Program: GS-2011B-Q-3

For over a century two classes of optical transients - nova eruptions and supernova explosions - have been studied in great detail. These two classes occupy narrow ranges of absolute magnitudes, around -8 and -18 mag (+/-2 mag) respectively. However, in recent years several transients have been discovered in the wide nova-SN gap and at very high luminosity (<-20 mag), suggesting that new classes of optical transients remain to be discovered. The origin of these events is hotly debated: they are argued to represent massive star eruptions, deficient white dwarf thermonuclear explosions, eta Carina-like ejections, and possibly new SN mechanisms (electron-capture, fallback, pair-instability). The Pan-STARRS project provides an unprecedented opportunity to explore this sparsely-sampled phase-space thanks to its unmatched depth and areal coverage. Here we propose to continue our successful TOO spectroscopy of Pan-STARRS transients in the nova-SN gap and at high luminosity to classify and characterize their origin for the first time. Our Gemini program recently led to the discovery of the most luminous SN-like event to date (z=1.4 with a peak of -24 mag!), as well as other high- and intermediate-luminosity events. Pan-STARRS will continue to discover many such transients and Gemini spectroscopy will determine their origin.