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 ∼2 to ∼200 d after explosion. The first spectrum is characterized by a blue continuum with a blackbody temperature of ∼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 α, we deduce that the mass-loss rate is larger than ${\sim}3.7\times 10^{-3}\, {\rm M_\odot \, yr^{-1}}$. The disappearance of the flash features in the first week after explosion indicates that the circumstellar material is confined within ∼6 × 1014 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 MV = −18.11 ± 0.08 mag and a fast post-peak decline with a V-band decay of 1.22 ± 0.09 mag within ∼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 56Ni (${\sim}0.009\, {\rm M_\odot }$) 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.