Over the decades leading up to the explosion of the rare and luminous Type Ia supernova SN 2022erq, the mass-loss rate of its progenitor system rose sharply, according to new observations. The finding provides new observational evidence for understanding how Type Ia supernova progenitor systems evolve during their final stages.

Led by Dr. ZHAI Qian from Yunnan Observatories (YNAO) of the Chinese Academy of Sciences (CAS), the study was published in The Astrophysical Journal.

SN 2022erq was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS). Shortly thereafter, the research team conducted spectroscopic observations with the Lijiang 2.4 m Telescope at Yunnan Observatories and identified it as an SN Ia-CSM — a Type Ia supernova strongly interacting with circumstellar material (CSM). Such events are extremely rare, accounting for only about 0.1% of all Type Ia supernovae.

Following the identification, the team coordinated observations with multiple telescopes worldwide and continuously monitored SN 2022erq for three years. By analyzing the spectroscopic and photometric data, they reconstructed the mass-loss history of the progenitor system leading up to the explosion.

About 60 years before the explosion, the system was ejecting material at a rate of approximately 0.04 solar masses per year. By the final few years before the explosion, that rate had soared to about 0.6 solar masses per year. The expelled material accumulated into a massive gas shell with a total mass of about three solar masses, extending to approximately 350 billion kilometers.

The observations challenge the existing theoretical models of Type Ia supernova progenitors. Although current mainstream models can produce modest amounts of circumstellar material, they struggle to simultaneously explain both the enormous CSM mass and the extreme mass-loss rate observed in SN 2022erq.

The researchers suggest that the progenitor was likely a close binary system consisting of a white dwarf and an intermediate-mass non-degenerate companion.

Shortly before the explosion, the system may have undergone a brief and violent evolutionary phase — such as common-envelope evolution with intense mass transfer, or the rapid merger of the white dwarf with the core of a giant companion. Such strong binary interaction could account for the sharp increase in mass loss shortly before the explosion. The unusual physical conditions required may also explain why SNe Ia-CSM are so rare.

According to the researchers, SN 2022erq provides a high-quality observational benchmark for this rare class of supernovae while placing clear quantitative constraints on theoretical models.

In particular, the observations raise a key question: What binary evolutionary pathway can eject several solar masses of material within only decades before a thermonuclear explosion? Answering this question could deepen our understanding of Type Ia supernova progenitors and the violent processes that binary systems may undergo during their final evolutionary stages before explosion.

The study used observations from more than a dozen telescopes worldwide. It was supported by the National Key Research and Development Program of China, the Strategic Priority Research Program of CAS, the National Natural Science Foundation of China, the Yunnan Provincial Science and Technology Projects, the Yunnan "Xingdian Talents" Program, and the International Centre of Supernovae (ICESUN).

Schematic illustration of the progenitor system, circumstellar environment, and post-explosion interaction of SN 2022erq. The top panel shows an artist's impression of the progenitor system; the lower-left panel illustrates the SN-CSM interaction; and the lower-right panel presents the measured CSM density profile. (Image by ZHAI Qian)