  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.
