We report the detection of shock-excited far-infrared emission of H _ { 2 } O , OH , and CO from the supernova remnant 3C 391 , using the ISO Long-Wavelength Spectrometer .
This is the first detection of thermal H _ { 2 } O and OH emission from a supernova remnant .
For two other remnants , W 28 and W 44 , CO emission was detected but OH was only detected in absorption .
The observed H _ { 2 } O and OH emission lines arise from levels within \sim 400 K of the ground state , consistent with collisional excitation in warm , dense gas created after the passage of the shock front through the dense clumps in the pre-shock cloud .
The post-shock gas we observe has a density \sim 2 \times 10 ^ { 5 } cm ^ { -3 } and temperature 100-1000 K , and the relative abundances of CO : OH : H _ { 2 } O in the emitting region are 100:1:7 for a temperature of 200 K. The presence of a significant column of warm H _ { 2 } O suggests that the chemistry has been significantly changed by the shock .
The existence of significant column densities of both OH and H _ { 2 } O , which is at odds with models for non-dissociative shocks into dense gas , could be due to photodissociation of H _ { 2 } O or a mix of fast and slow shocks through regions with different pre-shock density .