Astronomers discover binary companion for Type Ia supernova, prove decades-old theory

Freya Preimesberger

UT astronomers and a flash of blue light revealed the story behind some exploding stars.

While working at Harvard in 2012, UT research scientist Howie Marion and his team of astronomers found evidence supporting a decades-old space theory. The discovery provides insight into the formation of Type Ia supernovae, the explosive end-stage of some stars’ lives, according to a study published in The Astrophysical Journal. Researchers have used Type Ia supernovae to study dark energy and measure the expansion of the universe.

Astronomers have long argued over the origins of these supernovae. In one long-standing theory, Type Ia supernovae emerge from a binary system of two stars: a white dwarf and a main-sequence star.

Main-sequence stars are younger, ordinary stars, such as the sun, that burn hydrogen. If their initial mass doesn’t reach a certain size, they won’t become hot enough to reach the next stage and burn carbon. These stars become dense white dwarfs. 

“A white dwarf is sort of like this cinder that’s left behind when the star burns out,” Marion said.

In the binary system, gravity transfers mass from the companion star to the white dwarf. Once the white dwarf’s mass hits a critical point, it explodes and forms a Type Ia supernova. 

“When the white dwarf blows up, some of the material should come out at great speeds, nearly the speed of light, and smack into this companion star, and it should get hot and it should get bright and emit some extra radiation,” Craig Wheeler, a UT professor of astronomy and co-author of the study, said.

After explosive material hits the companion star, it emits additional light, which researchers discovered in the form of excess blue light around Supernova 2012cg. This matches theoretical models for the interaction, confirming the existence of a companion star. 

“The excitement is [because] we’ve been working toward this for decades,” Wheeler said. “This is the first time it’s been done.”

The first discovery of a binary companion verified a series of hypotheses that astronomers have long assumed to be true.

Type Ia supernovae have similar brightnesses each time they form, which allows astronomers to calculate their distance by how dim they appear from Earth. This property led previous researchers to use distance of supernovae to discover that the universe’s accelerating expansion is driven by dark energy. 

Scientists have trouble observing supernovae after the initial explosion because the early stages are dim and brief, Wheeler said. 

To make collecting observations easier, the astronomical community collaborates and shares their supernovae discoveries. Astronomers at a University of California system observatory initially detected Supernova 2012cg in 2012 and shared their discovery with other researchers, including Marion.

Researchers made observations using several telescopes, including the Hobby-Eberly Telescope at UT’s McDonald Observatory. Collaborators on this paper came from 20 institutions, including Harvard, A&M, Princeton, Texas Tech, the University of Tokyo and Aarhus University. 

Although this is the first evidence for a companion star, researchers urge that this isn’t a definitive model for Type Ia supernovae formation. Scientists will collect more observations on Type Ia supernovae and continue to study their formation.

“In some sense, this is sort of the opening salvo in what’s going to be a major enterprise over the next decade,” Wheeler said.