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 56Ni distribution, with most 56Ni at low velocity and a small amount at high velocity. The rapid postmaximum decline requires a large fraction of the γ-rays to escape from the 56Ni-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 γ-ray deposition in the He layer due to enhanced γ-ray escape from the 56Ni-dominated region may explain both the delayed strengthening and the increasing velocity of the He lines. The SN has massive ejecta (~6-7 M☉), normal kinetic energy [~(1.0-1.5) × 1051 ergs], a high peak bolometric luminosity (~5 × 1042 ergs s-1) for an epoch as late as ~ 40 days, and a large 56Ni mass (~0.32 M☉). These properties and the presence of a small amount of H suggest that the progenitor was initially massive (M ~ 25-30 M☉) and had lost most of its H envelope, and was 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.