SN 2018hna: Adding a piece to the puzzles of the explosion of blue supergiants

We present extensive optical/ultraviolet observations and modelling analysis for the nearby SN 1987A-like peculiar Type II supernova (SN) 2018hna. Both photometry and spectroscopy covered phases extending to >500 d after the explosion, making it one of the best-observed SN II of this subtype. SN 2018hna is obviously bluer than SN 1987A during the photospheric phase, suggesting higher photospheric temperature, which may account for weaker Ba ii lambda 6142 lines in its spectra. Analysis of early-time temperature evolution suggests a radius of similar to 45 R-circle dot for the progenitor of SN 2018hna, consistent with a blue supergiant (BSG). By fitting the bolometric light curve with hydrodynamical models, we find that SN 2018hna has an ejecta mass of similar to(13.7-17.7) M-circle dot, a kinetic energy of similar to(1.0-1.2) x 10(51) erg, and a Ni-56 mass of about 0.05 M-circle dot. Moreover, based on standard stellar evolution and the oxygen mass (0.44-0.73 M-circle dot) deduced from nebular [O i] lines, the progenitor of SN 2018hna is expected to have an initial main-sequence mass <16 M-circle dot. In principle, such a relatively low-mass star cannot end as a BSG just before core-collapse, except some unique mechanisms are involved, such as rapid rotation, restricted semiconvection, etc. On the other hand, binary scenario may be more favourable, like in the case of SN 1987A. While the much lower oxygen mass inferred for SN 2018hna may imply that its progenitor system also had much lower initial masses than that of SN 1987A.

Automated summary

We present extensive observations and analysis for the nearby SN 1987A-like peculiar Type II supernova (SN) 2018hna. The observations cover phases extending to >500 days after the explosion, making it one of the best-observed SN II of this subtype. The analysis suggests that SN 2018hna has a higher photospheric temperature and a smaller progenitor radius compared to SN 1987A. By fitting the bolometric light curve with hydrodynamical models, the study estimates the ejecta mass, kinetic energy, and Ni-56 mass of SN 2018hna. The study also discusses the possible mechanisms and scenarios for the formation of SN 2018hna.