Herschel Planetary Nebula Survey (HerPlaNS) - First Detection of OH+ in Planetary Nebulae

Abstract

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 51 and 672$\mu$m to look for new detections. OH${}^{+}$ rotational emission lines 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 to 47 K and 2$\times$10${}^{10}$ to 4 $\times$10${}^{11}$ cm${}^{-2}$, 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${}_{eff}$ > 100000 K), suggesting that high-energy photons may play a role in the OH+ formation and its line excitation in these objects, as it seems to be the case for ultraluminous galaxies.

arXiv comment9 pages, 7 figures; accepted for publication in A&A

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• An Imaging Spectroscopic Survey of the Planetary Nebula NGC 7009 with MUSE
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• Electron-impact excitation of diatomic hydride cations II: OH${}^{+}$ and SH${}^{+}$
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• High Resolution Laboratory Spectroscopy
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