The Peculiar Type II Supernova 2000cb

Unusual supernovae (SNe) shed light on the more mundane objects by allowing us to explore the edges of the physical conditions that can lead a star to its violent demise. SN 1987A revealed that blue supergiants can explode as Type II SNe, along with the red supergiants that produce the bulk of that population. This revolutionized our perspective on the possible outcomes of massive stellar evolution. We present here optical photometry and spectroscopy gathered on SN 2000cb, which is neither a standard Type II SN nor a SN 1987A analog. The light curve of SN 2000cb is reminiscent of that of SN 1987A in shape, with a slow rise to a late optical peak, but the timescales and color evolution of these two objects are quantifiably different. Spectroscopically, SN 2000cb seems like a normal Type II but with photospheric velocities that far exceed those measured for SN 1987A or normal SNe II, above 18,000 km/s for the H-alpha absorption line at early times. The red colors, high velocities, and late peak all point toward a scenario involving the high-energy explosion of a small-radius star that has shed a significant fraction of its hydrogen envelope. We derive a rate for this loosely defined class of blue supergiant explosions on the order of 2% of the core-collapse SN rate.

We are grateful for the support of NSF grant AST-0908886, AST-0907903, and the TABASGO Foundation.