Warm water deuterium fractionation in IRAS 16293-2422. The high-resolution ALMA and SMA view

Abstract

Context. 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.

Aims: To measure the water deuterium fractionation in the warm gas of the deeply-embedded protostellar binary IRAS 16293-2422.

Methods: Observations toward IRAS 16293-2422 of the 5$_{3,2}$ - 4$_{4,1}$ transition of H$_{2}$$^{18}$O at 692.07914 GHz from Atacama Large Millimeter/submillimeter Array (ALMA) as well as the 3$_{1,3}$ - 2$_{2,0}$ of H$_{2}$$^{18}$O at 203.40752 GHz and the 3$_{1,2}$ - 2$_{2,1}$ transition of HDO at 225.89672 GHz from the Submillimeter Array (SMA) are presented.

Results: The 692 GHz H$_{2}$$^{18}$O line is seen toward both components of the binary protostar. Toward one of the components, ''source B'', the line is seen in absorption toward the continuum, slightly red-shifted from the systemic velocity, whereas emission is seen off-source at the systemic velocity. Toward the other component, ''source A'', the two HDO and H$_{2}$$^{18}$O lines are detected as well with the SMA. From the H$_{2}$$^{18}$O transitions the excitation temperature is estimated at 124 {plusmn} 12 K. The calculated HDO/H$_{2}$O ratio is (9.2 {plusmn} 2.6) { imes} 10$^{-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. Conclusions: 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/H$_{2}$O 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. Appendix A is available in electronic form at http://www.aanda.org