SN 2023zaw: an ultra-stripped, nickel-poor supernova from a low-mass progenitor

We present SN 2023zaw $-$ a sub-luminous (Mᵣ = -16.7 mag) and rapidly-evolving supernova (t1/2,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 NIR 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 ultra-stripped SN, which originates from a mass-losing low mass He-star (ZAMS 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.