Abstract
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 (approximate to 0.002 M-circle dot) 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 similar to 10,000-12,000 km s(-1). The late-time spectra show prominent narrow He I emission lines at similar to 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 approximate to 0.2 M-circle dot and an envelope radius of approximate to 50 R-circle dot. The extremely low nickel mass and low ejecta mass (approximate to 0.5 M-circle dot) suggest an ultrastripped SN, which originates from a mass-losing low-mass He-star (zero-age main-sequence mass < 10 M-circle dot) 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-circle dot) stripped-envelope supernovae, which can be explored with deep and high-cadence transient surveys.