Context :

Aims : We report the first detections of OH ^ { + } emission in planetary nebulae ( PNe ) .

Methods : As part of an imaging and spectroscopy survey of 11 PNe in the far-IR using the PACS and SPIRE instruments aboard the Herschel Space Observatory , we performed a line survey in these PNe over the entire spectral range between 51 \mu m and 672 \mu m to look for new detections .

Results : The rotational emission lines of OH ^ { + } at 152.99 , 290.20 , 308.48 , and 329.77 \mu m were detected in the spectra of three planetary nebulae : NGC 6445 , NGC 6720 , and NGC 6781 .
Excitation temperatures and column densities derived from these lines are in the range of 27–47 K and 2 \times 10 ^ { 10 } –4 \times 10 ^ { 11 } cm ^ { -2 } , respectively .

Conclusions : In PNe , the OH ^ { + } rotational line emission appears to be produced in the photodissociation region ( PDR ) in these objects .
The emission of OH ^ { + } is observed only in PNe with hot central stars ( T _ { \mathrm { eff } } > 100 000 K ) , suggesting that high-energy photons may play a role in the OH ^ { + } formation and its line excitation in these objects , as seems to be the case for ultraluminous galaxies .