observations of water isotopologues using (sub)millimeter wavelength interferometers have the potential to shed light on this matter. To measure the water deuterium fractionation in the warm gas of the deeply-embedded protostellar binary IRAS 16293-2422. Observations toward IRAS 16293-2422 of the 5₃₂-4₄₁ transition of H₂¹⁸O at 692.07914GHz from Atacama Large Millimeter/submillimeter Array (ALMA) as well as the 3₁₃-2₂₀ of H₂¹⁸O at 203.40752GHz and the 3₁₂-2₂₁ transition of HDO at 225.89672GHz from the Submillimeter Array (SMA) are presented. The 692GHz H₂¹⁸O line is seen toward both components of the binary protostar. Toward one of the components, ``source B'', the line is seen in absorption toward the continuum, slightly red-shifted from the systemic velocity, whereas emission is seen off-source at the systemic velocity. Toward the other component, ``source A'', the two HDO and H₂¹⁸O lines are detected as well with the SMA. From the H₂¹⁸O transitions the excitation temperature is estimated at 124±12K. The calculated HDO/H₂O ratio is (9.2±2.6)x10^–4 - significantly lower than previous estimates in the warm gas close to the source. It is also lower by a factor of ∼5 than the ratio deduced in the outer envelope. Our observations reveal the physical and chemical structure of water vapor close to the protostars on solar-system scales. The red-shifted absorption detected toward source B is indicative of infall. The excitation temperature is consistent with the picture of water ice evaporation close to the protostar. The low HDO/H₂O ratio deduced here suggests that the differences between the inner regions of the protostars and the Earth's oceans and comets are smaller than previously thought.