Type IIb Supernova Progenitors in 3D: Variability and Episodic Mass Loss Revealed by Radiation Hydrodynamics Simulations

Abstract We present the first 3D radiation hydrodynamics simulations of partially stripped ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">core</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>10</mml:mn> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> </mml:math> , M env ∼ 0.1–1 M ⊙ ) yellow supergiant ( L ∼ 10 5 L ⊙ , T eff ≈ 5000–8000 K) envelopes, constructed with Athena++ . These envelope models represent the progenitors of type IIb supernovae (SNe IIb), which have lost a substantial fraction of their H-rich envelope before undergoing core collapse. The luminosity-to-mass ratio is high in these extended envelopes, and convection is strongly driven by hydrogen and helium opacity peaks. This surface convection, coupled with changes in the opacity, sustains large-amplitude low-azimuthal-order radial pulsations, creating order-of-magnitude variability in the stellar luminosity on a timescale of tens of days. If persistent prior to the SN, these variations will be detectable with dedicated monitoring of SN IIb progenitor candidates in nearby galaxies and within deep all-sky time-domain surveys, such as the Vera Rubin Observatory’s Legacy Survey of Space and Time. Supersonic fluid motions across the outer layers of the star lead to both successful and failed mass-ejection events, which shape the circumstellar environment and drive episodic mass loss (∼10 −6 –10 −5 M ⊙ yr −1 , in outbursts). The resulting 3D gas distribution in the outer atmosphere, responsible for early-time SN shock-breakout and shock-cooling emission, shows orders-of-magnitude fluctuations in both space and time at any given radial location. This intrinsically complex halo of bound and unbound material complicates predictions for early SN IIb light curves relative to spherically symmetric models. However, it does provide a natural, self-consistent explanation for the presence and diversity of dense circumstellar material observed or inferred around pulsating evolved stars.