Merger Of Two White Dwarfs Produces A Bizarre Star Only 150 Light-Years From Us

Eddie Gonzales Jr. – MessageToEagle.com – Astronomers have discovered an unusual ultra-massive white dwarf around 150 light-years from us with an atmospheric composition never seen before.

This celestial object has been identified using its atmospheric composition as a clue.

Artist’s impression of two white dwarfs in the process of merging. Depending on the combined mass, the system may explode in a thermonuclear supernova, or coalesce into a single heavy white dwarf, as with WDJ0551+4135. Credit: University of Warwick/Mark Garlick

The star, named WDJ0551+4135, was identified in a survey of data from the European Space Agency’s Gaia telescope. The international team of astronomers followed up with spectroscopy taken using the William Herschel Telescope, focusing on those white dwarfs identified as particularly massive—a feat made possible by the Gaia mission.

“This star stood out as something we had never seen before,” said lead author Dr. Mark Hollands, from the University of Warwick Department of Physics.

To solve the puzzle, the astronomers turned detective to uncover the star’s true origins.

White dwarfs are the remains of stars like our own Sun that have burnt out all their fuel and shed their outer layers. Most are relatively lightweight, around 0.6 times the mass of our Sun, but this one weighs in at 1.14 solar masses, nearly twice the average mass. Despite being heavier than our Sun, it is compacted into two-thirds the diameter of Earth.

The age of the white dwarf is also a clue. Older stars orbit the Milky Way faster than younger ones, and this object is moving faster than 99% of the other nearby white dwarfs with the same cooling age, suggesting that this star is older than it looks.

“We have a composition that we can’t explain through normal stellar evolution, a mass twice the average for a white dwarf, and a kinematic age older than that inferred from cooling,” Dr. Hollands said.

“We’re pretty sure of how one star forms one white dwarf and it shouldn’t do this. The only way you can explain it is if it was formed through a merger of two white dwarfs.”

While white dwarf mergers have been predicted to occur, this one would be particularly unusual. Most of the mergers in our galaxy will be between stars with different masses, whereas this merger appears to be between two similarly sized stars. There is also a limit to how big the resulting white dwarf can be: at more than 1.4 solar masses it is thought that it would explode in a supernova though it may be possible for that these explosions can occur at slightly lower masses, so this star is useful in demonstrating how massive a white dwarf can get and still survive.

Because the merging process restarts the cooling of the star, it is difficult to determine how old it is. The white dwarf probably merged around 1.3 billion years ago but the two original white dwarfs may have existed for many billions of years prior.

It is one of only a handful of merged white dwarfs to be identified so far, and the only one via its composition.

“There aren’t that many white dwarfs this massive, although there are more than you would expect to see which implies that some of them were probably formed by mergers,” Dr. Hollands added.

Written by Eddie Gonzales Jr. – MessageToEagle.com Staff