Wave-Driven Mass Loss in the Last Year of Stellar Evolution: Setting the Stage for the Most Luminous Core-Collapse Supernovae

Abstract: During the late stages of stellar evolution in massive stars (C fusion and later), the fusion luminosity in the core of the star exceeds the star's Eddington luminosity. This can drive vigorous convective motions which in turn excite internal gravity waves. The local wave energy flux excited by convection is itself well above Eddington during the last few years in the life of the star. We suggest that an interesting fraction of the energy in gravity waves can, in some cases, convert into sound waves as the gravity waves propagate (tunnel) towards the stellar surface. The subsequent dissipation of the sound waves can unbind up to several $M_\odot$ of the stellar envelope. This wave-driven mass loss can explain the existence of extremely large stellar mass loss rates just prior to core-collapse, which are inferred via circumstellar interaction in some core-collapse supernovae (e.g., SNe 2006gy and PTF 09uj, and even Type IIn supernovae more generally). An outstanding question is understanding what stellar parameters (mass, rotation, metallicity, age) are the most susceptible to wave-driven mass loss. This depends on the precise internal structure of massive stars and the power-spectrum of internal gravity waves excited by stellar convection.

• The paper demonstrates that vigorous convective motions excite gravity waves which exceed the Eddington limit, driving large envelope ejections.
• It uses a 40 M☉ star model to detail how wave energy converts to acoustic waves, leading to dissipation near the stellar surface.
• The study implies that pre-supernova mass loss creates dense circumstellar environments crucial for interpreting luminous, interaction-driven supernova observations.

Overview of Wave-Driven Mass Loss in Massive Stellar Evolution

The dynamics of stellar mass loss in the final years of massive stellar evolution is a critical facet that influences the optical properties of core-collapse supernovae. The paper by Quataert and Shiode presents a comprehensive analysis of wave-driven mass loss in massive stars during the late stages of their life cycle, particularly focusing on the phases of carbon fusion and beyond. The study underscores the transient and intense nature of mass loss episodes that are driven by internal stellar processes involving fusion exceeding the Eddington luminosity.

Key Findings

At the core of the paper's analysis is the proposition that massive stars undergoing late-stage fusion can generate vigorous convective motion, which excites internal gravity waves. The convective energy flux exceeds the Eddington limit, sufficiently vigorous to transport energy as gravity waves. A fraction of this wave energy can convert into acoustic (sound) waves through tunneling effects, while propagating towards the stellar surface. The authors estimate that the dissipation of these sound waves near the stellar surface can unbound up to several solar masses of the stellar envelope.

The paper uses a 40 $M_\odot$ star model to outline a potential stellar structure during oxygen fusion. Results indicate that in certain regions, dissipation of wave energy through radiative diffusion and inefficient convective energy transport can lead to critical mass ejections. Such mass loss rates are pivotal in producing circumstellar environments conducive to interaction-driven luminous supernovae. The study ties these phenomena to supernovae displaying significant circumstellar interaction, such as SN 2006gy and PTF 09uj, where precursory mass loss shapes the luminosity.

Theoretical and Practical Implications

The implications of the study span both theoretical modeling and observational insights into massive stellar phenomena. It addresses the "fine-tuning" problem regarding why significant mass loss occurs just prior to supernova events, something not accounted for in line-driven wind models. The findings suggest that internal stellar dynamics, specifically wave dynamics, play a crucial role in this behavior.

Theoretically, this approach necessitates re-evaluating mass loss scenarios in stellar evolution models, prompting a deeper exploration of convective and wave dynamics in stars. Practically, understanding these mechanisms may allow observers to predict and better interpret the pre-explosion signatures of massive stars through variable circumstellar medium structures.

Speculation on Future Developments

Looking forward, the incorporation of refined wave excitation parameters such as those determining mode energy distributions and leakage thresholds could substantially enhance the predictive capability of stellar models. Also, more sophisticated treatment of the feedback mechanisms, where wave energy deposition alters stellar structure before supernova events, stands to improve the understanding of mass-loss variability across different progenitor types.

In conclusion, Quataert and Shiode's paper sheds light on the significant role of wave-driven dynamics in massive stellar evolution's culminating phase. This mechanism not only contributes to substantial envelope mass loss but also shapes the observational reality of core-collapse supernovae through interaction with circumstellar material. As simulation and observational tools evolve, further elucidating these processes could markedly redefine current models of stellar evolution and supernovae.