A research team from the Yunnan Observatories of the Chinese Academy of Sciences (CAS), in collaboration with domestic and international partners, has carried out observational studies on SN 2024gy—a high-velocity Type Ia supernova (SN Ia)—using the Lijiang 2.4-meter telescope.

Their findings, recently published in The Astrophysical Journal, offer new observational evidence supporting the "delayed-detonation" model of Type Ia supernovae (SNe Ia).

SNe Ia originate from thermonuclear explosions of carbon-oxygen white dwarfs within binary star systems. Regarded as critical "standard candles" in the universe, they were pivotal to the discovery of cosmic accelerated expansion and play an indispensable role in measuring cosmological distances. However, their progenitor systems and explosion mechanisms have long been subjects of scientific debate. Notably, observations reveal that some SNe Ia display prominent high-velocity features (HVFs) in their early-phase spectra; the physical origins of these HVFs and their links to explosion mechanisms remain a focus of active research.

To address this gap, the research team deployed the Lijiang 2.4-meter telescope at Yunnan Observatories, complemented by data from a suite of facilities. These include the Xinglong 2.16-meter telescope (National Astronomical Observatories, CAS), the 1.6-meter Multi-Channel Photometric Survey Telescope (Yunnan University), the 0.8-meter Tsinghua–NAOC Telescope, the Shane 3-meter telescope at Lick Observatory in California, USA, and the Keck I telescope in Hawaii. Through this multi-facility collaboration, the team obtained multi-band photometric and spectroscopic evolution data for SN 2024gy, spanning from approximately two days to over 400 days post-explosion.

Analysis of the observational data confirms that SN 2024gy is a normal-luminosity SN Ia, characterized by strong calcium HVFs in its early-phase spectra—features independent of the photospheric component. The study tracked the velocity evolution of various spectral components in SN 2024gy's early phases, suggesting that marked velocity differences may reflect distinct ionization states of intermediate-mass elements in the outermost ejecta.

The observed HVFs of calcium in the spectra of SN 2024gy, along with the layered velocity structure between silicon and calcium, align closely with effects such as ionization suppression or density enhancement predicted by the delayed-detonation model in the outer ejecta.

Furthermore, the nickel-to-iron abundance ratio, derived from emission lines of iron-group elements in the supernova's late nebular-phase spectra, falls within the theoretical range projected by the delayed-detonation model. These consistent lines of evidence—encompassing early-phase dynamical characteristics and late-phase nucleosynthesis products—provide support for the delayed-detonation physical scenario.

This study delivers key observational constraints for understanding the outer-layer structure and explosion mechanisms of SNe Ia. A deeper grasp of early spectral features, such as high-velocity components, is essential to unraveling the nature of their progenitor systems and refining explosion models. Such advances will, in turn, boost the precision of SNe Ia as tools for measuring cosmological distances.

This work was supported by the National Key R&D Program of China, the CAS B-type Strategic Priority Program, and other sources.