Physical properties of long-rising type II supernovae -- Bayesian analytic modeling and spectrophotometric correlations

Abstract

Supernova (SN) 1987A, with its long-rising ($\gtrsim$40 days) light curve, defines a rare subclass of type II SNe known as 1987A-like events. Representing only 1-3 percent of all core-collapse SNe and often found in low-metallicity environments, their large diversity suggests a wide range of progenitor and explosion properties. This study aims to improve the understanding of 1987A-like SNe by characterizing their explosion parameters, including kinetic energy, ejected mass, progenitor radius at explosion, and synthesized ${}^{56}$Ni mass. Additionally, it seeks to identify systematic trends in both the physical properties and the observed features of these peculiar events. A new Bayesian parameter estimation method, based on our ${}^{56}$Ni-dependent analytical model for hydrogen-rich SNe, is applied to derive explosion parameters from the light curves and expansion velocities of one of the largest and most comprehensive 1987A-like SN samples to date. These data are measured through a consistent analysis of observations available in the literature. The analysis reveals a heterogeneous population that nevertheless clusters into two main groups: (i) lower-energy explosions with modest ${}^{56}$Ni yields ($\sim$0.07 M${}_{\odot }$), similar to SN 1987A, and (ii) more energetic events (up to $\sim$5 foe) with larger nickel production and, in some cases, unusually extended progenitors. We confirm a robust correlation between ${}^{56}$Ni mass, peak luminosity, and explosion energy, as well as between ejecta mass and recombination timescale. An anti-correlation between Ba II line strength and photospheric velocity indicates that stronger Ba II absorptions in 1987A-like SNe arise from more compact, slowly expanding ejecta. Our study underscores the need to extend analytical frameworks to include additional power sources, enabling scalable and accurate modeling of the growing number of peculiar transients.

18 pages, 8 figures, accepted for publication in Astronomy & Astrophysics - Main Journal (Accepted 2026 February 7; received 2026 January 21; in original form 2025 October 27)

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