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Stellar alumna

Astronomy scholar discovers new type of star


By Kelly Foss

A Memorial Faculty of Science alumna has made a discovery that can potentially reveal what the universe is made of.

A woman with long brown hair and burgundy glasses sits in the Bruneau building
While completing her joint major in physics and applied mathematics at Memorial, Anna O’Grady spent a lot of quiet study time in the Bruneau building.
Photo: Rich Blenkinsopp

Anna O’Grady (B.Sc.’16), originally of Kilbride, is currently an astronomy PhD student at the University of Toronto.

In October she published a paper in The Astrophysics Journal confirming the existence of a new type of star known as a super asymptotic giant branch, or super-AGB, star.

“We were looking at an unusual star and thought it might be something called a Thorne Żytkow Object, which is this really crazy hypothetical class of stars that are essentially stars within stars – a neutron star that is inside a massive red star,” Ms. O’Grady said. “It’s the weirdest thing I’ve heard of in all of astronomy.”

Didn’t meet requirements

In trying to find more stars that looked like that one, Ms. O’Grady and her collaborators found nine more, and were soon able to determine they weren’t what they first believed.

Through the researchers’ analysis, looking at how bright the stars were, their colour and temperature, they eventually came to the conclusion the stars didn’t meet the requirements.

“Thorne Żytkow Objects have to be at least 15 times the mass of our sun. If they aren’t, they can’t support themselves. Whereas these stars all fell into the five to 12 solar mass range, so they were too small. But all of their properties, including mass, lined up with being super-AGB stars.”

Tipping point

Prior to her findings, there was only one other candidate for a super-AGB star – a star in our galaxy that also appears to meet the qualifications.

A woman with long brown hair and burgundy glasses sits in front of a building.
Anna O’Grady says none of the 10 stars she found are in the Milky Way.
Photo: Rich Blenkinsopp

“However, none of the 10 stars I found are in our galaxy. They’re either in the Small or Large Magellanic Cloud, which are small satellite galaxies that orbit our Milky Way.”

She says super-AGB stars are interesting because they are at the tipping point or dividing line between low mass stars and high mass stars.

“Low mass stars, like our sun, don’t explode when they reach the end of their lives, while high mass stars will generally explode as supernova,” explained Ms. O’Grady.

The elements that make up the universe, and many of the rare elements found on Earth, are created as stars reach the end of their lives, with different elements formed from low mass stars dying than from supernova explosions.

“There’s always this passing the ball back and forth . . . that make up the continuing thread of astronomy.” — Anna O’Grady

Ms. O’Grady says understanding the mass at which stars start to explode, and whether that matters in their environment, can help scientists figure out the abundance of particular elements that appear in the universe.

Passing the ball

She now hopes that theoreticians will take the data from her paper, and the number of super-AGB stars she’s identified, and determine if she’s found them all.

“There’s always this passing the ball back and forth, between the theory and the observations, that make up the continuing thread of astronomy.”

Ms. O’Grady is currently working on a follow-up paper that delves deeper into confirming these stars are super-AGB stars and not Thorne Żytkow Objects.

“The paper has a couple of different techniques that will check off a few remaining boxes of whether or not there is evidence of them being Thorne Żytkow Objects, or if the hypothesis of them being super-AGB stars holds,” she said.

“And, spoiler alert . . . for all of the analysis I’ve done so far, they continue to look like super-AGB stars.”

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