Optical Observations of the Nearby Type Ia Supernova 2021hpr

Abstract

We present the optical photometric and spectroscopic observations of the nearby Type Ia supernova (SN) 2021hpr. The observations covered the phase of −14.37 to +63.68 days relative to its maximum luminosity in the B band. The evolution of multiband light/color curves of SN 2021hpr is similar to that of normal Type Ia supernovae (SNe Ia) with the exception of some phases, especially a plateau phase that appeared in the V − R color curve before peak luminosity, which resembles that of SN 2017cbv. The first spectrum we observed at t ∼ −14.4 days shows a higher velocity for the Si ii λ6355 feature (∼21,000 km s−1) than that of other normal velocity (NV) SNe Ia at the same phase. Based on the Si ii λ6355 velocity of ∼12,420 km s−1 around maximum light, we deduce that SN 2021hpr is a transitional object between high velocity (HV) and NV SNe Ia. Meanwhile, the Si ii λ6355 feature shows a high velocity gradient (HVG) of about 800 km s−1 day−1 from roughly −14.37 to −4.31 days relative to the B-band maximum, which indicates that SN 2021hpr can also be classified as an HVG SN Ia. Despite SN 2021hpr having a higher velocity for the Si ii λ6355 and Ca ii near-IR (NIR) triplet features in its spectra, its evolution is similar to that of SN 2011fe. Including SN 2021hpr, there have been six supernovae observed in the host galaxy NGC 3147; the supernovae explosion rate in the last 50 yr is slightly higher for SNe Ia, while for SNe Ibc and SNe II it is lower than expected rate from the radio data. Inspecting the spectra, we find that SN 2021hpr has a metal-rich (12 + log(O/H) ≈ 8.648) circumstellar environment, where HV SNe tend to reside. Based on the decline rate of SN 2021hpr in the B band, we determine the distance modulus of the host galaxy NGC 3147 using the Phillips relation to be 33.46 ± 0.21 mag, which is close to that found by previous works.