The influence of binarity on dust obscuration events in the planetary nebula M 2-29 and its analogues^⋆

¹ Centre for Astrophysics Research, STRI, University of Hertfordshire, College Lane Campus, Hatfield AL10 9AB, UK
e-mail: b.miszalski@herts.ac.uk
² Observatoire Astronomique, Université de Strasbourg, 11 rue de l’Université, 67000 Strasbourg, France
³ Department of Physics, Macquarie University, NSW 2109, Sydney, Australia
⁴ Nicolaus Copernicus Astronomical Centre, Bartycka 18, 00716 Warsaw, Poland
e-mail: mikolaj@camk.edu.pl
⁵ International Space University, Parc d’Innovation, 1 Rue Jean-Dominique Cassini, 67400 Illkirch-Graffenstaden, France
⁶ Institut für Theorestische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany
⁷ Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Sand 1, 72076 Tübingen, Germany
⁸ Radio Astronomy Laboratory, University of California, Berkeley, CA 94720, USA
⁹ Dépt. de physique, Univ. de Montréal C.P. 6128, Succ. Centre-Ville, Montréal, QC H3C 3J7, and Centre de recherche en astrophysique de Québec, Canada
¹⁰ Australian Astronomical Observatory, NSW 1710, Epping, Australia
¹¹ School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand
¹² Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warsaw, Poland

Received: 24 July 2010
Accepted: 24 January 2011

Abstract

The central star of the planetary nebula (CSPN) M 2-29 shows an extraordinary R Coronae Borealis-like fading event in its optical lightcurve. The only other CSPN to show these events are CPD-56°8032 (Hen 3-1333) and V651 Mon (NGC 2346). Dust cloud formation in the line of sight appears responsible but the exact triggering mechanism is not well understood. Understanding how planetary nebulae (PNe) trigger dust obscuration events may help understand the same process in a wide range of objects including Population-I WC9 stars, symbiotic stars and perhaps asymptotic giant branch (AGB) stars with long secondary periods (LSPs). A binary scenario involving an eccentric, wide companion that triggers dust formation via interaction at periastron is a potential explanation that has been suggested for LSP variables. Model fits to the lightcurves of CPD-56°8032 and M 2-29 show the dust forms in excess of 70 AU at the inner edge of a dust disk. In the case of CPD-56°8032 this radius is far too large to coincide with a binary companion trigger, although a binary may have been responsible for the formation of the dust disk. We find no direct evidence to support previous claims of binarity in M 2-29 either from the OGLE lightcurve or deep medium-resolution VLT FLAMES spectroscopy of the CSPN. We classify the CSPN as Of(H) with T_eff = 50 ± 10 kK and log g = 4.0 ± 0.3. We find a mean distance of 7.4 ± 1.8 kpc to M 2-29 at which the M_V =  −0.9 mag CSPN could potentially hide a subgiant luminosity or fainter companion. A companion would help explain the multiple similarities with D′-type symbiotic stars whose outer nebulae are thought to be bona-fide PNe. The 7.4 kpc distance, oxygen abundance of 8.3 dex and Galactic coordinates (ℓ = 4.0, b =  −3.0) prove that M 2-29 is a Galactic Bulge PN and not a Halo PN as commonly misconceived.