THE HDO/H2O RATIO IN GAS IN THE INNER REGIONS OF A LOW-MASS PROTOSTAR

The HDO/H2O abundance ratio is thought to be a key diagnostic for the evolution of water during the star and planet formation process and thus for its origin on Earth. We here present millimeter-wavelength high angular resolution observations of the deeply embedded protostar NGC 1333-IRAS4B from the Submillimeter Array targeting the 3(12)-2(21) transition of HDO at 225.6 GHz (E-u = 170 K). We do not (or only very tentatively) detect the HDO line toward the central protostar, contrasting the previous prominent detection of a line from another water isotopologue, (H2O)-O-18, with similar excitation properties using the IRAM Plateau de Bure Interferometer. The non-detection of the HDO line provides a direct, model-independent, upper limit to the HDO/H2O abundance ratio of 6x10(-4) (3 sigma) in the warm gas associated with the central protostar. This upper limit suggests that the HDO/H2O abundance ratio is not significantly enhanced in the inner approximate to 50 AU around the protostar relative to what is seen in comets and Earth's oceans and does not support previous suggestions of a generally enhanced HDO/H2O ratio in these systems.