A Type Ia (One-A) supernova is the nuclear explosion of a white dwarf star. This type of supernova is well known; these supernovae are used by astronomers to measure cosmological distances and the expansion of the Universe. But the explosion mechanism of Type Ia supernovae is not well understood. Solitary white dwarfs don’t explode, so it is thought that mass accretion from a neighboring companion star plays a role in triggering the explosion. The accreted mass is the outer layer of the companion star, so it is normally composed mostly of hydrogen, but it was thought that it should also be possible for a white dwarf to accrete helium from a companion star which had lost its outer layer of hydrogen.
As the white dwarf strips matter from its companion star, not all of the material falls onto the white dwarf; some of it forms a cloud of circumstellar material around the binary star system. When a white dwarf explodes in a cloud of circumstellar material, it is expected that the shockwaves from the explosion traveling through the circumstellar material will excite atoms, causing them to emit strong radio waves. However, although many Type Ia supernovae have been observed exploding within a cloud of circumstellar material, so far astronomers had yet to observe radio wave emissions associated with a Type Ia supernova.
An international team of researchers, including members from Stockholm University and the National Astronomical Observatory of Japan, performed detailed observations of a Type Ia supernova which exploded in 2020. They revealed that this supernova was surrounded by circumstellar material consisting mainly of helium, and also succeeded in detecting radio waves from the supernova. Comparing the observed radio wave strength with theoretical models revealed that the progenitor white dwarf star had been accreting material at a rate of about 1/1000 the mass of the Sun every year. This is the first confirmed Type Ia supernova triggered by mass accretion from a companion star with an outer layer consisting primarily of helium.
It is expected that this observation of radio waves from a helium-rich Type Ia supernova will deepen our understanding of the explosion mechanism and the conditions before a Type Ia supernova. Now the team plans to search for radio emissions from other Type Ia supernovae to elucidate the evolution which leads to the explosion.
These results appeared as Kool et al. “A radio-detected Type Ia supernova with helium-rich circumstellar material” in the journal Nature on May 17, 2023.