A study led by Dr. ZHANG Jujia from Yunnan Observatories of the Chinese Academy of Sciences interprets the major observational features of luminous Type Ia supernovae through the over-abundance of nickel in outer ejecta.

The study was published in The Astrophysical Journal on Jan. 10. It was based on Lijiang One hour per Night Supernova program (LiONS) project.

Type Ia supernovae (SNe Ia) are widely accepted as the results of the explosion of a carbon-oxygen white dwarf in a binary system. Their high and standardizable luminosity make them excellent cosmological distance indicators.

Based on the function in distance measurement, SNe Ia provide the first evidence for the acceleration of cosmic expansion and, hence, the current dark energy-dominated cosmic mass-energy inventory.

SNe Ia continue to be a major cosmographic tool, with future efforts focusing on determining the evolution on cosmological timescales of the dark energy equation of state. Thus, how to improve the role as a distance indicator is the major problem in SNe Ia. However, some luminous SNe Ia with higher luminosity than the expectation of the empirical relation reduces the precision in distance measurements.

LiONS team obtained the essential data of a Type Ia supernova SN 2015bq based on the high-cadence monitoring campaign at the Lijiang 2.4 m telescope. They found evidences of over-abundance of nickel in outer ejecta during the explosion, such as the early flux excess, the high ionized absorptions and the monotonous evolution in the color curves. These indicated that the common channel combined with the over-abundance of nickel in outer ejecta could almost interpret the particular natures of the luminous SNe Ia.

Besides, they also found that the luminosity of luminous SNe Ia was related to the velocity and density distribution of ejecta, considering the previous work of LiONS. The more expanding at the early phase, the higher luminosity at the peak. Thus, it is possible to standardize these luminous SNe Ia under the structure of their ejecta.

LiONS team suggests this subclass of SNe Ia can also be applied as an excellent indicator to measure the farther cosmological distance due to the higher luminosity than the normal SNe Ia. A new standardizable method considering ejecta construction needs to be introduced to achieve this purpose.

This work reveals that the luminous SNe Ia might share a common mechanism as the normal SNe Ia. The significant characters are seen in the former origins from the construction and abundance in the outer ejecta.