SN 2025dys: A luminous supernova explosion from a partially stripped helium star
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 $L_{\rm opt}=8.57\times10^{42}\,\mathrm{erg\,s^{-1}}$. Early X-ray emission was identified, displaying a luminosity of $7.55\times10^{40}\,\mathrm{erg\,s^{-1}}$. The observational investigation points to a dense, hydrogen-rich circumstellar medium with a mass of $M_{\rm CSM}\approx0.5$--$0.7\,\mathrm{M_\odot}$, suggesting a partially stripped, hydrogen-poor inner ejecta with a relatively low mass of $M_{\rm ej}\approx2.5\,\mathrm{M_\odot}$. The explosion possessed a kinetic energy of $E_{\rm k}\approx0.3\text{--}0.5$~foe and yielded a synthesized nickel mass of $M_{\rm Ni}\approx0.042_{-0.011}^{+0.012}\,\mathrm{M_\odot}$. These physical parameters indicate an extreme mass-loss episode before the explosion, characterized by a high mass-loss rate of $\dot{M} \gtrsim 0.01\,\mathrm{M_\odot\,yr^{-1}}$. 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.