The luminous and rapidly evolving SN 2018bcc

Clues toward the origin of Type Ibn SNe from the Zwicky Transient Facility

¹ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
e-mail: emir.k@astro.su.se
² Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
³ Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
⁴ IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
⁵ DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195, USA
⁶ Oskar Klein Centre, Department of Physics, Stockholm University, 106 91 Stockholm, Sweden
⁷ Benoziyo Center for Astrophysics, Weizmann Institute of Science, Rehovot, Israel
⁸ Humboldt-Universität zu Berlin, Newtonstraße 15, 12489 Berlin, Germany
⁹ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark

Received: 13 July 2019
Accepted: 5 November 2019

Abstract

Context. Supernovae (SNe) Type Ibn 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 Type Ibn.

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 Type Ibn 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 He I 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^+0.2 days in the restframe with a rising shape power-law index close to 2, and seems to be a typical rapidly evolving Type Ibn 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 ∼ 2000 km s⁻¹. The luminous and rapidly evolving LC could be powered by CSM interaction but not by the decay of radioactive ⁵⁶Ni. Modeling of the He I lines indicated a dense and optically thick CSM that can explain the P-Cygni profiles.

Conclusions. Like other rapidly evolving Type Ibn 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 Type Ibn spectral classes. We also note the remarkable observational match to pulsational pair instability SN models.