SN 2017hpa: A carbon-rich type Ia supernova

Abstract

We present the optical (UBVRI) and ultraviolet (Swift-UVOT) photometry, and optical spectroscopy of Type Ia supernova SN 2017hpa. We study broadband UV+optical light curves and low resolution spectroscopy spanning from $-13.8$ to $+108$~d from the maximum light in $B$-band. The photometric analysis indicates that SN 2017hpa is a normal type Ia with $\hat{I}”m_B(15)=0.98\pm 0.16$ mag and $M_B=-19.45\pm 0.15$ mag at a distance modulus of $\hat{I}¼=34.08\pm 0.09$ mag. The $(uvw1-uvv)$ colour evolution shows that SN 2017hpa falls in the NUV-blue group. The $(B-V)$ colour at maximum is bluer in comparison to normal type Ia supernovae. Spectroscopic analysis shows that the Si II 6355 absorption feature evolves rapidly with a velocity gradient, $\dot{v}=128\pm 7$ km s${}^{-1}$ d${}^{-1}$. The pre-maximum phase spectra show prominent C II 6580 à absorption feature. The C II 6580 à line velocity measured from the observed spectra is lower than the velocity of Si II 6355 à , which could be due to a line of sight effect. The synthetic spectral fits to the pre-maximum spectra using syn++ indicate the presence of a high velocity component in the Si II absorption, in addition to a photospheric component. Fitting the observed spectrum with the spectral synthesis code TARDIS, the mass of unburned C in the ejecta is estimated to be $\sim 0.019$~$M_{\odot }$. The peak bolometric luminosity is $L_{peak}^{bol}=1.43\times 10^{43}$ erg s${}^{-1}$. The radiation diffusion model fit to the bolometric light curve indicates $0.61\pm 0.02$ $M_{\odot }$ of ${}^{56}$Ni is synthesized in the explosion.

15 pages, 13 figures, Accepted for publication in MNRAS