Looking back on exploding white dwarfs

In the early universe white dwarfs often exploded at lower masses than they do today, US astronomers have found. 

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Image of a Type Ia supernova. Credit: Zwicky Transient Facility

They’re not sure why, but say it indicates that these small dense stars could explode from a variety of causes, and they don’t necessarily need to reach a critical mass before exploding.

The findings, by a team from the California Institute of Technology led by Evan Kirby, will be published in a paper in the Astrophysical Journal. It is currently available on the pre-print server arXiv.

Near the end of their lives, a majority of stars dwindle down into dim white dwarfs, with all their mass packed into a space about the size of Earth. Sometimes, they explode in what’s called a Type Ia supernova.

Studying these supernovae is tricky, as they flare into existence and fade back into darkness within a few months.

Kirby and his team chose to study long-gone supernovae and the white dwarfs that produced them – “fossils” as they call them – using what is known as “galactic archaeology”.

Using the Keck II telescope in Hawaii, they first looked at certain ancient galaxies, those that ran out of material to form stars in the first billion years of the universe’s life. 

Most of the stars in these galaxies, they found, had relatively low nickel content. This meant that the exploded white dwarfs that gave them that nickel must have been relatively low mass – about as massive as the Sun. 

However, the nickel content was higher in more recently formed galaxies, meaning that as more time elapsed after the Big Bang, white dwarfs had begun to explode at higher masses.

Understanding the processes that result in Type Ia supernovae is important, the researchers say, because the explosions themselves are useful tools for making measurements of the universe. 

“Type Ia supernovae have been very useful in calculating things like the rate of expansion of the universe,” Kirby says. 

“We use them all the time in cosmology. So, it’s important to understand where they come from and characterise the white dwarfs that generate these explosions.”

The next step is to study elements other than nickel, in particular manganese.

Manganese production is very sensitive to the mass of the supernova that produces it, and therefore gives a precise way to validate the conclusions drawn by the nickel content.

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