When nature tries to trick us

An eclipsing eccentric close binary superposed on the central star of the planetary nebula M 3-2^⋆,⋆⋆

¹ European Southern Observatory, Karl-Schwarzschild-str. 2, 85748 Garching, Germany
e-mail: hboffin@eso.org
² Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁴ Department of Physics and Astronomy, University College London, Gower St, London, WC1E 6BT, UK
⁵ Las Campanas Observatory, Carnegie Institution of Washington, La Serena, Chile
⁶ South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
⁷ Southern African Large Telescope Foundation, PO Box 9, Observatory 7935, South Africa
⁸ European Southern Observatory, Alonso de Córdova 3107, Casilla, 19001, Santiago Chile
⁹ Departamento de Ciencias Físicas, Universidad Andres Bello, Fernández Concha 700, Las Condes, Santiago
¹⁰ GRANTECAN, Cuesta de San José s/n, 38712 Breña Baja, La Palma, Spain
¹¹ Observatorio Astronómico Nacional (OAN-IGN), C/Alfonso XII 3, 28014 Madrid, Spain

Received: 21 June 2018
Accepted: 27 July 2018

Abstract

Bipolar planetary nebulae (PNe) are thought to result from binary star interactions and, indeed, tens of binary central stars of PNe have been found, in particular using photometric time-series that allow for the detection of post-common envelope systems. Using photometry at the NTT in La Silla we have studied the bright object close to the centre of PN M 3-2 and found it to be an eclipsing binary with an orbital period of 1.88 days. However, the components of the binary appear to be two A or F stars, of almost equal mass, and are therefore too cold to be the source of ionisation of the nebula. Using deep images of the central star obtained in good seeing conditions, we confirm a previous result that the central star is more likely much fainter, located 2″ away from the bright star. The eclipsing binary is thus a chance alignment on top of the planetary nebula. We also studied the nebular abundance and confirm it to be a Type I PN.