Abstract
AbstractWe 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 H2O 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 H2O molecules when compared to the terminal outflow shocks. The very low ortho-to-para ratio suggests that H2O around the protostar originates from primordial envelope material that has never been thermally processed before.
Highlights
Observations of the abundance ratio of nuclear spin isomers of H2O can possibly provide clues about the formation conditions and the thermal processing of molecules
We report on the detection of a rich water reservoir in the protostellar envelope of the Class 0 source HH211
The very low ortho-to-para ratio suggests that H2O around the protostar originates from primordial envelope material that has never been thermally processed before
Summary
Observations of the abundance ratio of nuclear spin isomers of H2O can possibly provide clues about the formation conditions and the thermal processing of molecules. Low OPR can be associated with the formation of H2O at very low temperature environments. Processes leading to the desorption of water into the gas phase can alter the low-temperature OPR and modify it toward the thermal equilibrium value of 3. Non-destructive photo-desorption of H2O is believed to preserve the original formation value of OPR (van Dishoeck et al 2013), providing clues about the thermal history of the molecule. We here report on the detection of an unusually low water - OPR in the envelope of HH 211, indicating that the water originates from primordial, unprocessed envelope material
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