Pre-Supernova Outbursts via Wave Heating in Massive Stars II: Hydrogen-poor Stars

Abstract: Pre-supernova (SN) outbursts from massive stars may be driven by hydrodynamical wave energy emerging from the core of the progenitor star during late nuclear burning phases. Here, we examine the effects of wave heating in stars containing little or no hydrogen, i.e., progenitors of type IIb/Ib SNe. Because there is no massive hydrogen envelope, wave energy is thermalized near the stellar surface where the overlying atmospheric mass is small but the optical depth is large. Wave energy can thus unbind this material, driving an optically thick, super-Eddington wind. Using 1D hydodynamic MESA simulations of $\sim ! 5 \, M_\odot$ He stars, we find that wave heating can drive pre-SN outbursts composed of a dense wind whose mass loss rate can exceed $\sim ! 0.1 \, M_\odot/{\rm yr}$. The wind terminal velocities are a few $100 \, {\rm km}/{\rm s}$, and outburst luminosities can reach $\sim ! 10^6 \, L_\odot$. Wave-driven outbursts may be linked with observed or inferred pre-SN outbursts of type Ibn/transitional/transformational SNe, and pre-SN wave-driven mass loss is a good candidate to produce these types of SNe. However, we also show that non-linear wave breaking in the core of the star may prevent such outbursts in stars with thick convective helium-burning shells. Hence, only a limited subset of SN progenitors are likely to experience wave-driven pre-SN outbursts.

Wave Heating in Hydrogen-Poor Stars as Drivers of Pre-Supernova Outbursts

The paper examines the phenomenon of pre-supernova (SN) outbursts in hydrogen-poor massive stars, specifically addressing the contribution of hydrodynamical wave energy originating from a star's core during late phases of nuclear burning. This theoretical exploration provides insights into the mechanisms by which wave energy, primarily gravity and acoustic waves, instigates outflows by thermalizing near the stellar surface, thereby driving optically thick and super-Eddington winds.

Key Findings and Technical Aspects

The authors have employed 1D hydrodynamic simulations within the MESA framework to simulate the behavior and evolution of $\sim 5 \, M_\odot$ helium stars, representative of hydrogen-poor progenitors of type IIb/Ib supernovae. The study implements wave excitation from core convection, determining that near core collapse, the luminosity of convectively generated waves vastly exceeds that of standard stellar luminosity—with wave heating rates surpassing $10^7 \, L_\odot$. These waves can prompt winds with mass loss rates of $\sim 0.1 \, M_\odot/{\rm yr}$ and terminal velocities around several hundred kilometers per second.

The authors stipulate that stars lacking thick hydrogen envelopes thermalize wave energy closer to the surface, where the low stellar binding energy allows such energy to unbind atmospheric mass. This leads to the formation of an expansive and dense optically-thick wind. The concept of wave energy transport, originating from convection zones during terminal phases of stellar evolution, emphasizes the potential for waves to carry significant energy—often dissipating near the surface to result in observable outbursts.

Theoretical and Observational Implications

The paper suggests that wave-driven mechanisms may account for the enhanced pre-SN mass loss observed in type Ibn and transitional/transformational SNe. Observationally, this can manifest as luminous pre-SN outbursts similar to those witnessed for SN 2006jc. However, not all SN progenitors will demonstrate wave-driven pre-outbursts; the presence of a convective helium-burning shell may impede wave escape from the core, limiting outbursts only to progenitors where wave core reflection is minimized.

This research delineates how wave heating significantly influences progenitor luminosity and mass loss, which are pivotal observational parameters. By dramatically altering surface luminosities and atmospheric dynamics long before core-collapse, these phenomena expand our understanding of SN progenitor variability and diversity, providing viable pathways for further investigation into hydrodynamical processes preceding SN events.

Speculations on Future AI Developments

Continuous improvement in AI-driven simulations may enable more intricate modeling of wave propagation and dissipation dynamics by overcoming the current limitations of mechanical simplifications, such as assuming linear propagation dynamics. Advancements could deepen our understanding of non-linear wave interactions, thereby refining predictive accuracy concerning wave-driven outflows and outbursts.

Concluding Remarks

This paper contributes pivotal insights juxtaposing stellar structure with hydrodynamical wave processes as precursors to supernovae in hydrogen-poor massive stars. The adoption of rigorous hydrodynamic simulations provides a deeper comprehension of mass loss dynamics, potentially informing future observational strategies and enhancing theoretical models for SN evolution. Further interdisciplinary explorations are necessitated to fully integrate observational data and refine models to encompass other progenitor structures and masses.