SN 2017hpa: A Nearby Carbon-Rich Type Ia Supernova with a Large Velocity Gradient

TL;DR

SN 2017hpa is a nearby, normal Type Ia supernova with unusual spectral features, including prominent carbon lines and a large velocity gradient, suggesting a possible violent white dwarf merger origin.

Contribution

This paper provides extensive photometric and spectroscopic data of SN 2017hpa, highlighting its unique spectral evolution and velocity features that challenge standard models of Type Ia supernovae.

Findings

Displays prominent early carbon absorption features.

Exhibits a large velocity gradient of ~130 km/s/day.

Shows spectral characteristics consistent with a white dwarf merger.

Abstract

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 $M_{\rm max}^{B} \approx$ -19.12$\pm$0.11 mag and a post-peak decline rate \mb\ = 1.02$\pm$0.07 mag. According to the quasibolometric light curve, we derive a peak luminosity of 1.25$\times$10$^{43}$ erg s$^{-1}$ and a $^{56}$Ni mass of 0.63$\pm$0.02 $M_{\odot}$. The spectral evolution of SN 2017hpa is similar to that of normal SNe Ia, while it exhibits unusually rapid velocity evolution resembling that of SN 1991bg-like SNe Ia or the high-velocity subclass of SNe Ia, with a post-peak velocity gradient of $\sim$ 130$\pm$7 km s$^{-1}$ d$^{-1}$. Moreover, its early spectra ($t < -7.9$ d) show prominent \CII~$\lambda$6580 absorption feature, which disappeared in near-maximum-light spectra but reemerged at phases from $t \sim +8.7$ d to $t \sim +11.7$ d after maximum light. This implies that some unburned carbon may mix deep into the inner layer, and is supported by the low \CII~$\lambda$6580 to \SiII~$\lambda$6355 velocity ratio ($\sim 0.81$) observed in SN 2017hpa. The \OI~$\lambda$7774 line shows a velocity distribution like that of carbon. The prominent carbon feature, low velocity seen in carbon and oxygen, and 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 modelling is still needed to reveal the nature of SN 2017hpa.