Discovery of superthermal hydroxyl (OH) in the HH211 outflow

TL;DR

This paper reports the detection of highly excited hydroxyl (OH) emission in the HH211 outflow, revealing the highest rotational excitation of OH observed outside Earth, likely caused by water photodissociation in shock regions.

Contribution

First observation of superthermal OH emission in a protostellar outflow, demonstrating extreme excitation levels and linking it to UV-driven water dissociation in shocks.

Findings

Detected OH emission lines up to E/k~28200K

Observed high excitation of OH outside Earth

Linked OH excitation to water photodissociation in shocks

Abstract

We present a 5-37 micron infrared spectrum obtained with the Spitzer Space Telescope toward the southeastern lobe of the young protostellar outflow HH211. The spectrum shows an extraordinary sequence of OH emission lines arising in highly excited rotational levels up to an energy E/k~28200K above the ground level. This is, to our knowledge, by far the highest rotational excitation of OH observed outside Earth. The spectrum also contains several pure rotational transitions of H2O (v=0), H2 (v=0) S(0) to S(7), HD (v=0) R(3) to R(6), and atomic fine-structure lines of [Fe II], [Si II], [Ne II], [S I], and [Cl I]. The origin of the highly excited OH emission is most likely the photodissociation of H2O by the UV radiation generated in the terminal outflow shock of HH211.