Recently, Doctor LI Liping (postdoctoral fellow) and Professor ZHANG Jujia from the Southern Astronomical Observing Base of Yunnan Observatories, together with their collaborators, conducted a detailed optical observational study of the young Type Ia supernova SN 2021fxy. Their work reveals the unusual velocity evolution of high-velocity features (HVFs) that are clearly separated from the photosphere in its early spectra, providing new observational constraints on the explosion physics of Type Ia supernovae. The results were recently published in The Astrophysical Journal.
Type Ia supernovae originate from the thermonuclear explosion of a carbon–oxygen white dwarf in a binary system. As crucial “standard candles” in the universe, they played a key role in the discovery of cosmic acceleration. Observationally, some Type Ia supernovae exhibit prominent HVFs in their early spectra absorption lines of elements that are distinctly detached from the photospheric component. Whether these HVFs arise from intrinsic ejecta structures produced by the explosion itself or from interactions with circumstellar material remains a hotly debated topic.
Using the Lijiang 2.4 m telescope of Yunnan Observatories and other facilities, the research team obtained multi-band photometric and spectroscopic data of SN 2021fxy during the very early phase after explosion. In its early spectra, they identified extremely strong, well-separated HVFs of silicon and calcium, distinct from the photospheric component. Through precise measurements of the velocity evolution of these detached HVFs, the team found that the velocity of the Si II λ6355 HVF declines very slowly, following a power-law index of β ≈ 0.1. This rate of velocity decrease is significantly slower than that of the photospheric component over the same period (β ≈ 0.22), and also slower than the theoretical expectation for a typical outer density profile under the assumption of homologous expansion. Such a slow velocity evolution suggests that the ejecta giving rise to the HVFs do not simply come from the outermost continuous layers of the ejecta; instead, they form an independent density structure that is at least partially decoupled from the main outer ejecta. This structure may correspond to clumps produced by burning fingers in the delayed-detonation model, or to the ash from helium-shell burning in the double-detonation scenario. This finding provides important observational constraints on the outer structure and explosion mechanism of Type Ia supernovae.
This work made use of the Lijiang 2.4 m telescope and Tsinghua-NAOC 0.8 meter telescope. It was supported by the National Key R&D Program of China, the B-type Strategic Priority Program of the Chinese Academy of Sciences, the National Natural Science Foundation of China, the Yunnan Revitalization Talent Support Program，the International Centre of Supernovae (ICESUN), etc.
Contact:
LI Liping
Yunnan Observatories, CAS
e-mail:liliping@ynao.ac.cn
