Sn 2017hpa: A Nearby Carbon-Rich Type Ia Supernova With A Large Velocity Gradient

We present extensive, well-sampled optical and ultraviolet photometry and optical spectra of the Type Ia supernova (SN Ia) 2017hpa. The light curves indicate that SN 2017hpa is a normal SN Ia with an absolute peak magnitude of M-max(B) approximate to -19.12 +/- 0.11 mag and a postpeak decline rate Delta m(15)(B) = 1.02 +/- 0.07 mag. According to the quasi-bolometric light curve, we derive a peak luminosity of 1.25 x 10(43) erg s(-1) and a Ni-56 mass of 0.63 +/- 0.02M(circle dot). The spectral evolution of SN 2017hpa is similar to that of normal Ia supernovae (SNe Ia), while it exhibits an unusually rapid velocity evolution resembling that of SN 1991bg-like SNe Ia or the high-velocity subclass of SNe Ia, with a postpeak velocity gradient of similar to 130 +/- 7 km s(-1) day(-1). Moreover, its early spectra (t < - 7.9 days) show a prominent C II lambda 6580 absorption feature, which disappeared in near-maximum-light spectra but reemerged at phases from t similar to + 8.7 days to t similar to + 11.7 days after maximum light. This implies that some unburned carbon may mix deep into the inner layer and is supported by the low C II lambda 6580-to-Si II lambda 6355 velocity ratio (similar to 0.81) observed in SN 2017hpa. The O I lambda 7774 line shows a velocity distribution like that of carbon. The prominent carbon feature, the low velocity seen in carbon and oxygen, and the large velocity gradient make SN 2017hpa stand out from other normal SNe Ia and are more consistent with predictions from a violent merger of two white dwarfs. Detailed modeling is still needed to reveal the nature of SN 2017hpa.