2014A&A...566A..79A

We report the first detections of OH⁺ emission in planetary nebulae (PNe). 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 51µm and 672µm to look for new detections. The rotational emission lines of OH⁺ at 152.99, 290.20, 308.48, and 329.77µm were detected in the spectra of three planetary nebulae: NGC6445, NGC6720, and NGC6781. Excitation temperatures and column densities derived from these lines are in the range of 27-47K and 2x10¹⁰-4x10¹¹cm^–2, respectively. 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_eff>100000K), suggesting that high-energy photons may play a role in OH⁺ formation and its line excitation in these objects, as seems to be the case for ultraluminous galaxies.