A Model for the Fast Blue Optical Transient AT2018cow: Circumstellar Interaction of a Pulsational Pair-instability Supernova

The fast blue optical transient (FBOT) ATLAS18qqn (AT2018cow) has a light curve as bright as that of superluminous supernovae (SLSNe) but rises and falls much faster. We model this light curve by circumstellar interaction of a pulsational pair-instability (PPI) supernova (SN) model based on our PPISN models studied in previous work. We focus on the 42 M_⊙ He star (core of a 80 M_⊙ star) which has circumstellar matter (CSM) of mass 0.50 M_⊙. With the parameterized mass cut and the kinetic energy of explosion E, we perform hydrodynamical calculations of nucleosynthesis and optical light curves of PPISN models. The optical light curve of the first ~20 days of AT2018cow is well reproduced by the shock heating of CSM for the 42 M_⊙ He star with E = 5 × 10⁵¹ erg. After day 20, the light curve is reproduced by the radioactive decay of 0.6 M_⊙⁵⁶Co, which is a decay product of ⁵⁶Ni in the explosion. We also examine how the light-curve shape depends on the various model parameters, such as CSM structure and composition. We also discuss (1) other possible energy sources and their constraints, (2) the origin of the observed high-energy radiation, and (3) how our result depends on the radiative transfer codes. Based on our successful model for AT2018cow and the model for SLSN with CSM mass as large as 20 M⊙, we propose the working hypothesis that PPISN produces SLSNe if the CSM is massive enough and FBOTs if CSM is less than ~1 M_⊙.