2025ApJ...986...39F

The red supergiant (RSG) problem refers to the observed dearth of luminous RSGs identified as progenitors of Type II supernovae (SNe II) in pre-SN imaging. Understanding this phenomenon is essential for studying pre-SN mass loss and the explodability of core-collapse SNe. In this work, we reassess the RSG problem using late-phase spectroscopy of a sample of 50 SNe II. The [O I] λλ6300,6363 emission in the spectra is employed to infer the zero-age main-sequence (ZAMS) mass distribution of the progenitors, which is then transformed into a luminosity distribution via an observation-calibrated mass-luminosity relation. The resulting luminosity distribution reveals an upper cutoff at ${\mathrm{log}}\,L/{L}_{\odot }\,=\,5.2{1}_{-0.07}^{+0.09}\,$dex, and the RSG problem is statistically significant at the 2σ-3σ level. Assuming single RSG progenitors that follow the mass-luminosity relation of KEPLER models, this luminosity cutoff corresponds to an upper ZAMS mass limit of $20.6{3}_{-1.64}^{+2.42}\,{M}_{\odot }$. Comparisons with independent measurements, including pre-SN imaging and plateau-phase light curve modeling, consistently yield an upper ZAMS mass limit below ∼25 M_⊙, with a significance level of 1σ-3σ. While each individual method provides only marginal significance, the consistency across multiple methodologies suggests that the lack of luminous RSG progenitors may reflect a genuine physical problem. Finally, we discuss several scenarios to account for this issue should it be confirmed as a true manifestation of stellar physics.