Assigned ID: 881 ----- Attempting Submission 1 (Fri Feb 24 23:04:47 GMT 2012) ----- ----- Attempting Submission 1 (Tue Jul 03 21:06:08 GMT 2012) ----- Staring into the Beasts' Lair: HST Observations of the Host Galaxies of Pan-STARRS Ultra-luminous Supernovae The advent of wide-field optical time-domain surveys is providing an opportunity to discover and decipher new classes of astronomical transient phenomena. One of the most unexpected results from Pan-STARRS and other time-domain surveys is the discovery of ultra-luminous supernovae (ULSNe), with bolometric luminosities up to 100 times higher than normal core-collapse and Type Ia supernovae (SNe), and with spectra that do not match known SN classes. These ULSNe represent a new challenge to our understanding of the deaths of massive stars, the standard core-collapse picture, and the mechanism for powering optical emission in SNe. Progress in our understanding of these mysterious explosions requires detailed studies of their light curves and spectra (available from our Pan-STARRS data and follow-up), and studies of their galactic and sub-galactic environments - the focus of this HST proposal. Here we propose rest-frame UV and optical observations of 5 Pan-STARRS ULSNe at z~1-1.5, whose hosts remain undetected from the ground to limits of r,i~25 mag. The observations will reach about 0.02 L* in the UV/optical (SFR~0.2 Msun/yr and M*~10^9 Msun), comparable to the least luminous hosts of GRBs and SNe. Taking advantage of HST's angular resolution we will also study the locations of the ULSNe relative to their host UV light distribution as a probe of the progenitor population (similar studies of GRBs and core-collapse SNe suggest distinct types of massive star progenitors). Thus, with a modest allocation of 8 HST orbits (combined with existing data on the transients from Pan-STARRS), we will begin to address one of the key new mysteries in time-domain astrophysics. Our targets are five ULSN host galaxies, at redshifts 0.9-1.4, that are undetected from the ground to a limit r,i>25 mag. We aim to image each target in two filters, corresponding to rest-frame UV and optical emission. Two of our targets are at redshift ~0.9, and we use ACS/WFC F606W and F850LP as our two filters, fit into a single orbit for each target, doing a two-point dither in each filter. Our remaining three targets are at slightly higher redshifts (1.2-1.4), and so we do one orbit with ACS/WFC F606W to get rest-frame UV, and one orbit with WFC3/IR F110W to get the rest-frame optical, using a standard box dither pattern for each for optimal PSF subsampling. 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