SN 2025dys: A luminous supernova explosion from a partially stripped helium star

Abstract

We present well-sampled multi-band imaging and low- to medium-resolution spectroscopic observations of the early-discovered Type~II supernova (SN)~2025dys. The photometric evolution exhibits a rise time of $\sim$\,18 days ($r$ band) and a rapid decline transitioning to the radioactive decay tail at $\sim$\,70 days. SN~2025dys is among the high-luminosity events with a peak pseudo-bolometric luminosity of $L_{\mathrm{opt}}=8.57\times 10^{42}\mathrm{erg}{\mathrm{s}}^{-1}$. Early X-ray emission was identified, displaying a luminosity of $7.55\times 10^{40}\mathrm{erg}{\mathrm{s}}^{-1}$. The observational investigation points to a dense, hydrogen-rich circumstellar medium with a mass of $M_{\mathrm{CSM}}\approx 0.5$--$0.7{\mathrm{M}}_{\odot }$, suggesting a partially stripped, hydrogen-poor inner ejecta with a relatively low mass of $M_{\mathrm{ej}}\approx 2.5{\mathrm{M}}_{\odot }$. The explosion possessed a kinetic energy of $E_{\mathrm{k}}\approx 0.3\text{–}0.5$~foe and yielded a synthesized nickel mass of $M_{\mathrm{Ni}}\approx 0.042_{-0.011}^{+0.012}{\mathrm{M}}_{\odot }$. These physical parameters indicate an extreme mass-loss episode before the explosion, characterized by a high mass-loss rate of $\dot{M}\gtrsim 0.01{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$. SN~2025dys represents a stripped progenitor, intrinsically resembling a Type~IIb or even a Type~Ib SN, that exploded within a dense, hydrogen-rich circumstellar environment.

arXiv comment22 Pages, 9 Figures, 2 Tables, submitted