SN 2022ann: a Type Icn supernova from a dwarf galaxy that reveals helium in its circumstellar environment

We present optical and near-infrared (NIR) observations of the Type Icn supernova (SN Icn) 2022ann, the fifth member of its newly identified class of SNe. Its early optical spectra are dominated by narrow carbon and oxygen P-Cygni features with absorption velocities of ~800 km s^-1; slower than other SNe Icn and indicative of interaction with a dense, H/He-poor circumstellar medium (CSM) that is outflowing slower than typical Wolf-Rayet wind velocities of >1000 km s^-1. We identify helium in NIR spectra 2 weeks after maximum and in optical spectra at 3 weeks, demonstrating that the CSM is not fully devoid of helium. Unlike other SNe Icn, the spectra of SN 2022ann never develop broad features from SN ejecta, including in the nebular phase. Compared to other SNe Icn, SN 2022ann has a low luminosity (o-band absolute magnitude of ~-17.7), and evolves slowly. The bolometric light curve is well-modelled by 4.8 M_⊙ of SN ejecta interacting with 1.3 M_⊙ of CSM. We place an upper limit of 0.04 M_⊙ of ⁵⁶Ni synthesized in the explosion. The host galaxy is a dwarf galaxy with a stellar mass of 10^7.34 M_⊙ (implied metallicity of log(Z/Z_⊙) ≈ 0.10) and integrated star-formation rate of log (SFR) = -2.20 M_⊙ yr^-1; both lower than 97 per cent of galaxies observed to produce core-collapse supernovae, although consistent with star-forming galaxies on the galaxy Main Sequence. The low CSM velocity, nickel and ejecta masses, and likely low-metallicity environment disfavour a single Wolf-Rayet progenitor star. Instead, a binary companion is likely required to adequately strip the progenitor and produce a low-velocity outflow.