A luminous and fast-expanding type Ib supernova SN 2012au

We present a set of photometric and spectroscopic observations of a bright Type Ib supernova SN 2012au from -6 days until + 150 days after maximum. The shape of its early R-band light curve is similar to that of an average Type Ib/c supernova. The peak absolute magnitude is M_R = -18.7 ± 0.2 mag, which suggests that this supernova belongs to a very luminous group among Type Ib supernovae. The line velocity of He I λ5876 is about 15,000 km s^-1 around maximum, which is much faster than that in a typical Type Ib supernova. From the quasi-bolometric peak luminosity of (6.7 ± 1.3) × 10⁴² erg s^-1, we estimate the ⁵⁶Ni mass produced during the explosion as 0.30 M. We also give a rough constraint to the ejecta mass 5-7 M and the kinetic energy (7-18) × 10⁵¹ erg. We find a weak correlation between the peak absolute magnitude and He I velocity among Type Ib SNe. The similarities to SN 1998bw in the density structure inferred from the light-curve model as well as the large peak bolometric luminosity suggest that SN 2012au had properties similar to energetic Type Ic supernovae.