The HDO/H2O ratio in gas in the inner regions of a low-mass protostar

• Astrophysical Journal

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 3₁₂-2₂₁ 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, H¹⁸₂O, 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 6 × 10^–4 (3σ) 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 ≈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.