Hydrogen-Rich to Stripped-Envelope:Observational Continuity and Biases in CCSNe

TL;DR

Core-collapse supernovae likely occupy a multidimensional continuum rather than discrete classes, governed by envelope composition, pre-SN mass loss, and circumstellar interaction. The review synthesizes observational and theoretical evidence across II, SE-SNe, and interaction-dominated subclasses, arguing that envelopes, winds, binary evolution, and engine power produce overlapping outcomes. It proposes viewing traditional subtypes as reference points along a continuum defined by a small set of physical parameters, while highlighting biases that can obscure true transitions. Realizing this continuum as predictive science will require large, homogeneous time-domain surveys combined with advanced modeling to map progenitor pathways to observable SN properties.

Abstract

Although historically classified into discrete subclasses, there is growing evidence that indicates that core-collapse supernovae (CCSNe) categories often overlap, reflecting continuous variations in progenitor structure, mass-loss history, and circumstellar environments rather than strictly distinct channels. In this review, we explore the proposed continua that link hydrogen-rich Type II SNe to stripped-envelope explosions (IIb-Ib-Ic), and that extend further into interaction-dominated and superluminous events. We discuss the physical processes-stellar winds, binary interaction, eruptive outbursts, and circumstellar interaction-that may produce graded outcomes across classes, while highlighting where observational evidence supports or challenges smooth transitions. We propose that CCSNe are better viewed as a multidimensional continuum of explosion outcomes, where traditional subclasses act as reference points rather than strict boundaries. Future progress will rely on large, homogeneous datasets and advanced modeling to disentangle true evolutionary sequences from apparent overlaps, ultimately connecting progenitor pathways to the observed diversity of explosions

Figures (2)

• Figure 1: Schematic illustration of the current classification scheme of supernovae based on spectral features, light curve morphology and luminosity.Because spectral classification can overlap with photometric classification (e.g. SLSN IIn) we refrain from placing such classes, instead we place "superluminous" as a class based on luminosity and "narrow line" as a class based on spectral lines
• Figure 2: Luminosity $L$ and expansion velocity $v_{\rm SN}$ as functions of phase. CCSNe spread all over the phase space in the luminosities for different subtypes while for velocities, the velocity has higher value for SESNe than the interacting SNe as the latter are dominated by electron scattering wings. Error bars denote $1\sigma$ uncertainties. Note that SNe II include a large variety of subclasses including SNe IIn (see Sect.\ref{['supp:lumvel']}). Different markers are used to separate the interacting and the non-interacting classes. Cautionary note: velocity tracers for interacting SNe are not directly comparable to photospheric velocities of non-interacting SNe. Measured velocities for interacting events (derived from narrow or intermediate-width lines) reflect slow CSM or post-shock gas, rather than ejecta photospheric speeds. Consequently, comparisons of ($v_{\text{SN}}$) across classes are not strictly homogeneous. The apparent lack of a clear continuum in the plotted luminosity–velocity distribution may partly reflect measurement bias, rather than disproving an underlying physical continuum.