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What is a supernova?
A supernova is the explosion of a star. It is the largest explosion that takes place in space
What Causes a Supernova?
A supernova happens where there is a change in the core, or
center, of a star. A change can occur in two different ways, with both resulting in a supernova.
The first type of supernova happens in binary star systems. Binary stars are two stars that orbit the same point. One of the stars, a carbon-oxygen white dwarf, steals matter from its companion star. Eventually, the white dwarf accumulates too much matter. Having too much matter causes the star to explode, resulting in a supernova.
The second type of supernova occurs at the end of a single star’s lifetime. As the star runs out of nuclear fuel, some of its mass flows into its core. Eventually, the core is so heavy that it cannot withstand its own gravitational force. The core collapses, which results in the giant explosion of a supernova. The sun is a single star, but it does not have enough mass to become a supernova.
When a White Dwarf steals mass, can there be multiple explosion scenarios?
When a white dwarf accretes gas from a binary companion and gains enough mass to exceed the Chandrasekhar limit, it can ignite in a cataclysmic explosion. This is the typical scenario for a Type Ia supernova, a common curtain call for low- to intermediate-mass stars in binary systems.
However, this isn’t the only way a Type Ia supernova can happen.
A short artist's video depicting a kind of Type Ia explosion mechanism. The single degenerate model is where the white dwarf accretes mass from a companion star until it approaches the Chandrasekhar mass limit and explodes.
In the double-detonation model, the explosion of the white dwarf is triggered by the ignition of an accreted helium shell. In this case, the white dwarf can be far less massive than the Chandrasekhar limit, leading to unexpectedly dim explosions. Past studies have explored the minimum helium shell mass necessary (~0.01 solar mass) for this process and found that helium-shell detonations can efficiently cause core detonations, but there’s still plenty we don’t know about these events. The best way to learn about supernovae — double-detonation or otherwise — is to spot them soon after they happen.
A Double detonation candidate?
In May 2019, an unusual supernova was discovered by ATLAS. ATLAS is an asteroid impact early warning system developed by the University of Hawaii and funded by NASA. It consists of four telescopes (Hawaii ×2, Chile, South Africa), which automatically scan the whole sky several times every night looking for moving objects. After the discovery, Las Cumbres Observatory (LCO) began an observing campaign where photometry and spectra of the object were taken.
The object showed extremely unusual features, in particular SN 2019eix, which revealed extreme reddening, or suppression of flux at short wavelengths. An unusual observation for typical SNe Ia, likely attributed to iron group elements synthesized from helium burning. Padilla Gonzalez and collaborators compared SN 2019eix to double detonation models and found that this supernova was likely caused by an ignition of 0.15 helium solar mass, causing a converging shock wave travel to the core of the white dwarf of 0.76 solar mass that resulted in a secondary detonation. This supernova is extremely unique since there is only a handful of these events recorded in literature.
Citation
Padilla Gonzalez, E., et al. "Peculiar Spectral Evolution of the Type I Supernova 2019eix: A Possible Double Detonation from a Helium Shell on a Sub-Chandrasekhar-mass White Dwarf." The Astrophysical Journal, vol. 953, no. 1, 2023, p. 25.