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 lightmaximum. The maximum phase is broad and occurred around 2005 May 7, about fortydays after the shock breakout. SN 2005bf has a peak bolometric magnitudeM_{bol}=-18.0\pm 0.2: while this is not particularly bright, it occurred at anepoch significantly later than other SNe Ibc, indicating that the SN possiblyejected ~0.31 M_{sun} of 56Ni, which is more than the typical amount. Thespectra of SN 2005bf around maximum are very similar to those of the Type IbSNe 1999ex and 1984L about 25-35 days after explosion, displaying prominent HeI, Fe II, Ca II H & K and the near-IR triplet P Cygni lines. Except for thestrongest lines, He I absorptions are blueshifted by <~6500 km/s, and Fe II by\~7500-8000 km/s. No other SNe Ib have been reported to have their Fe IIabsorptions blueshifted more than their He I absorptions. Relatively weakH-alpha and very weak H-beta may also exist, blueshifted by ~15,000 km/s. Wesuggest that SN 2005bf was the explosion of a massive He star, possibly with atrace of a hydrogen envelope.Comment: 13 pages, 4 figures. Accepted for publication in ApJ (Letters