Type 1a supernova, caught in the act of exploding, raises new questions

An artist’s impression of a white dwarf siphoning material from a companion star. Once a critical mass is reached, the white dwarf will explode in a Type 1a supernova. Astronomers caught one in the act of exploding earlier this year. Image: NASA

An international team of astronomers using ground-based telescopes and NASA’s exoplanet-hunting Kepler spacecraft managed to catch a long-awaited glimpse of a Type 1a supernova just minutes after the explosion occurred 170 million light years away.

The observations of SN 2018oh (also known as ASASSN-18bt) generally support the long-held idea that Type 1a supernovae are triggered when a compact white dwarf siphons off enough material from a companion star to reach a critical mass, setting off a titanic thermonuclear blast. But SN 2018oh exhibited a faster-than-expected brightening that could indicate a more complex mechanism at work.

The research is critical because Type 1a supernovae act as fundamental tools for cosmologists. They seem to explode with the same overall luminosity, allowing them to be used as “standard candles” to measure distances across the universe. Type 1a supernovae were instrumental in helping astronomers measure how the expansion of the cosmos is accelerating due to dark energy.

Catching one of these doomed star systems in the act of blowing up has been a long-standing research objective. Astronomers finally spotted one in its initial moments on 4 February using the All-Sky Automated Survey for Supernovae (ASAS-SN), a global network of telescopes headquartered at the Ohio State University. The Kepler space telescope collected complementary data using ultra-sensitive detectors designed to ferret out exoplanets.

“Kepler, in its final days before running out of fuel and being retired, observed the minute changes in brightness of the star’s explosion from its very beginnings, while the ground-based telescopes detected changes in colour and the atomic make-up of this dying star,” said Brad Tucker, an researcher with the Australian National University’s school of astronomy and astrophysics.

“With the combined data from these telescopes, astronomers achieved what they had hoped for, an unprecedented observation of the onset of a star’s death.”

A garden-variety Type 1a supernova typically brights over a period of about three weeks before slowly fading away. But SN 2018oh exhibited an unusual brightening a few days after the explosion, ramping up about three times faster than expected.

These six images show the galaxy hosting a Type 1a suoernova known as SN 2018oh. The top three images were taken before the explosion with the two at left from ground-based telescope and the one at right from the Kepler space telescope. The lower left image shows the supernova light alone after subtracting other light. To the right is the initial discovery image with a post-explosion shot from Kepler at lower right. Image: Pan-STARRS/ASAS-SN/Kepler/Carnegie Observatories

Telescopes in Hawaii and Chile saw the supernova briefly shining with an intensity indicating temperatures of billions of degrees.

“It’s possible in the case of SN 2018oh that the shock wave from the exploding white dwarf ran into the companion star, creating an extremely hot and bright halo that accounts for the added brightness and heat we observed,” Tucker said.

But maybe not. Analysis by Anthony Piro at the Carnegie Institution raises another possibility.

“While the steep increase in ASASSN-18bt’s early brightness could indicate that the explosion collides with another star, our follow-up data don’t fit predictions for how this should look,” said Carnegie’s Tom Holoien.

“Other possibilities, such as an unusual distribution of radioactive material in the exploded star, are a better explanation for what we saw,” he said. “More observations of ASASSN-18bt and more early discoveries like this one will hopefully help us differentiate between different models and better understand the origins of these explosions.”

Added Piro: “Nature is always finding new ways to surprise us, and unique observations like this are great for motivating creative new approaches to how we think about these explosions. As a theorist at the Carnegie Observatories, it’s so helpful and inspiring to be right near the observers who are making these key measurements.”

The observations appear to support a hypothesis based on recent work by the Carnegie Supernova Project that suggests there may be two distinct populations of Type 1a supernovae, ones that show this same sort of early brightening and those that do not.