SN 2006tf: Precursor Eruptions and the Optically Thick Regime of Extremely Luminous Type IIn Supernovae

SN2006tf is the third most luminous SN discovered so far, after SN2005ap and SN2006gy. SN2006tf is valuable because it provides a link between two regimes: (1) luminous type IIn supernovae powered by emission directly from interaction with circumstellar material (CSM), and (2) the most extremely luminous SNe where the CSM interaction is so optically thick that energy must diffuse out from an opaque shocked shell. As SN2006tf evolves, it slowly transitions from the second to the first regime as the clumpy shell becomes more porous. This link suggests that the range in properties of the most luminous SNe is largely determined by the density and speed of H-rich material ejected shortly before they explode. The total energy radiated by SN2006tf was at least 7e50 ergs. If the bulk of this luminosity came from the thermalization of shock kinetic energy, then the star needs to have ejected ~18 Msun in the 4-8 yr before core collapse, and another 2-6 Msun in the decades before that. A Type Ia explosion is therefore excluded. From the H-alpha emission-line profile, we derive a blast-wave speed of 2,000 km/s that does not decelerate, and from the narrow P Cygni absorption from pre-shock gas we deduce that the progenitor's wind speed was ~190 km/s. This is reminiscent of the wind speeds of LBVs, but not of RSGs or WR stars. We propose that like SN2006gy, SN2006tf marked the death of a very massive star that retained its H envelope until the end of its life, and suffered extreme LBV-like mass loss in the decades before it exploded.