Massive stars on the verge of exploding: the properties of oxygen sequence Wolf-Rayet stars^⋆,⋆⋆

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, PO Box 94249, 1090 GE Amsterdam The Netherlands
² ASTRON, The Netherlands Institute for Radio Astronomy, PO Box 2, 7790 AA Dwingeloo, The Netherlands
³ Argelander Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁶ Armagh Observatory, College Hill, BT61 9 DG Armagh, UK

Received: 22 November 2014
Accepted: 30 June 2015

Abstract

Context. Oxygen sequence Wolf-Rayet (WO) stars are a very rare stage in the evolution of massive stars. Their spectra show strong emission lines of helium-burning products, in particular highly ionized carbon and oxygen. The properties of WO stars can be used to provide unique constraints on the (post-)helium burning evolution of massive stars, and their remaining lifetimes and the expected properties of their supernovae.

Aims. We aim to homogeneously analyze the currently known presumed-single WO stars to obtain the key stellar and outflow properties and to constrain their evolutionary state.

Methods. We use the line-blanketed non-local thermal equilibrium atmosphere code cmfgen to model the X-Shooter spectra of the WO stars and to deduce the atmospheric parameters. We calculate dedicated evolutionary models to determine the evolutionary state of the stars.

Results. The WO stars have extremely high temperatures that range from 150 kK to 210 kK, and very low surface helium mass fractions that range from 44% down to 14%. Their properties can be reproduced by evolutionary models with helium zero-age main sequence masses of M_He,ini = 15−25 M_⊙ that exhibit a fairly strong (a few times 10^-5M_⊙ yr^-1), homogeneous (f_c> 0.3) stellar wind.

Conclusions. WO stars represent the final evolutionary stage of stars with estimated initial masses of M_ini = 40−60 M_⊙. They are post core-helium burning and predicted to explode as type Ic supernovae within a few thousand years.