Appraising the Necklace: A post-common-envelope carbon dwarf inside an apparently carbon-poor planetary nebula

¹ Instituto de Astrofísica de Canarias E-38205 La Laguna, Spain
² Departamento de Astrofísica, Universidad de La Laguna E-38206 La Laguna, Spain
³ GRANTECAN Cuesta de San José s/n E-38712 Breña Baja La Palma, Spain
⁴ Instituto de Astronomía (IA), Universidad Nacional Autónoma de México Apdo. postal 106 C.P. 22800 Ensenada Baja California, México
⁵ Instituto de Ciencias Físicas (ICF), Universidad Nacional Autónoma de México Av. Universidad s/n 62210 Cuernavaca Morelos, México
⁶ Department of Astronomy, University of Washington Seattle WA 98195-1580, USA
⁷ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park 146 Brownlow Hill Liverpool L3 5RF, UK
⁸ Department of Physics, University of Warwick Gibbet Hill Road Coventry CV4 7AL, United Kingdom
⁹ Centro de Astrobiología (CSIC/INTA) 28850 Torrejón de Ardoz Madrid, Spain
¹⁰ Observatorio Astronómico Nacional (OAN-IGN), Alfonso XII 3 28014 Madrid, Spain
¹¹ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The University of Manchester Oxford Road Manchester M13 9PL, UK
¹² Department of Physics and Astronomy, Valparaiso University Valparaiso IN 46383, USA
¹³ Department of Physics, University of Oxford Parks Rd Oxford OX1 3PU, United Kingdom
¹⁴ Nordic Optical Telescope Rambla José Ana Fernández Pérez 7 38711 Breña Baja, Spain
¹⁵ School of Physics, Trinity College Dublin, The University of Dublin, College Green Dublin 2 Dublin, Ireland

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Received: 21 October 2025
Accepted: 26 January 2026

Abstract

Context. The Necklace nebula is a bipolar, post-common-envelope planetary nebula, the central star of which has been shown to have a dwarf carbon star companion.

Aims. We aim to understand the origins of the Necklace and its dwarf carbon central star.

Methods. We study the carbon abundance of the nebula through far-ultraviolet spectroscopy obtained with the Hubble Space Telescope. Furthermore, through simultaneous modelling of multi-band light and velocity curves, we attempt to constrain the parameters of the central star system.

Results. Puzzlingly, we find that the region of the inner nebula observed with the Hubble Space Telescope is seemingly not carbon-rich, at odds with the dwarf carbon star nature of the companion of the central star. The initial mass of the nebular progenitor was likely very close to the limit to become carbon-rich, perhaps experiencing a very late thermal pulse. The dwarf carbon star companion is found to be significantly inflated with respect to that expected for an isolated main-sequence star of the same mass.

Conclusions. The properties of the central binary are consistent with the progenitor having become carbon-rich and its companion having accreted a significant amount of that carbon-enriched material. However, it is unclear how this evolutionary hypothesis can be reconciled with the inner nebula potentially being carbon-poor.