Rapidly evolving faint transients from stripped-envelope electron-capture supernovae

We investigate the expected rates and bolometric light-curve properties of stripped-envelope electron-capture supernovae (ECSNe) using stellar models from the Binary Population and Spectral Synthesis (BPASS) code. We find that 0.8 per cent (Z = 0.020) and 1.2 per cent (Z = 0.004) of core-collapse supernovae are stripped-envelope ECSNe. Their typical ejecta masses are estimated to be about 0.3 Msun (Z = 0.020) and 0.6 Msun (Z = 0.004). Assuming ECSN explosion properties from numerical explosion simulations, an explosion energy of 1.5e50 erg and a 56Ni mass of 2.5e-3 Msun, we find that stripped-envelope ECSNe have a typical rise time of around 7 days (Z = 0.020) or 13 days (Z = 0.004) and peak luminosity of around 1e41 erg/s (-13.8 mag, Z = 0.020) or 7e40 erg/s (-13.4 mag, Z = 0.004). Their typical ejecta velocities are around 7000 km/s (Z = 0.020) or 5000 km/s (Z = 0.004). Thus, stripped-envelope ECSNe are observed as rapidly-evolving faint transients with relatively small velocities. SN 2008ha-like supernovae, which are the faintest kind of SN 2002cx-like (a.k.a. Type Iax) supernovae, may be related to stripped-envelope ECSNe.