The Peculiar Type Ib Supernova SN 2005bf: Explosion of a Massive He Star with a Thin Hydrogen Envelope?

We present BVRI photometry and optical spectroscopy of SN 2005bf near maximum light. The maximum phase is broad and occurred around 2005 May 7, about 40 days after the shock breakout. SN 2005bf has a peak bolometric magnitude Mbol = -18.0 ± 0.2; while this is not particularly bright, it occurred at an epoch significantly later than other Type Ibc supernovae (SNe Ibc), indicating that the SN possibly ejected ~0.31 M☉ of 56Ni, which is more than the typical amount. The spectra of SN 2005bf around maximum are very similar to those of the Type Ib SNe 1999ex and 1984L about 25-35 days after explosion, displaying prominent He I, Fe II, Ca II H and K, and the near-IR triplet P Cygni lines. Except for the strongest lines, He I absorptions are blueshifted by ≲6500 km s-1, and Fe II absorptions are blueshifted by ~7500-8000 km s-1. No other SNe Ib have been reported to have their Fe II absorptions blueshifted more than their He I absorptions. Relatively weak Hα and very weak Hβ may also exist, blueshifted by ~15,000 km s-1. We suggest that SN 2005bf was the explosion of a massive He star, possibly with a trace of a hydrogen envelope.