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ABSTRACT The early phases of the observed evolution of the supernovae (SNe) are expected to be dominated by the shock breakout and ‘flash’ ionization of the surrounding circumstellar medium. This material arises from the last stages of the evolution of the progenitor, such that photometry and spectroscopy of SNe at early times can place vital constraints on the latest and fastest evolutionary phases leading up to stellar death. These signatures are erased by the expansion of the ejecta within ∼5 d after explosion. Here we present the earliest constraints, to date, on the polarization of 10 transients discovered by the Zwicky Transient Facility (ZTF), between 2018 June and 2019 August. Rapid polarimetric follow-up was conducted using the Liverpool Telescope RINGO3 instrument, including three SNe observed within <1 d of detection by the ZTF. The limits on the polarization within the first 5 d of explosion, for all SN types, is generally $\lt 2{ per\ cent}$, implying early asymmetries are limited to axial ratios >0.65 (assuming an oblate spheroidal configuration). We also present polarimetric observations of the Type I Superluminous SN 2018bsz and Type II SN 2018hna, observed around and after maximum light. 

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 56 Ni. 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. 

In this paper, we discuss the outcomes of the follow-up campaign of SN 2018ijp, discovered as part of the Zwicky Transient Facility survey for optical transients. Its first spectrum shows similarities to broad-lined Type Ic supernovae around maximum light, whereas later spectra display strong signatures of interaction between rapidly expanding ejecta and a dense H-rich circumstellar medium, coinciding with a second peak in the photometric evolution of the transient. This evolution, along with the results of modeling of the first light-curve peak, suggests a scenario where a stripped star exploded within a dense circumstellar medium. The two main phases in the evolution of the transient could be interpreted as a first phase dominated by radioactive decays, and a later interaction-dominated phase where the ejecta collide with a pre-existing shell. We therefore discuss SN 2018jp within the context of a massive star depleted of its outer layers exploding within a dense H-rich circumstellar medium.