For the first time, strong helium emission lines have been found in the spectrum of a Type Ia supernova, proving that the white dwarf that exploded had a helium-rich companion. After decades of debate about what causes this type of explosion, the findings prove that despite the famous consistency in their brightness, they can have differing causes.
Before we knew what supernovas were, we distinguished them by the presence or absence of emission lines in their spectra. Like other type I supernovas, Ia supernovas have no hydrogen.
We've since learned that Type Ia explosions are caused by thermal runaway events when white dwarfs exceed their maximum stable mass, but the source of the extra material has been a matter of great debate. A study of supernova 2020eyj, reported in Nature, appears to settle that question, at least in one case.
One theory of Ia events has the white dwarf locked in a tight orbit with an ordinary gas star. The immense gravity of the hyper-dense white dwarf pulls material off the companion until the dwarf explodes in a scaled-up version of ordinary novas. An alternative explanation proposes that two white dwarfs collide in a more modest version of the neutron star mergers known as kilonovas.
SN 2020eyj looked like any other Type Ia supernova until a strong helium component was found in its spectrum, identifying the companion as a helium star that had lost its outer layers before the explosion.
“Once we saw the signatures of strong interaction with the material from the companion we tried to also detect it in radio emission”, said Dr Erik Kool, of Stockholm University, in a statement. “The detection in radio is the first one of a Type Ia supernova – something astronomers have tried to do for decades.”
Radio signals are expected if the companion is a conventional star. The failure to find them in other explosions has bolstered the merging white dwarfs theory.
The general public probably associates supernovas with giant stars collapsing in on themselves, as we impatiently wait for Betelgeuse to do. However, the white dwarf explosions known as Type Ia have proven more important for our understanding of the universe, thanks to their exceptionally useful feature of consistent intrinsic brightness. By measuring the apparent brightness of Type Ia supernovas, we can determine their distance, and therefore that of the galaxy in which they lie. This process has led to the greatest revolution in cosmology in recent decades: the discovery of dark energy.
Consequently, the quest to understand Type Ias’ cause has become one of astrophysics highest priorities, but the quest has not been easy.
When other types of supernova are found, we can look back through archival images in search of the star that made them. Identification is not always possible, particularly when the galaxy in question is sufficiently distant, but enough examples have been found to confirm our expectations of the sort of star that causes these events.
The white dwarfs that cause Type Ia explosions, on the other hand, are so faint they don't show up in before-and-after shots. Even their companion stars may not be visible and are almost impossible to identify if they are. Consequently, what we know about Type Ia events comes from a mix of theoretical modeling, and occasional strokes of luck such as a meteorite that may have been formed in the wake of one. SN 2020eyj may be another such lucky break.
It might be thought that since Type Ia supernovas have the same intrinsic brightness, they would also have the same cause. However, SN 2020eyj’s helium lines disprove this. “This is clearly a very unusual Type Ia supernova, but still related to the ones we use to measure the expansion of the universe,” said Joel Johansson. “This supernova tells us that there are many different pathways to a white dwarf star explosion.”
The study is published in Nature.