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

Abstract

We present SN 2023zaw $-$ a sub-luminous (${\mathrm{M}}_{\mathrm{r}}=-16.7$ mag) and rapidly-evolving supernova (${\mathrm{t}}_{1/2,\mathrm{r}}=4.9$ days), with the lowest nickel mass ($\approx 0.002$ ${\mathrm{M}}_{\odot }$) 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 $\sim 10000-12000$ $\mathrm{km}{\mathrm{s}}^{-1}$. The late-time spectra show prominent narrow He I emission lines at $\sim$1000$\mathrm{km}{\mathrm{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 $\approx 0.2$ ${\mathrm{M}}_{\odot }$ and an envelope radius of $\approx 50$ ${\mathrm{R}}_{\odot }$. The extremely low nickel mass and low ejecta mass ($\approx 0.5$ ${\mathrm{M}}_{\odot }$) suggest an ultra-stripped SN, which originates from a mass-losing low mass He-star (ZAMS mass $<$ 10 ${\mathrm{M}}_{\odot }$) 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 ($<0.005$ ${\mathrm{M}}_{\odot }$) stripped-envelope supernovae, which can be explored with deep and high-cadence transient surveys.

Accepted for publication in ApJL