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

TL;DR

This study uses high-resolution millimeter observations to constrain the HDO/H2O ratio in the inner regions of a low-mass protostar, finding it similar to that in comets and Earth's oceans, challenging previous assumptions.

Contribution

It provides the first direct upper limit on the HDO/H2O ratio in the warm inner regions of a low-mass protostar using high-resolution data.

Findings

HDO line not detected, setting an upper limit on the ratio

HDO/H2O ratio in the inner 50 AU is not significantly enhanced

Results challenge previous suggestions of elevated HDO/H2O ratios in such systems

Abstract

The HDO/H2O abundance ratio is thought to be a key diagnostic on the evolution of water during the star- and planet-formation process and thus on its origin on Earth. We here present millimeter-wavelength high angular resolution observations of the deeply embedded protostar NGC1333-IRAS4B from the Submillimeter Array targeting the 3(1,2)-2(2,1) transition of HDO at 225.6 GHz (Eu = 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, H2-18-O, 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 6e(-4) (3sigma) 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.