Mass Loss: Its Effect on the Evolution and Fate of High-Mass Stars

Abstract: Our understanding of massive star evolution is in flux, due to recent upheavals in our view of mass loss, and observations of a high binary fraction among O-type stars. Mass-loss rates for standard metallicity-dependent winds of hot stars are now thought to be lower by a factor of 2-3 compared to rates adopted in modern stellar evolution models, due to the influence of clumping. Weaker line-driven winds shift the burden of H-envelope removal elsewhere, so that the dominant modes of mass loss are the winds, pulsations, and eruptions of evolved supergiants, as well as binary mass transfer. Studies of stripped-envelope supernovae, in particular, require binary mass transfer. Dramatic examples of eruptive mass loss are seen in Type IIn supernovae, which have massive shells ejected just a few years before core collapse. The shifting emphasis from steady winds to episodic mass loss is a major change for low-metallicity regions, since eruptions and binary mass transfer are less sensitive to metallicity. We encounter the predicament that the most important modes of mass loss are also the most uncertain, undermining the predictive power of single-star evolution models beyond core H burning. Moreover, the influence of winds and rotation in models has been evaluated by testing single-star models against observed statistics that, as it turns out, are heavily influenced by binary evolution. This alters our view about the most basic outcomes of massive-star mass loss --- are WR stars and SNe Ibc the products of single-star winds, or are they mostly the result of binary evolution and eruptive mass loss? This paradigm shift has far-reaching impact on a number of other areas of astronomy. (abridged)

• The paper reassesses mass-loss rates, finding them 2-3 times lower due to the effects of clumping in stellar winds.
• The study highlights binary mass transfer as a critical mechanism for H-envelope removal in massive stars.
• The findings challenge traditional models and call for integrated approaches combining single-star and binary evolution.

Mass Loss: Its Effect on the Evolution and Fate of High-Mass Stars

This paper explores the pivotal role of mass loss in the evolutionary trajectories and endpoints of high-mass stars. Recent reassessments of mass-loss rates and observations of extensive binarity among O-type stars have significantly disrupted traditional understandings of massive star evolution. The focus is on the implications of revised mass-loss rates, particularly for standard line-driven winds, and how these shifts demand reconsidering the mechanisms responsible for H-envelope removal.

Revisiting Mass Loss and Clumping

Recent studies have revealed that mass-loss rates for metallicity-dependent line-driven winds in hot stars are lower by a factor of 2-3 than those traditionally adopted in stellar evolution models. The empirical reduction in mass-loss estimates stems from clumping's influence, which affects density diagnostics used in deriving mass-loss rates. This re-evaluation shifts the responsibility for H-envelope removal to other mechanisms, such as pulsations, eruptions in evolved supergiants, and especially binary mass transfer.

Binary Mass Transfer and Stellar Evolution

Binary systems play a crucial role in the mass-loss processes of high-mass stars, particularly through mass transfer effects. Observations indicate that a high fraction of O-type stars are in binary systems, implying a dramatic role for binarity in massive star evolution. The reduced effectiveness of single-star models in explaining observed phenomena suggests that binary interactions may more frequently produce Wolf-Rayet stars and stripped-envelope supernovae (SESNe).

Challenges in Predictive Modeling

The most significant modes of mass loss — namely, LBV eruptions, rotational influences, and binary interactions — are simultaneously the least well-constrained, posing challenges for predictive stellar evolution models beyond core hydrogen burning. The limitations in current single-star models become evident when confronted with binary evolution's contributions to observed stellar statistics. A prominent question remains regarding the balance between single-star wind and binary evolution in producing H-poor end states such as WR stars and SESNe.

Implications for Broader Astrophysics

The shifted paradigms in understanding high-mass stellar evolution carry implications far beyond stellar astrophysics. They impact predictions for ionizing radiation and wind feedback in stellar populations, possibly necessitating revisions in assumptions for star formation rates and initial mass functions across the cosmos. Understanding the origins of compact remnants such as black holes and neutron stars, and interpreting supernovae as evolutionary probes over cosmic distances, are also influenced by these evolving insights.

Future Directions

The paper suggests a multi-faceted approach to improve the understanding of high-mass stellar evolution. This includes using binary evolution models alongside updated single-star tracks with properly adjusted mass-loss prescriptions that account for clumping. Moreover, direct observational studies of mass transfer and merger events in binaries, as well as more sophisticated modeling of episodic mass loss and its triggers, could further illuminate the complex processes shaping the evolution and ultimate fate of massive stars. The integration of these elements is crucial for developing a coherent and comprehensive picture of high-mass stellar evolution.

In summarizing, the paper provides a robust reassessment of mass loss in high-mass stars, driven by both observations and theoretical considerations. By emphasizing the increasing relevance of binary interactions and refined mass-loss predictions, it forms a foundation for more accurate models that align with observed stellar phenomena, necessitating a careful consideration of the interplay between single-star and binary evolution in shaping the cosmos.