What Type of Star Made the One-of-a-kind Supernova iPTF14hls?

iPTF14hls is an onging nearby supernova with spectral features identical to those of the common Type IIP class but more slowly evolving, remaining luminous for over 600 days with at least five distinct peaks in its light curve and showing evidence for multiple pre-explosion eruptions. The observed properties of iPTF14hls are unique among all known supernovae, challenge all existing explosion models and likely indicate pre-explosion eruptions with time scales and energies never observed before. Such eruptions are theorized to occur in 95-130 solar mass stars which experience the pulsational pair instability. If iPTF14hls is indeed the first observed case of such a supernova, then determining its progenitor mass and metallicity will provide the first observational constraints of stellar evolutionary models in these mass ranges. Since no pre-explosion high-resolution imaging of the location of iPTF14hls exist, the only way to constrain the progenitor properties (independent of the supernova) is through HST studies of the site after the event fades. Such studies will require knowing the position of iPTF14hls to few-parsec precision. We propose a single-orbit HST observation to obtain high resolution localization of iPTF14hls before it fades. These images will serve as registration anchors for future post-event observations at the required precision (not possible with ground-based AO imaging). These observations will also allow us to measure the very late-time decline rate of iPTF14hls, constraining possible power sources, and to test an alternative explanation for iPTF14hls as a lensed normal supernova, by constraining any departure from point-source emission.