What drives heavy mass loss in the final years before collapse?
In plain words
Many supernovae explode into thick gas that the star shed only years to decades before dying, far more than normal stellar winds can explain.
Precise statement
Early spectra with narrow, short-lived emission lines (flash ionization) show dense circumstellar material within $\sim 1\mathrm{e}15\,\mathrm{cm}$ in a large fraction of Type II supernovae, implying mass-loss rates ~1e-4 to 1e-2 M_sun/yr in the last years, far above steady red supergiant winds of ~1e-6 M_sun/yr. Identify the mechanism (wave energy transport from core convection in the last burning stages, binary interaction, pulsation-driven winds) and whether it also produces observed pre-explosion outbursts. Answer: a mechanism predicting circumstellar mass and extent versus progenitor mass and evolutionary stage.
What would settle it
Pre-explosion monitoring of many nearby supernova progenitors combined with early spectra, compared with predicted outburst timing and circumstellar masses.
Status in the literature
2023: SN 2023ixf in M101, the nearest Type II supernova in a decade, showed dense, confined circumstellar material.