Reconciling the Tension between Light-curve Modeling of Type II Supernovae and Neutrino-driven Core-collapse Supernovae Models with Late-phase Spectroscopy

Abstract Type II supernovae (SNe II) are the most frequently observed outcome of core-collapse explosions and provide a valuable window into the physical mechanisms governing the deaths of massive stars. However, estimates of explosion properties based on optical light-curve modeling often show tension with the predictions of modern neutrino-driven explosion models. In particular, when based on light curves from the explosions of red supergiants (RSGs) tied to specific stellar wind models, many SNe II are found to originate from low-mass progenitors yet exhibit unusually high explosion energies ( E K ), far exceeding theoretical predictions. In this study, we incorporate late-phase (nebular) spectroscopy to estimate the helium core mass of the progenitor ( <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">He</mml:mi> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">core</mml:mi> </mml:mrow> </mml:msub> </mml:math> ), which serves as an additional constraint to break degeneracies in light-curve modeling. This approach is applied to a sample of 32 well-observed SNe II, using a light-curve model grid constructed from RSGs with arbitrarily stripped hydrogen-rich envelopes, rather than assuming a fixed wind model. Examining the resulting correlations among the physical parameters, we find that the tension between the observed <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">He</mml:mi> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">core</mml:mi> </mml:mrow> </mml:msub> </mml:math> – E K and E K – M Ni relations and those predicted by neutrino-driven explosion models has significantly lessened by incorporating nebular spectroscopy in light-curve modeling. This study highlights the crucial role of nebular spectroscopy in interpreting SNe II observations and provides support to the neutrino-driven explosion mechanism as the dominant engine powering these events.