The chemical composition of TS 01, the most oxygen-deficient planetary nebula^*,**,***

AGB nucleosynthesis in a metal-poor binary star

¹ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France e-mail: grazyna.stasinska@obspm.fr
² Instituto de Astronomia, Universidad Nacional Autonoma de Mexico, Apdo. Postal 70264, Mexico D.F., 04510 Mexico
³ Instituto de Astronomia, Universidad Nacional Autonoma de Mexico, Apdo. Postal 877, Ensenada, Baja California, 22800 Mexico
⁴ Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Sand 1, 72076 Tübingen, Germany
⁵ N. Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland
⁶ Argelander Institute for Astronomy (AIfA), Auf dem Hügel 71, 53121 Bonn, Germany
⁷ Geneva Observatory, University of Geneva, ch. des Maillettes 51, 1290 Sauverny, Switzerland and Laboratoire d'Astrophysique de Toulouse-Tarbes, CNRS UMR 5572, Université de Toulouse, 14, Av. E.Belin, 31400 Toulouse, France
⁸ Institute of Astronomy of the Russian Academy of Sciences, 48 Pyatniskaya Str., 119017 Moscow, Russia
⁹ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL109AB, UK
¹⁰ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain

Received: 30 April 2009
Accepted: 15 December 2009

Abstract

The planetary nebula TS 01 (also called PN G 135.9+55.9 or SBS 1150+599A) with its record-holding low oxygen abundance and its double degenerate close binary core (period 3.9 h) is an exceptional object located in the Galactic halo.
We have secured observational data in a complete wavelength range to pin down the abundances of half a dozen elements in the nebula. The abundances are obtained via detailed photoionization modelling which takes into account all the observational constraints (including geometry and aperture effects) using the pseudo-3D photoionization code Cloudy_3D. The spectral energy distribution of the ionizing radiation is taken from appropriate model atmospheres. Incidentally we find from the new observational constraints that both stellar components contribute to the ionization: the “cool” one provides the bulk of hydrogen ionization, while the “hot” one is responsible for the presence of the most highly charged ions, which explains why previous attempts to model the nebula experienced difficulties. 
The nebular abundances of C, N, O, and Ne are found to be 1/3.5, 1/4.2, 1/70, and 1/11 of the solar value respectively, with uncertainties of a factor 2. Thus the extreme O deficiency of this object is confirmed. The abundances of S and Ar are less than 1/30 of solar. The abundance of He relative to H is 0.089 ± 0.009. 
Standard models of stellar evolution and nucleosynthesis cannot explain the abundance pattern observed in the nebula. To obtain an extreme oxygen deficiency in a star whose progenitor has an initial mass of about 1  requires an additional mixing process, which can be induced by stellar rotation and/or by the presence of the close companion. We have computed a stellar model with an initial mass of 1 , appropriate metallicity, and initial rotation of 100 km s^-1, and find that rotation greatly improves the agreement between the predicted and observed abundances.