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

Abstract

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

30 pages, 21 figures, 5 tables