2024ApJ...969L..11D

We present SN 2023zaw-a subluminous (M_r = -16.7 mag) and rapidly evolving supernova (t_1/2,r = 4.9 days), with the lowest nickel mass (≈0.002 M_⊙) measured among all stripped-envelope supernovae discovered to date. The photospheric spectra are dominated by broad He I and Ca near-infrared emission lines with velocities of ∼10,000-12,000 km s^–1. The late-time spectra show prominent narrow He I emission lines at ∼1000 km s^–1, indicative of interaction with He-rich circumstellar material. SN 2023zaw is located in the spiral arm of a star-forming galaxy. We perform radiation-hydrodynamical and analytical modeling of the lightcurve by fitting with a combination of shock-cooling emission and nickel decay. The progenitor has a best-fit envelope mass of ≈0.2 M_☉ and an envelope radius of ≈50 R_⊙. The extremely low nickel mass and low ejecta mass (≈0.5 M_⊙) suggest an ultrastripped SN, which originates from a mass-losing low-mass He-star (zero-age main-sequence mass < 10 M_⊙) in a close binary system. This is a channel to form double neutron star systems, whose merger is detectable with LIGO. SN 2023zaw underscores the existence of a previously undiscovered population of extremely low nickel mass (<0.005 M_☉) stripped-envelope supernovae, which can be explored with deep and high-cadence transient surveys.