SN 2025aico: Early observations of a faint type IIb supernova with a low-mass envelope

Abstract

Originating from massive stars that have lost most of their hydrogen envelopes, type IIb supernovae are stellar explosions of partially stripped stars. Due to this extensive stripping, their spectra exhibit transitional signatures that bridge the gap between hydrogen-rich (type II) and helium-rich (type Ib) events, making them crucial astrophysical probes for investigating the final evolutionary stages of massive stars. We aimed to investigate the physical properties and the underlying explosion mechanisms of the type IIb SN 2025aico. Through a comprehensive analysis of early-phase optical light curves and spectroscopic data, we aimed to constrain the fundamental explosion parameters and evaluate the physical state of the event. We present early multi-band optical imaging and low-resolution optical spectroscopic follow-up observations of the type IIb SN 2025aico, spanning sim70 days from the explosion. We constrained the properties of SN 2025aico using a hybrid model that combines shock-cooling emission and radioactively powered diffusion, as well as through an analysis of its spectroscopic evolution. We used various approaches to constrain 56 Ni mixing from early data and compared our spectra with models to constrain the properties of the progenitor. The explosion epoch of SN 2025aico is estimated to be Modified Julian Date (MJD) 61032.69, while the rise time in the r_ Ni mass of M_ band is 22.30 =(4.07 , . The fitting yields a moderate to relatively low days. The peak pseudo-bolometric luminosity in the optical bands is L_ opt 41 -1 56 Ni = 0.033_ -0.004 ^ +0.006 , $ and an ejecta mass of M_ = 2.79_ M_⊙ ej -0.18 ^ +0.21 , . The photospheric velocity near the bolometric peak, measured from the łambda5169 line, is 6450_ M_⊙ Fe II -160 ^ +180 , . The derived envelope properties suggest a compact He-star progenitor possessing an H-rich envelope of M_ -1 env M_⊙ and a radius of R_ env ≈ 6,--$10, . R_⊙ The derived physical properties of SN 2025aico indicate an origin from a moderate-mass, stripped He-star in a compact binary system, characterized by a minimal residual hydrogen envelope. The explosion itself demonstrates weak to moderate 56 Ni mixing throughout the ejecta.