[Paper Review] Wolf-Rayet stars

T. Shenar|arXiv (Cornell University)|Oct 6, 2024

History and Developments in Astronomy4 citations

TL;DR

This chapter reviews the observed properties and evolutionary pathways of Wolf-Rayet (WR) stars, emphasizing their role as massive, hot, wind-driven stars that dominate feedback in stellar populations. It examines the formation of classical WR stars via single-star mass loss or binary interactions, highlighting unresolved challenges in wind dynamics and progenitor pathways.

ABSTRACT

Massive Wolf-Rayet (WR) stars comprise a spectroscopic class characterized by high temperatures (Teff > ~30 kK) and powerful and rapid stellar winds. Hydrogen-rich WR stars represent the most massive stars in existence (M > ~100 Msun), while classical WR stars are hydrogen-depleted, evolved massive stars which probe the final evolutionary stages of massive stars prior to core collapse. They dominate entire stellar populations in terms of radiative and mechanical feedback, and are thought to give rise to powerful transients such as hydrogen-stripped supernovae (type Ibc SNe) and long-duration gamma-ray bursts (LGRBs). In this chapter, we summarize the main observed properties of WR populations in our Galaxy and nearby galaxies, and discuss open problems in our understanding of their structure and formation

Motivation & Objective

• To summarize the observed properties of WR stars in the Milky Way and nearby galaxies.
• To examine the formation mechanisms of classical WR stars through single-star mass loss or binary interactions.
• To identify and discuss open problems in WR wind dynamics and stellar structure.
• To assess the role of WR stars in powering transients like type Ibc supernovae and long-duration gamma-ray bursts.
• To evaluate the contribution of WR stars to radiative and mechanical feedback in stellar populations.

Proposed method

• Synthesis of observational data from Galactic and extragalactic WR populations.
• Analysis of spectral energy distributions and emission-line profiles to infer wind properties and effective temperatures.
• Use of hydrodynamic models to simulate radiatively driven winds and assess instabilities and non-radial outflows.
• Comparison of theoretical models with observed binary fractions and mass-loss rates in WR systems.
• Application of the Eddington limit and Rosseland mean opacity to constrain wind acceleration and mass-loss rates.
• Evaluation of the Conti scenario (single-star evolution) and binary channel (common envelope evolution) for WR formation.

Experimental results

Research questions

• RQ1 What are the dominant formation channels for classical WR stars—single-star mass loss or binary interactions?
• RQ2 How do non-radial instabilities and complex wind velocity fields challenge 1D modeling of WR winds?
• RQ3 Why is the observed binary fraction of cWR stars approximately 40–50% and independent of metallicity?
• RQ4 To what extent do WR stars dominate the radiative and mechanical feedback in massive stellar populations?
• RQ5 What is the role of WR stars as progenitors of type Ibc supernovae and long-duration gamma-ray bursts?

Key findings

• Approximately 700 WR stars are known in the Milky Way, with an estimated total population of around 2,000, and thousands more in external galaxies.
• Classical WR stars are hydrogen-depleted, evolved massive stars with masses between 8 and 30 M☉, formed from initial masses >20 M☉ at solar metallicity.
• The observed binary fraction of cWR stars is ~40–50%, suggesting both single-star and binary formation channels are significant.
• WR stars dominate the radiative and mechanical feedback in stellar populations due to their extreme luminosities and powerful winds.
• WR stars are key progenitors of type Ibc supernovae and potential direct progenitors of black holes, including those detected via gravitational waves.
• Strong instabilities and non-radial outflows in WR winds challenge traditional 1D hydrodynamic models, necessitating 3D simulations for accurate parameter derivation.