Discovery of the Closest Ultra-stripped Supernova: SN 2021agco in UGC 3855

Abstract We present the discovery and studies of the helium-rich, fast-evolving supernova (SN) 2021agco at a distance of ∼40 Mpc. Its early-time flux is found to rise from half peak to the peak of −16.06 ± 0.42 mag in the r band within 2.4 − 1.0 + 1.5 days, and the post-peak light curves also decline at a much faster pace relative to normal stripped-envelope supernovae (SNe) of Type Ib/Ic. The early-time spectrum of SN 2021agco ( t ≈ 1.0 days after the peak) is characterized by a featureless blue continuum superimposed with a weak emission line of ionized C iii , and the subsequent spectra show prominent He i lines. Both the photometric and spectroscopic evolution show close resemblances to SN 2019dge, which is believed to have an extremely stripped progenitor. We reproduce the multicolor light curves of SN 2021agco with a model combining shock-cooling emission with 56 Ni decay. The best-fit results give an ejecta mass of ≈0.3 M ⊙ and a synthesized nickel mass of ≈2.2 × 10 −2 M ⊙ . The progenitor is estimated to have an envelope radius of R env ≈ 80 R ⊙ and a mass of M env ≈ 0.10 M ⊙ . All these suggest that SN 2021agco can be categorized as an ultra-stripped SN Ib, representing the closest object of this rare subtype. This SN is found to explode in the disk of a Sab-type galaxy with an age of ∼10.0 Gyr and low star-forming activity. Compared to normal SNe Ib/c, the host galaxies of SN 2021agco and other ultra-stripped SNe tend to have relatively lower metallicity, which complicates the properties of their progenitor populations.