SN 2006tf: Precursor eruptions and the optically thick regime of extremely luminous type IIn supernovae

SN 2006tf is the third most luminous supernova (SN) discovered so far, after SN 2005ap and SN 2006gy. SN 2006tf is valuable because it provides a link between two regimes: (1) luminous Type Un 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 SN 2006tf 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 hydrogen-rich material ejected shortly before they explode. The total energy radiated by SN 2006tf was at least 7 × 10⁵⁰ ergs. If the bulk of this luminosity came from the thermalization of shock kinetic energy, then the star needs to have ejected ∼18 M_⊙ in the 4-8 yr before core collapse, and another 2-6 M_⊙ in the decades before that. AType Ia explosion is therefore excluded. From the Hα emission-line profile, we derive a blast wave speed of 2000 km s^-1 that does not decelerate, and from the narrow P Cygni absorption from preshock gas we deduce that the progenitor's wind speed was ∼190 km s^-1. This is reminiscent of the wind speeds of luminous blue variables (LBVs), but not of red supergiants or Wolf-Rayet stars. We propose that like SN 2006gy, SN 2006tf marked the death of a very massive star that retained a hydrogen envelope until the end of its life and suffered extreme LBV-like mass loss in the decades before it exploded.