16215

SN2016iet is an unprecedented supernova in terms of its temporal evolution comma spectra comma and environment. The light curve exhibits two distinct peaks separated by 100 days comma and slowly decline by only 5 mag in 2.5 years. The spectra are dominated by a blue continuum and low velocity calcium and oxygen lines with an unusually high Ca-O ratio comma but lack hydrogen or helium. The SN is located at an unusually large offset (16 kpc comma 5 effective radii) from a low metallicity dwarf galaxy (0.1 Zsun comma 3e8 Msun). Modeling of the SN data indicates that it is powered by interaction of 20-85 Msun of ejecta with 35 Msun of H-He-free circumstellar material (CSM) comma ejected in the final decade before explosion. Thus comma the progenitor.s CO core shortly before explosion was 55-120 Msun comma placing it in the regime of pulsational pair instability or pair instability explosions comma further supported by the low metallicity. This is the first robust case for such an origin based directly on a large progenitor mass and low metallicity. We request observations of this unprecedented SN to achieve several goals that can only be done with HSTdoublePoint (i) track the SN emission at >1000 days post explosion comma as it fades below ground-based capabilities comma to track the large-scale CSM (ii) use NIR data to search for the first time for evidence of dust formation in H-He-poor CSM interaction (iii) measure the UV brightness of the host galaxy to precisely determine its star formation activity and escape fraction of ionizing radiation and (iv) search for a potential underlying galaxy at the SN position comma which from existing limits will be <1e-3 L*. This will be accomplished with 4 orbits in Cycle 27 and 3 orbits in Cycle 28.