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

Observations and modeling for the light curve (LC) and spectra of supernova (SN) 2005bf are reported. This SN showed unique features: the LC had two maxima, and declined rapidly after the second maximum, while the spectra showed strengthening He lines whose velocity increased with time. The double-peaked LC can be reproduced by a double-peaked $^{56}$Ni distribution, with most $^{56}$Ni at low velocity and a small amount at high velocity. The rapid post-maximum decline requires a large fraction of the $\gamma$-rays to escape from the $^{56}$Ni-dominated region, possibly because of low-density ``holes''. The presence of Balmer lines in the spectrum suggests that the He layer of the progenitor was substantially intact. Increasing $\gamma$-ray deposition in the He layer due to enhanced $\gamma$-ray escape from the $^{56}$Ni-dominated region may explain both the delayed strengthening and the increasing velocity of the He lines. The SN has massive ejecta ($\sim6-7\Msun$), normal kinetic energy ($\sim 1.0-1.5\times 10^{51}$ ergs), high peak bolometric luminosity ($\sim 5\times 10^{42}$ erg s$^{-1}$) for an epoch as late as $\sim$ 40 days, and a large $^{56}$Ni mass ($\sim0.32\Msun$). These properties, and the presence of a small amount of H suggest that the progenitor was initially massive (M$\sim 25-30 \Msun$) and had lost most of its H envelope, and was possibly a WN star. The double-peaked $^{56}$Ni 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.