Recently, Dr. ZHANG Jujia from Yunnan Observatories of Chinese Academy of Sciences, and his collaborators, published their new finding on Type II supernovae (SNe II) in Monthly Notices of the Royal Astronomical Society. The study interpreted the observational diversities of SNe II via the interaction scenario, which is essential to understand the natures of SNe II and to improve the precision of cosmology.

Similar to Type Ia supernovae, SNe II is another excellent cosmological tool to study the evolution of the universe via distance measurement. It is further divided into Type IIP (SNe IIP) and Type IIL (SNe IIL) based on the light curve shape.

SNe IIP is characterized by a relatively constant optical luminosity plateau lasting for about three months after the explosion, followed by a rapid drop to the radioactive tail. The thermalization of the initial shock wave and the ionized hydrogen recombination provide sources to power the plateau light curve.

SNe IIL was named after their linear (in magnitudes) light-curve decay that starts soon after peak brightness. Based on the understanding of SNe IIP, the absence of a plateau in the light curves of SNe IIL might suggest less energy input and lower-mass hydrogen envelope in their progenitors. However, the higher peak brightness of SNe IIL and the possible drop disfavors this hypothesis.

In this study, the researchers found a strong stellar-wind mass-loss history of SN 2018zd via optical monitoring. This wind created a massive (0.18 M_⊙) circumstellar material (CSM) shell at about 2000 R_⊙ from the progenitor's center. Supernova interacted with this CSM produces higher luminosity than the normal.

Considering the evolution of SN 2018zd, they suggest that SNe IIP with short-lived interaction can create a luminous peak and fast decline, as seen in the light curve of SNe IIL. This scenario also matches the luminosity drops seen in both SNe IIP and SNe IIL.

This study indicated that SN-CSM interaction variation is essential to produce the observational diversity of SNe II. However, the previous work might lose the flashed signatures of interaction due to the slow response in spectral classification, which causes confusion to understand the natures of SNe II.

Thanks to the flexible time allocation policy and prompt response of the Lijiang 2.4 m telescope (LJT), the critical data of SN 2018zd has been captured at only a few hours after the discovery. Besides, benefiting from the faint object detection power of LJT and international cooperation, this study investigated the nature of the explosion via long-time monitoring, which lasted 456 days. At the last LJT observation, the brightness of this SN is only 1‰ to the peak.

In brief, the diversity of SNe II originates from SN-CSM interaction. It is a significant error source in the SNe II cosmology. Thus, the fast response as LJT shown in this study is essential to improve distance measurement precision.