Astronomers have observed radio waves from a Type Ia supernova, an explosion originating from a white dwarf star, for the first time. This provides important clues into how white dwarfs explode. A Type Ia (One-A) supernova is the nuclear explosion of a white dwarf star. This type of supernovae is widely known; astronomers use these supernovae to measure cosmological distances and the expansion of the universe. However, the explosion mechanism of Type Ia supernovae is not well understood.
Lone white dwarfs do not explode, so it is thought that accretion of mass from a nearby companion star plays a role in triggering the explosion. The accreted material is the outer layers of the companion star and so is usually composed mainly of hydrogen, but it is thought that it is also possible for a white dwarf to accrete helium from a companion star that has lost its outer hydrogen layers.
When a white dwarf strips material from its companion star, not all of it falls onto the white dwarf; some forms a cloud of orbiting material around the binary system. When a white dwarf explodes in a cloud of circumstellar material, the shock wave from the explosion is expected to pass through the circumstellar material and excite the atoms, causing them to emit intense radio waves. However, although many Type Ia supernovae have been observed exploding in circumstellar clouds of material, astronomers have so far not observed the radio wave emission associated with Type Ia supernovae.
Artistic impression of a binary star system: A compact white dwarf absorbs material from a helium-rich donor companion, surrounded by a high density of dusty peristellar material. It is the interaction between the exploded star and the remaining material of the companion star that generates strong radio signals and forms obvious helium lines in the optical spectrum of SN2020eyj. Source: Adam Makarenko/W.M. Keck Observatory
An international research team composed of members from Stockholm University and the National Astronomical Observatory of Japan (NAOJ) has made detailed observations of a Type Ia supernova that exploded in 2020. They found that the supernova was surrounded by circumstellar material composed mainly of helium and successfully detected radio waves from the supernova. Comparing the observed radio wave intensity with theoretical models, it was found that the primordial white dwarf accreted material at a rate of approximately 1/1000 of the solar mass per year. This is the first confirmed Type Ia supernova caused by the accretion of mass from a companion star whose outer layers are mainly composed of helium.
This observation of radio waves from helium-rich Type Ia supernovae is expected to deepen our understanding of the explosion mechanism and pre-explosion conditions of Type Ia supernovae. Now, the team plans to search for radio emissions from other Type Ia supernovae to shed light on the evolution that led to the explosions.
These results were published in the journal Nature under the title "Aradio-detected Type Iasupernovawithhelium-richcircumstellarmaterial" by Kool et al.