The metamorphosis of the Type Ib SN 2019yvr: late-time interaction

We present observational evidence of late-time interaction between the ejecta of the hydrogen-poor Type Ib supernova (SN) 2019yvr and hydrogen-rich circumstellar material (CSM), similar to the Type Ib SN 2014C. A narrow Hα emission line appears simultaneously with a break in the light-curve decline rate at around 80-100 d after explosion. From the interaction delay and the ejecta velocity, under the assumption that the CSM is detached from the progenitor, we estimate the CSM inner radius to be located at  6.5-9.1 × 10^15 cm. The Hα emission line persists throughout the nebular phase at least up to +420 d post-explosion, with a full width at half maximum of  2000 km/s. Assuming a steady mass-loss, the estimated mass-loss rate from the luminosity of the Hα line is  3-7 × 10^-5 M_⊙yr^-1. From hydrodynamical modelling and analysis of the nebular spectra, we find a progenitor He-core mass of 3-4 M_⊙, which would imply an initial mass of 13-15 M_⊙. Our result supports the case of a relatively low-mass progenitor possibly in a binary system as opposed to a higher mass single star undergoing a luminous blue variable phase.