Massive stars on the verge of exploding: the properties of oxygen sequence Wolf-Rayet stars (1507.00839v1)

Abstract: Context. Oxygen sequence Wolf-Rayet (WO) stars represent 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, as well as their remaining lifetime and the expected properties of their supernovae. Aims. We aim to homogeneously analyse 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 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 $M_{\mathrm{He, ini}} = 15-25 M_{\odot}$ that exhibit a fairly strong (on the order of a few times $10^{-5} M_{\odot} \mathrm{yr}^{-1}$), homogeneous ($f_\mathrm{c} > 0.3$) stellar wind. Conclusions. WO stars represent the final evolutionary stage of stars with estimated initial masses of $M_{\mathrm{ini}} = 40-60 M_{\odot}$. They are post core-helium burning and predicted to explode as type Ic supernovae within a few thousand years.

• The paper reveals that WO stars exhibit extremely high temperatures (150–210 kK) and low helium mass fractions (14–44%), marking a late evolutionary stage.
• It employs cmfgen modeling of high-resolution X-Shooter spectra to determine mass-loss rates (0.8–1.5×10⁻⁵ M⊙ yr⁻¹) that challenge existing models.
• The study implies that enhanced mass-loss mechanisms and binary interactions may accelerate these stars' evolution toward type Ic supernova explosions.

Overview of Oxygen Sequence Wolf-Rayet Stars and Their Evolutionary Significance

This paper provides a comprehensive analysis of the rare class of massive stars known as oxygen sequence Wolf-Rayet (WO) stars. Emphasizing their evolutionary stage, surface properties, and potential explosive outcomes, the work builds on observations from the VLT's X-Shooter instrument, aiming to elucidate the status and future of these intriguing celestial objects.

Characteristics and Analysis of WO Stars

WO stars represent one of the final evolutionary phases in the life cycle of massive stars, as they are characterized by surface layers rich in helium-burning products such as carbon and oxygen. This paper homogeneously models the currently known presumed-single WO stars, employing the cmfgen code to decipher their X-Shooter spectra.

The findings reveal that WO stars exhibit extremely high temperatures ranging from 150 kK to 210 kK, accompanied by remarkably low surface helium mass fractions, between 44% and as low as 14%. These characteristics suggest a significant evolution from stronger winds at earlier stellar phases. The authors align these observations with theoretical models where stars have initial helium-core masses of 15-25 M⊙ and a robust, homogeneous stellar wind.

Evolutionary Context and Implications

The evolutionary models proposed in the paper deduce that WO stars are likely in the post-core helium burning phase, with expected initial masses ranging from 40-60 M⊙. These models predict that WO stars will culminate their lifecycle in a type Ic supernova. Given their imminent explosive fate within several thousand years, WO stars provide a unique snapshot of the terminal evolution of massive stars.

This paper challenges existing evolutionary tracks by highlighting discrepancies that suggest current mass-loss models, such as those by Nugis or Hamann, may underestimate the mass-loss rates especially for massive stars in stages preceding the WO phase. Such insights imply that WO stars might require earlier or more intense mass-loss mechanisms, potentially {through} LBV phases or binary interactions, to reach their observed states.

Numerical Results and Comparisons

The paper presents strong numerical outcomes with WO stars exhibiting mass-loss rates between 0.8-1.5 × 10^{-5} M⊙ yr^{-1}. This surpasses the predictions by existing WR star mass-loss models, highlighting potential areas for adjustment in those formulations. The terminal wind velocities were observed to scale with metallicity, further reinforcing the connection between a star's environment and its wind dynamics.

Future Prospects and Theoretical Considerations

While the data provide a significant step in mapping the life cycle of massive stars, the research opens avenues for further refinement of nuclear reaction rates like the $^{12}\mathrm{C}(\alpha,\gamma)^{16}\mathrm{O}$ thermonuclear reaction, which remains weakly constrained but critically influences late-stage stellar evolution and supernova yields.

These insights also prompt further investigations into how binary evolution scenarios could shape the lifecycles of WO predecessors. The rare observations of these stars, coupled with advanced modeling approaches, continue to sharpen our understanding of massive star evolution, paving the way for new theoretical and observational advancements in stellar astrophysics.