SN 2017fgc: A Fast-Expanding Type Ia Supernova Exploded in Massive Shell\n Galaxy NGC 474

Abstract

We present extensive optical photometric and spectroscopic observations of\nthe high-velocity (HV) Type Ia supernova (SN Ia) 2017fgc, covering the phase\nfrom $\\sim$ 12 d before to $\\sim 389$ d after maximum brightness. SN 2017fgc is\nsimilar to normal SNe Ia, with an absolute peak magnitude of $M_{\\rm max}^{B}\n\\approx$ $-19.32 \\pm 0.13$ mag and a post-peak decline of ${\\Delta}m_{15}(B)$ =\n$1.05 \\pm 0.07$ mag. Its peak bolometric luminosity is derived as $1.32 \\pm\n0.13) \\times 10^{43} $erg s$^{-1}$, corresponding to a $^{56}$Ni mass of $0.51\n\\pm 0.03 M_{\\odot}$. The light curves of SN 2017fgc are found to exhibit excess\nemission in the $UBV$ bands in the early nebular phase and pronounced secondary\nshoulder/maximum features in the $RrIi$ bands. Its spectral evolution is\nsimilar to that of HV SNe Ia, with a maximum-light Si II velocity of $15,000\n\\pm 150 $km s$^{-1}$ and a post-peak velocity gradient of $\\sim$ $120 \\pm 10\n$km s$^{-1} $d$^{-1}$. The Fe II and Mg II lines blended near 4300 {\\AA} and\nthe Fe II, Si II, and Fe III lines blended near 4800 {\\AA} are obviously\nstronger than those of normal SNe Ia. Inspecting a large sample reveals that\nthe strength of the two blends in the spectra, and the secondary peak in the\n$i/r$-band light curves, are found to be positively correlated with the\nmaximum-light Si II velocity. Such correlations indicate that HV SNe~Ia may\nexperience more complete burning in the ejecta and/or that their progenitors\nhave higher metallicity. Examining the birthplace environment of SN 2017fgc\nsuggests that it likely arose from a stellar environment with young and\nhigh-metallicity populations.\n