The post-common-envelope, binary central star of the planetary nebula Hen 2-11 ^⋆

¹ European Southern Observatory, Alonso de Córdova 3107, 19001 Casilla, Santiago, Chile
e-mail: djones@eso.org; hboffin@eso.org
² Universidad de Atacama, Copayapu 485, Copiapó, Chile
³ South African Astronomical Observatory, PO Box 9, Observatory, 7935 Cape Town, South Africa
⁴ Southern African Large Telescope Foundation, PO Box 9, Observatory, 7935 Cape Town, South Africa
⁵ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁷ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK

Received: 4 October 2013
Accepted: 2 January 2014

Abstract

We present a detailed photometric study of the central star system of the planetary nebula Hen 2-11, selected for study because of its low-ionisation filaments and bipolar morphology – traits which have been strongly linked with central star binarity. Photometric monitoring with NTT-EFOSC2 reveals a highly irradiated, double-eclipsing, post-common-envelope system with a period of 0.609 d. Modelling of the lightcurve indicates that the nebular progenitor is extremely hot, while the secondary in the system is probably a K-type main sequence star. The chemical composition of the nebula is analysed, showing Hen 2-11 to be a medium-excitation non-Type i nebula. A simple photoionisation model is constructed determining abundance ratios of C/O and N/O which would be consistent with the common-envelope cutting short the AGB evolution of the nebular progenitor. The detection of a post-common-envelope binary system at the heart of Hen 2-11 further strengthens the link between binary progeny and the formation of axisymmetric planetary nebulae with patterns of low-ionisation filaments, clearly demonstrating their use as morphological indicators of central star binarity.