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 Ni-56 distribution, with most Ni-56 at low velocity and a small amount at high velocity. The rapid postmaximum decline requires a large fraction of the gamma-rays to escape from the Ni-56-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 Ni-56-dominated region may explain both the delayed strengthening and the increasing velocity of the He lines. The SN has massive ejecta (similar to 6-7 M.), normal kinetic energy [similar to(1.0-1.5) x 10(51) ergs], a high peak bolometric luminosity (similar to 5 x 10(42) ergs s(-1)) for an epoch as late as similar to 40 days, and a large Ni-56 mass (similar to 0.32 M.). These properties and the presence of a small amount of H suggest that the progenitor was initially massive (M similar to 25-30 M.) and had lost most of its H envelope, and was possibly a WN star. The double-peaked Ni-56 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.