SN 2015bf: a fast declining type II supernova with flash-ionised\n signatures

We present optical and ultraviolet photometry, as well as optical spectra,\nfor the type II supernova (SN) 2015bf. Our observations cover the phases from\n$\\sim 2$ to $\\sim 200$ d after explosion. The first spectrum is characterised\nby a blue continuum with a blackbody temperature of $\\sim 24,000$K and\nflash-ionised emission lines. After about one week, the spectra of SN 2015bf\nevolve like those of a regular SN II. From the luminosity of the narrow\nemission component of H$\\alpha$, we deduce that the mass-loss rate is larger\nthan $\\sim 3.7\\times10^{-3}\\,{\\rm M_\\odot\\,yr^{-1}}$. The disappearance of the\nflash features in the first week after explosion indicates that the\ncircumstellar material is confined within $\\sim 6 \\times 10^{14}$ cm. Thus, we\nsuggest that the progenitor of SN 2015bf experienced violent mass loss shortly\nbefore the supernova explosion. The multiband light curves show that SN 2015bf\nhas a high peak luminosity with an absolute visual magnitude $M_V = -18.11 \\pm\n0.08$ mag and a fast post-peak decline with a $V$-band decay of $1.22 \\pm 0.09$\nmag within $\\sim 50$ d after maximum light. Moreover, the $R$-band tail\nluminosity of SN 2015bf is fainter than that of SNe~II with similar peak by\n1--2 mag, suggesting a small amount of ${\\rm ^{56}Ni}$ ($\\sim 0.009\\,{\\rm\nM_\\odot}$) synthesised during the explosion. Such a low nickel mass indicates\nthat the progenitor of SN 2015bf could be a super-asymptotic-giant-branch star\nthat collapsed owing to electron capture.\n