SN 2021efd, SN 2019tsf photometry and spectra

The light curve evolution is similar to that of SN 2019tsf, whose previously unpublished spectrum at 400 days is also presented here. The nebular spectrum of SN 2021efd shows narrow emission lines (~1000km/s) in various species, such as OI, CaII, MgII, HeI, [OI], [CaII], and [SII]. Based on the observations, we studied the properties of the ejecta and CSM of SN 2021efd.

Our observations suggest that SN 2021efd is a Type Ib SN interacting with CSM with the following parameters: The estimated ejecta mass, explosion energy, and 56Ni-56 mass are 2.2M_{sun}_, 9.1*10^50 erg, of 0.14M_{sun}_, respectively, while the estimated CSM mass, composition, and distribution are at least a few times 0.1M_{sun}_, H-free, and clumpy, respectively. Based on the estimated ejecta properties, we conclude that this event is a transitional SN whose progenitor was experiencing He-layer stripping at the epoch of the explosion, and was on the way to becoming a carbon-oxygen star (as the progenitors of Type Ic SNe) from a He star (as the progenitors of Type Ib SNe).

The estimated CSM properties suggest that the progenitor had some episodic mass ejections with the rate of ~0.005-0.01M_{sun}_/yr for the last decade and slightly smaller before this final phase at least from ~200 years before the explosion, for the assumed CSM velocity of 100km/s. For the case of ~1000km/s, the necessary mass-loss rate would be increased by a factor of ten, and the timescales decreased by a factor of ten.