Peculiar Type II supernovae from blue supergiants

The vast majority of Type II supernovae (SNeII) are produced by red supergiants, but SN1987A revealed that blue supergiants (BSGs) can produce members of this class as well, albeit with some peculiar properties. This best-studied event revolutionized our understanding of SNe and linking it to the bulk of Type II events is essential. We present here the optical photometry and spectroscopy gathered for SN2000cb, which is clearly not a standard SNII and yet is not a SN1987A analogue. The light curve of SN2000cb is reminiscent of that of SN1987A in shape, with a slow rise to a late optical peak, but on substantially different time-scales. Spectroscopically, SN2000cb resembles a normal SNII, but with ejecta velocities that far exceed those measured for SN1987A or normal SNeII, above 18000kms^-1 for Hα at early times. The red colours, high velocities, late photometric peak and our modelling of this object all point towards a scenario involving the high-energy explosion of a small-radius star, most likely a BSG, producing 0.1M_⊙ of ⁵⁶Ni. Adding a similar object to the sample, SN2005ci, we derive a rate of ~2 per cent of the core-collapse rate for this loosely defined class of BSG explosions.