Luminous Fast Blue Optical Transients as very massive star core-collapse events

Abstract

Luminous Fast Blue Optical Transients (LFBOTs) are rare extragalactic events of unknown origin. Tidal disruptions of white dwarfs by intermediate mass black holes (BHs), mergers of BHs and Wolf-Rayet stars, and failed supernovae are among the suggestions. We explore the viability of very massive star core-collapse events as the origin of LFBOTs. The appeal of such a model is that the formation of massive BHs via core collapse may yield observational signatures that can match the disparate lines of evidence that point towards both core-collapse and tidal disruption origins. We explore the formation rate of massive BH in population synthesis models, and compare the metallicities of their progenitors with the observed metallicities of LFBOT host galaxies. We further examine the composition, mass loss rates and fallback masses of these stars, placing them in the context of LFBOT observations. The formation rate of BHs with mass greater than ~30-40Msol is similar to the observed LFBOT rate. The stars producing these BHs are biased to low metallicity (Z<0.3Zsol), are H and He-poor and have dense circumstellar media. However, some LFBOTs have host galaxies with higher metallicities than predicted, and others have denser environments (plausibly due to late mass loss not captured in the models). We find that long-lived emission from an accretion disc (as implicated in the LFBOT AT 2018cow) can only be produce in these events under maximal disc mass and angular momentum conditions. We conclude that (very) massive star core-collapse is a plausible explanation for at least some LFBOTs, but faces challenges. The preferred progenitors for LFBOTs in the failed supernova interpretation overlap with those predicted to produce super-kilonovae. We therefore suggest that LFBOTs are promising targets to search for super-kilonovae, and that they may contribute to the r-process enrichment of galaxies.