Unique 'cannibalising' binary system discovered

The Gaia satellite has spotted a unique binary star system that could help scientists learn more about Type Ia supernovae and the expansion of the Universe.

A diagram showing the binary system orbit of Gaia14aae

Credit: Fraser/Hodgkin/Campbell/BINSIM

Researchers have discovered a unique type of binary system in which a 'cannibalising' white dwarf is stealing gas from its companion star, but uniquely, neither star contains any hydrogen, the most common element in the Universe.

It is the first such star system in which one star completely eclipses the other, with both bodies orbiting each other and causing a total eclipse roughly every 50 minutes.

The team that discovered the binary system say it could be an important laboratory with which to learn more about how binary stars might explode at the end of their lives.

ESA’s Gaia satellite discovered the system behind the study, led by the University of Cambridge. Gaia is a global mission led by ESA to make a 3D map of the Galaxy by surveying over one billion stars.

The system, named Gaia14aae, is 730 lightyears away in the Draco constellation and was discovered by Gaia in August 2014, when the system suddenly became five times brighter over the course of a day.

A team of amateur astronomers led by the University of Cambridge looked at the data and concluded that this burst in energy was because the super dense white dwarf is devouring its companion. Data shows the white dwarf is so dense that a teaspoonful of its material would weigh as much as an elephant.

“It’s rare to see a binary system so well-aligned,” says Dr. Heather Campbell of Cambridge’s Institute of Astronomy, who led the follow-up campaign for Gaia14aae. “Because of this, we can measure the system with great precision in order to figure out what these systems are made of and how they evolved. It’s a fascinating system; there’s a lot to be learned from it.

“It’s really cool that the first time that one of these systems was discovered to have one star completely eclipsing the other, it was amateur astronomers who made the discovery and alerted us. This really highlights the vital contribution that amateur astronomers make to cutting edge scientific research.”

The team used spectroscopy from the William Herschel Telescope in the Canary Islands to conclude that Gaia14aae contains helium, but no hydrogen. This unusual absence means the system is a very rare type known as an AM Canum Venaticorum (AM CVn). AM CVn systems contain a small and hot white dwarf star that devours its bigger companion. These are known as cataclysmic variable systems, in which both stars have lost all of their hydrogen and regularly increase in brightness. AM CVn systems are thought to be key in discovering what causes Type Ia supernovae. This type of supernova explosion occurs in binary systems and their brightness provides a useful tool for measuring the expansion of the Universe.

But the Gaia14aae system is the first discovered in which one star totally eclipses the other. The white dwarf’s gravitational pull has made the companion star swell up and move towards it. The companion is about 125 times the volume of the Sun, while the white dwarf is about the size of Earth. However, the companion star weighs just one per cent of the white dwarf’s mass. It is unknown whether the two stars will collide and cause a supernova explosion, or whether the white dwarf will devour its companion before that can happen.

“Every now and then, these sorts of binary systems may explode as supernovae, so studying Gaia14aae helps us understand the brightest explosions in the universe,” says Dr. Morgan Fraser of the Institute of Astronomy.

“This is an exquisite system: a very rare type of binary system in which the component stars complete orbits faster than the minute hand of a clock, oriented so that one eclipses the other,” says Professor Tom Marsh of the University of Warwick. “We will be able to measure their sizes and masses to a higher accuracy than any similar system; it whets the appetite for the many new discoveries I expect from the Gaia satellite.”

Front image: artist's impression of Gaia14aae

Credit: Marisa Grove/Institute of Astronomy

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