The Unique Type Ib Supernova 2005bf: A WN Star Explosion Model for Peculiar Light Curves and Spectra

Photometric and spectroscopic observations of supernova (SN) 2005bf and theoretical modeling of the light curve and the spectra are reported. This SN showed unique features: the bolometric light curve had two maxima and declined rapidly after the second maximum, the spectra appeared to transform from those of a Type Ic to those of a Type Ib, the velocity of the helium lines increased with time. The double-peaked light curve can be reproduced by a double-peaked 56Ni distribution, with a large amount of 56Ni at low-velocity and a small amount at high-velocity. The rapid post maximum decline requires that a large fraction of the $\gamma$-rays can escape from the 56Ni-dominated region possibly because of the presence of many low-density ``holes''. Enhanced $\gamma$-ray escape from the 56Ni-dominated region yields rapidly increasing $\gamma$-ray deposition in the He layer, which may explain the increasing He line velocity. Balmer lines were actually present in the earlier spectrum, so the SN did not transform from Ib to Ic. The SN has rather massive ejecta ($\sim6-7\Msun$), a normal kinetic energy ($\sim 1.0-1.5\times 10^{51}$ ergs), and a large amount of 56Ni ($\sim0.31\Msun$). These properties, and the presence of a small amount of hydrogen suggest that the progenitor was a massive star, with initial mass $\sim 25-30\Msun$, that lost most of its H envelope, possibly a WN star. The double-peaked 56Ni distribution suggests that the explosion may have formed jets that did not reach the He layer. The properties of SN 2005bf resemble those of the explosion of Cassiopeia A.