Cannibalistic stars: a closer look at binary systems

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Scientists in Atlanta have directly detected the faint glow of stars that have been cannibalised by their orbital companions. Utilising the cutting-edge telescopes at Georgia State University’s Center for High Angular Resolution Astronomy (CHARA), scientists were able to survey a group of stars suspected of devouring most of the gas from their orbiting companion stars. Led by Postdoctoral Research Associate Robert Klement, the research is featured in the leading scientific peer-reviewed publication The Astrophysical Journal.

The stars that the researchers cast their telescopes upon were B-emission line stars (Be stars), extremely high energy, rapidly rotating stars thought to be accompanied by strange orbiting companions. Binary star systems, those with 2 stars that orbit one another, are not uncommon, especially among stars more massive than our sun. Be stars are formed in particularly violent interactions between the stars of a binary system. Pairs with small orbital separations are at risk of cannibalisation, a process in which the more massive star is able to strip its companion of most of its gas. The star pair is connected by a mass transfer stream, leaving behind the tiny hot core of the smaller star.

During cannibalisation, the more massive star is able to strip its companion of most of its gas

The research was conducted as a result of astronomer’s predictions that the mass transfer process results in Be stars spinning up and becoming some of the fastest rotating stars in the universe. So quickly in fact that some of the star’s gas is flung out, eventually forming an orbiting gas ring. Until now, this process has remained a mystery to astronomers as the separation between the stars is too small to distinguish with conventional telescopes and the smaller cannibalised stars are hidden in the glare of their companion. However, the CHARA array has offered researches a window into this mysterious and captivating phenomenon.

The CHARA array is a grouping of six optical telescopes atop the summit of Mt Wilson in California, spread out in such a way that they act like one giant telescope with an effective diameter of 330 metres. This process is called interferometry and it avoids the high cost and massive engineering challenge of building a 330-meter mirror. The advantage of such a large collecting surface is that astronomers can separate the light of binary star pairs with even the smallest of angular separations. Douglas Gies, director of CHARA praised the power of the array, “We are now seeing, for the first time, the result of the stellar feast that led to the stripped stars.”

The researchers hoped to determine if the Be stars had been spun up by mass transfer streams and if they were harbouring stripped, cannibalised companion stars. Observations were made over a two-year period at CHARA and the results were remarkable. Klement and his team discovered nine faint, stripped companion stars orbiting Be stars. They were able to follow the orbital motion of these stripped stars. Klement describing, “The orbits are important because they allow us to determine the masses of stellar pairs, our mass measurements indicate that stripped stars lost almost everything. In the case of the star HR2142, the stripped star probably went from 10 times the mass of the Sun down to about one solar mass.”

This groundbreaking research sheds light upon a previously elusive stage in the life of close binary star pairs, leading to a new understanding of the interaction of binary pairs and their life trajectories. Klement and his team are now expanding the search for cannibalised companions with help from the European Southern Observatory’s Very Large Telescope Interferometer in Chile and the NASA Hubble Space Telescope, in hopes of uncovering more of the secrets hidden by these fascinating cannibalistic duos.

Image: NASA, ESA, and G. Bacon (STScI) via Flickr

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