Unusual stars may not owe their origins to hypernovae after all

Evidence for hypernovae, theorized to be the most powerful explosions in the universe, has been disputed in two papers by University College London (UCL) researchers.

Previous work argued that unusual stars in the Milky Way’s halo (the cloud of stars surrounding the disk) and in a neighboring dwarf galaxy could only have formed from material blasted out by a stellar explosion at least 10 times more energetic than a typical supernova.

In two studies published in Monthly Notices of the Royal Astronomical Society, researchers found that ordinary supernovae could better explain the unusual makeup of these stars once the uneven way that material is flung into space was taken into account.

Uneven debris changes the stellar evidence

First author Anmol Aggarwal, a Ph.D. student at the Mullard Space Science Laboratory at UCL, said, “If we look at a supernova today, the star’s materials are not spread evenly. Oxygen might predominantly fly in one direction, sulfur in another. New stars might form from a mix of pieces of the supernova, not its overall mix of elements.

“We developed mathematical models that took account of this and found in all cases that stars with a very unusual mix of ingredients were most likely formed from ordinary supernovae. All the evidence we see for hypernovae is suddenly gone.”

A supernova occurs when a massive star runs out of fuel and collapses in on itself, causing an immense explosion. A hypernova—a much more energetic explosion—is theorized to occur when the same process happens to a more massive and rapidly spinning star.

These explosions fling material into nearby star-forming clouds that form new stars and planets. This process allows researchers to analyze a star’s composition and infer its family history (i.e., the stellar explosions that led to its formation).

Second author Dr. Ralph Schoenrich, also based at the Mullard Space Science Laboratory at UCL, said, “Just before a star goes supernova, it has layers like an onion, with heavier elements at the center and lighter ones farther out. Our model looks at how much material from each layer or each region would be needed to fit the observed star.

“We still don’t know how well the supernova material gets mixed before it forms new stars. That is despite decades of work in this area. Our research suggests some mixing goes on, but incomplete mixing.

“We need to take this into account in our models of how chemicals evolve in galaxies and of how the interstellar medium works.”

Four stars put to the test

Across two studies, the UCL team looked at four stars previously judged to have formed from material expelled by a hypernova. Three of these stars, they found in their first study, were more likely to have formed from supernovae (i.e., this was statistically favored over a hypernova).

In their second study, they looked at a single red giant in the Milky Way’s halo with a very peculiar chemical makeup. This included an abundance of certain heavy elements such as silver and uranium that can only be produced by neutron star mergers (the collisions of ultradense remnants of stars) or, in theory, the hypernova of a highly magnetized massive star that can form these heavy elements and also standard supernova elements, i.e., elements no heavier than iron and nickel.

Previous work argued that the star could not have been formed from neutron stars colliding. This was because the star is very metal-poor—that is, it has a very low proportion of heavier elements created in stellar explosions, and so formed from material that had barely been enriched by earlier generations of stars. The researchers used a Milky Way model that enriched its gas with heavy elements very quickly, meaning that, by the time the star was born, not enough time could have passed for neutron stars to form and then collide.

A dwarf galaxy offers another explanation

In the new analysis, Aggarwal and Schoenrich presented a different picture. They noted the star’s exceptional speed and the fact it orbited the Milky Way in the opposite direction to most other stars, and argued it originated in a small dwarf galaxy where metal-poor stars can form much later, many millions of years after the Big Bang, and only later got pulled into the Milky Way.

Using their mathematical model accounting for asymmetrical explosions, the researchers concluded the star most likely formed from a neutron star merger combined with a single ordinary supernova.

In tiny galaxies, stars form more slowly, and when they do explode, most of the material is blasted out into intergalactic space.

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