A barium central star binary in the Type I diamond ring planetary nebula Abell 70

Authors

  • B. Miszalski,

    Corresponding author
    1. South African Astronomical Observatory, PO Box 9, Observatory, 7935, South Africa
    2. Southern African Large Telescope Foundation, PO Box 9, Observatory, 7935, South Africa
    3. Centre for Astrophysics Research, STRI, University of Hertfordshire, College Lane Campus, Hatfield AL10 9AB
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  • H. M. J. Boffin,

    1. European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Santiago, Chile
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  • D. J. Frew,

    1. Department of Physics and Astronomy, Macquarie University, Sydney, NSW 2109, Australia
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  • A. Acker,

    1. Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l’Université, F-67000 Strasbourg, France
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  • J. Köppen,

    1. Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l’Université, F-67000 Strasbourg, France
    2. International Space University, Parc d’Innovation, 1 Rue Jean-Dominique Cassini, F-67400 Illkirch-Graffenstaden, France
    3. Institut für Theorestische Physik und Astrophysik, Universität Kiel, D-24098 Kiel, Germany
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  • A. F. J. Moffat,

    1. Déptartment de physique, Université de Montréal C.P. 6128, Succ. Centre-Ville, Montréal, QC H3C 3J7, and Centre de recherche en astrophysique du Québec, Canada
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  • Q. A. Parker

    1. Department of Physics and Astronomy, Macquarie University, Sydney, NSW 2109, Australia
    2. Australian Astronomical Observatory, Epping, NSW 1710, Australia
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Based on observations made with Gemini South under the programme GS-2009A-Q-35 and the Very Large Telescope (VLT) at Paranal Observatory under programmes 083.D-0654(A) and 085.D-0629(A).

E-mail: brent@saao.ac.za

ABSTRACT

Abell 70 (PN G038.1−25.4, hereafter A 70) is a planetary nebula known for its diamond ring appearance due to a superposition with a background galaxy. The previously unstudied central star is found to be a binary consisting of a G8IV–V secondary at optical wavelengths and a hot white dwarf at ultraviolet wavelengths. The secondary shows Ba ii and Sr ii features enhanced for its spectral type that, combined with the chromospheric Hα emission and possible 20–30 km s−1 radial velocity amplitude, firmly classifies the binary as a Barium star. The proposed origin of Barium stars is intimately linked to planetary nebulae (PNe) whereby wind accretion pollutes the companion with dredged-up material rich in carbon and s-process elements when the primary is experiencing thermal pulses on the asymptotic giant branch (AGB). A 70 provides further evidence for this scenario together with the other very few examples of Barium central stars. The nebula is found to have Type I chemical abundances with helium and nitrogen enrichment, which when combined with future abundance studies of the central star, will establish A 70 as a unique laboratory for studying s-process AGB nucleosynthesis. We also discuss guidelines to discover more binary central stars with cool secondaries in large orbits that are needed to balance our knowledge of binarity in PNe against the currently better studied post-common-envelope binary central stars.

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