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.

Automated summary

The study presents optical and ultraviolet photometry, as well as optical spectra of the type II supernova (SN) 2015bf. The observations reveal a high peak luminosity and fast post-peak decline of SN 2015bf, suggesting a small amount of synthesized Ni-56 during the explosion. The disappearance of flash features within the first week after explosion indicates violent mass loss of the progenitor shortly before the supernova explosion.