2021A&A...649A.163K

Context. Supernovae (SNe) TypeIbn are rapidly evolving and bright (M_R, peak_~-19) transients interacting with He-rich circumstellar material (CSM). SN 2018bcc, detected by the ZTF shortly after explosion, provides the best constraints on the shape of the rising light curve (LC) of a fast TypeIbn.
Aims. We used the high-quality data set of SN 2018bcc to study observational signatures of the class. Additionally, the powering mechanism of SN 2018bcc offers insights into the debated progenitor connection of TypeIbn SNe.
Methods. We compared well-constrained LC properties obtained from empirical models with the literature. We fit the pseudo-bolometric LC with semi-analytical models powered by radioactive decay and CSM interaction. Finally, we modeled the line profiles and emissivity of the prominent HeI lines, in order to study the formation of P-Cygni profiles and to estimate CSM properties.
Results. SN 2018bcc had a rise time to peak of the LC of 5.6_–0.1^+0.2days in the restframe with a rising shape power-law index close to 2, and seems to be a typical rapidly evolving TypeIbn SN. The spectrum lacked signatures of SN-like ejecta and was dominated by over 15 He emission features at 20 days past peak, alongside Ca and Mg, all with V_FWHM∼2000km/s. The luminous and rapidly evolving LC could be powered by CSM interaction but not by the decay of radioactive ⁵⁶Ni. Modeling of the HeI lines indicated a dense and optically thick CSM that can explain the P-Cygni profiles.
Conclusions. Like other rapidly evolving TypeIbn SNe, SN 2018bcc is a luminous transient with a rapid rise to peak powered by shock interaction inside a dense and He-rich CSM.Its spectra do not support the existence of two TypeIbn spectral classes. We also note the remarkable observational match to pulsational pair instability SN models.