SN 2015bf: A fast declining type II supernova with flash-ionized signatures

We present optical and ultraviolet photometry, as well as optical spectra, for the type II supernova (SN) 2015bf. Our observations cover the phases from similar to 2 to similar to 200 d after explosion. The first spectrum is characterized by a blue continuum with a blackbody temperature of similar to 24 000 K and flash-ionized emission lines. After about 1 week, the spectra of SN 2015bf evolve like those of a regular SN II. From the luminosity of the narrow emission component of H alpha, we deduce that the mass-loss rate is larger than similar to 3.7 x 10(-3) M-circle dot yr(-1). The disappearance of the flash features in the first week after explosion indicates that the circumstellar material is confined within similar to 6 x 10(14) cm. Thus, we suggest that the progenitor of SN 2015bf experienced violent mass loss shortly before the supernova explosion. The multiband light curves show that SN 2015bf has a high peak luminosity with an absolute visual magnitude M-V = -18.11 +/- 0.08 mag and a fast post-peak decline with a V-band decay of 1.22 +/- 0.09 mag within similar to 50 d after maximum light. Moreover, the R-band tail luminosity of SN 2015bf is fainter than that of SNe II with similar peak by 1-2 mag, suggesting a small amount of Ni-56 (similar to 0.009 M-circle dot) synthesized during the explosion. Such a low nickel mass indicates that the progenitor of SN 2015bf could be a super-asymptotic-giant-branch star that collapsed owing to electron capture.