Probing Massive Shell Eruptions of Superluminous Supernovae in the UV

Hydrogen-poor superluminous supernovae (SLSN-I) are a rare class of transients with peak luminosities 10-100 times those of ordinary supernovae and with unique spectra. Neither the energy source behind their enormous luminosities, nor their progenitor stars, are well understood. There is increasing evidence, however, that some SLSN-I experience significant mass-loss close to explosion, with two objects now having been found with fast-moving (v 3000 km/s) circumstellar shells of material. These shells are best explained by the pulsational pair-instability mechanism, and potentially link these supernovae to very massive progenitors (core mass 50 Msun). Analysis to date has been limited by not being able to constrain the ionization state, composition and ultimately the mass of these shells, however, which offers key tests of the proposed pulsational pair-instability scenario. This information can only be obtained through UV absorption spectroscopy while the supernova is near peak light. Here, we propose target-of-opportunity observations with COS/FUV and STIS/MAMA to obtain UV spectroscopy of the next SLSN-I with evidence of a CSM shell and at a suitable redshift and brightness for HST, which will allow for these crucial pieces of information to be determined. Beyond studying the shell, the supernova spectrum itself in the UV carries valuable information for both the potential powering source, and the metallicity of the progenitor. It will also add to the small but growing number of nearby, well-studied SLSNe with HST UV spectra, necessary for interpreting the rest-frame UV spectra of SLSNe found at higher redshifts in future surveys like LSST, WFIRST and even JWST.