A small international team of astronomers including Silvia Toonen (UvA) and Gijs Nelemans (Radboud University) has discovered two white dwarf stars orbiting each other that will merge and explode into a type Ia supernova. This missing link of astronomy has long been predicted and has finally been found. The researchers publish their findings Friday in Nature Astronomy.
Supernova-type Ia explosions are common in the universe. However, their precursors are hard to find. The most common idea is that these explosions occur when two burned-up heavy white dwarf stars merge. However, these precursors had never been seen before. Today’s publication is the first discovery.
Heavy enough and close enough together
The stars in question are just 150 light years away from us. Together, they weigh 1.56 times the mass of the sun. That’s heavy enough to go supernova. In the process, they are close enough to each other to ever merge. Their distance apart is a paltry 1/60th of the Earth-Sun distance.
When the two white dwarfs begin to merge, gas and material from one star will collide with the other, eventually creating an explosion that will cause the star to explode. The details are not yet clear, but if the two stars together are heavier than 1.44 times the mass of the sun, it is fairly certain that the system will explode into a supernova.
"Such a double white dwarf has been looked forward to for years, so when I saw this system, I was immediately elated," said first author and leader of the study James Munday of the University of Warwick (United Kingdom).
Series of telescopes, series of data
The astronomers used a series of telescopes and a range of data for their research. First, the Gaia satellite provided a list of promising candidates. Next, the researchers analyzed the starlight from these candidates using two Dutch-British telescopes on the Spanish island of La Palma: the William Herschel Telescope and the Isaac Newton Telescope. They also used the Nordic Optical Telescope (also on La Palma), the European Very Large Telescope (Chile) and data from space telescope Hubble and from Pan-STARRS in Hawaii.
Co-author Silvia Toonen (University of Amsterdam) has been studying binary stars for years. "More double stars have been seen before, but they never met the right specifications. That we now saw the perfect white double dwarf had to come at some point, but it did take a long time though."
Another 23 billion years of waiting
The two stars may be very close together: the eventual supernova is still going to take billions of years. Astronomers calculated that the stars currently orbit each other in 14 and a quarter hours. Over the next billions of years, the stars will emit gravitational waves, slowly spiraling toward each other. Only in 23 billion years will the stars have approached each other so closely that the supernova occurs.
The other Dutchman who participated in the research is Gijs Nelemans (Radboud University Nijmegen, KU Leuven and SRON). Among other things, he is involved in the LISA mission, which in about a decade’s time is to capture the gravitational waves of double white dwarfs. "The now-discovered double stars are still too far apart for LISA. But now that we have discovered one such system, and also so close, more are sure to follow."
Standard candle
From the brightness gradient of Type Ia supernovas, it is easy to predict their intrinsic brightness. As a result, astronomers can use the supernovas to accurately determine distances in the universe. The type Ia supernova is therefore sometimes called the standard candle of the universe. These types of supernovas have been used, for example, to show that the universe is expanding at an accelerated rate.
Image at top: An artistic impression of the explosion of a double white dwarf. This image shows the moment when the heaviest white dwarf (left) has just exploded. University of Warwick/Mark Garlick
Literature ReferenceThere is a super-Chandrasekhar mass type Ia supernova progenitor located 49 pc away that will detonate in 23 Gyr. By: James Munday, Ruediger Pakmor, Ingrid Pelisoli, David Jones, Snehaleta Sahu, Pier-Emmanuel Tremblay, Abinaya Swaruba Rajamuthukumar, Gijs Nelemans, Mark Magee, Silvia Toonen, Antoine Bédard & Tim Cunningham. In: Nature Astronomy.
