Herschel Planetary Nebula Survey (HerPlaNS)

First detection of OH⁺ in planetary nebulae^⋆

¹ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: aleman@strw.leidenuniv.nl
² Department of Physics and Astronomy, University of Denver, 2112 E. Wesley Ave., Denver, CO 80210, USA
³ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yosinodai, Chuo-ku, Sagamihara, 252-5210 Kanagawa, Japan
⁴ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁵ Chester F. Carlson Center for Imaging Science and Laboratory for Multiwavelength Astrophysics, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623, USA
⁶ Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
⁷ Department of Astronomy, University of Illinois, 1002 W. Green St., Urbana, IL 61801, USA
⁸ Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, 3037-5 Honjo, Kamogata, Asakuchi, 719-0232 Okayama, Japan
⁹ Jodrell Bank Centre for Astrophysics, Alan Turing Building, The University of Manchester, M13 9PL, UK
¹⁰ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
¹¹ N. Copernicus Astronomical Center, Rabianska 8, 87-100 Torun, Poland
¹² Royal Observatory of Belgium, Ringlaan 3, 1180 Brussel, Belgium
¹³ Universidad Autónoma de Madrid, Dpto. Física Teórica, Módulo 15, Facultad de Ciencias, Campus de Cantoblanco, 28049 Madrid, Spain
¹⁴ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
¹⁵ ESO, Karl Schwarzschild Str. 2, 85748 Garching bei München, Germany

Received: 29 October 2013
Accepted: 7 April 2014

Abstract

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 51μm and 672μm to look for new detections.

Results. 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: 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 × 10¹⁰–4 × 10¹¹ 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_eff> 100 000 K), 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.