Discovery of a primordial water reservoir in the envelope of HH 211

摘要

We report on the detection of a rich water reservoir in the protostellar envelope of the Class 0 source HH211. In striking contrast to all other molecules detected with Herschel/PACS, water emission peaks around the central source where both ortho and para forms are detected. The measured ortho-to-para ratio of just 0.65 indicates formation of water-ice at very low temperatures and a non-destructive photo-desorption process around the protostar. While part of the H_2O emission is likely related to collisional excitation, the centralized morphology around the protostar suggests that radiative excitation is also significant, despite the fact that radiation appears to have a very different impact on the H_2O molecules when compared to the terminal outflow shocks. The very low ortho-to-para ratio suggests that H_2O around the protostar originates from primordial envelope material that has never been thermally processed before.