SN 2018hna: Adding a Piece to the Puzzles of the Explosion of Blue Supergiants

TL;DR

This study provides detailed observations and modeling of SN 2018hna, a peculiar Type II supernova similar to SN 1987A, revealing its progenitor was likely a low-mass blue supergiant possibly influenced by binary interactions.

Contribution

It offers comprehensive photometric and spectroscopic data along with hydrodynamical modeling, enhancing understanding of low-mass blue supergiant progenitors in SN 1987A-like supernovae.

Findings

SN 2018hna is bluer than SN 1987A, indicating higher photospheric temperature.

Progenitor radius estimated at ~45 R_sun, consistent with a blue supergiant.

Ejecta mass around 13.7-17.7 M_sun, kinetic energy ~1.0-1.2 x 10^{51} erg, Ni-56 mass ~0.05 M_sun.

Abstract

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 days 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 BaII $\mathrm{\lambda}$6142 lines in its spectra. Analysis of early-time temperature evolution suggests a radius of $\sim$45 $\mathrm{R_{\odot}}$ 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 $\sim$(13.7--17.7) $\mathrm{M_{\odot}}$, a kinetic energy of $\sim$ (1.0--1.2) $\times 10^{51}$ erg, and a $^{56}$Ni mass of about 0.05 $\mathrm{M_{\odot}}$. Moreover, based on standard stellar evolution and the oxygen mass (0.44--0.73 $\mathrm{M_{\odot}}$) deduced from nebular [OI] lines, the progenitor of SN 2018hna is expected to have an initial main-sequence mass $<$16 $\mathrm{M_{\odot}}$. 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.