SN 2017hpa: a carbon-rich Type Ia supernova

Abstract

We present the optical (UBVRI) and ultraviolet (Swift-Ultraviolet and Optical Telescope) photometry, and optical spectroscopy of Type Ia supernova SN 2017hpa. We study broad-band ultraviolet (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 supernova with ΔmB(15) = 0.98 ± 0.16 mag and MB = −19.45 ± 0.15 mag at a distance modulus of μ = 34.08 ± 0.09 mag. The (uvw1 − uvv) colour evolution shows that SN 2017hpa falls in the near-UV (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 λ6355 Å absorption feature evolves rapidly with a velocity gradient, v˙=128±7 km s−1 d−1. The pre-maximum phase spectra show prominent C II λ6580 Å absorption feature. The C II 6580 Å line velocity measured from the observed spectra is lower than the velocity of Si II λ6355 Å, 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 ∼0.019 M⊙. The peak bolometric luminosity is Lbolpeak=1.43×1043 erg s−1. The radiation diffusion model fit to the bolometric light curve indicates 0.61 ± 0.02 M⊙ of 56Ni is synthesized in the explosion.