2005ApJ...633L..97T

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) x10⁵¹ ergs], a high peak bolometric luminosity (∼5x10⁴² ergs/s) 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.