21st October 2015 Hottest and heaviest contact binary stars discovered Astronomers using the Very Large Telescope have identified the hottest and most massive contact binary ever seen. This double star system – VFTS 352 – is located 160,000 light-years away.
Using the Very Large Telescope (VLT) operated by the European Southern Observatory (ESO) in Chile, astronomers have found the hottest and most massive known contact binary – a double star system with components so close that they touch each other. The two stars are likely to be heading for a dramatic end, during which they either coalesce to create a single giant star, or form a binary black hole. Known as VFTS 352, the system is located 160,000 light-years away in the Tarantula Nebula, which is part of the Large Magellanic Cloud, a dwarf satellite galaxy that orbits relatively close to the Milky Way. This remarkable region is the most active nursery of new stars in the local Universe and observations from the VLT reveal that this pair of young stars is among the most extreme and strangest yet found. VFTS 352 is composed of two very hot, bright and massive stars that orbit each other in little more than a single Earth day. The centres of the stars are separated by just 12 million kilometres (7.5 million miles). In fact, they are so close that their surfaces overlap and a bridge has formed between them. VFTS 352 is not only the most massive known in this rare and tiny class of "overcontact binaries" – with a combined mass about 57 times that of the Sun – but also contains the hottest components, with surface temperatures above 40,000°C (72,000°F). For comparison, our own Sun has a temperature of about 5,505°C (9,941°F)
Extreme stars like these play a key role in the evolution of galaxies and are thought to be the main producers of elements such as oxygen. Such double stars are also linked to exotic behaviour such as that shown by "vampire stars", where a smaller companion sucks matter from the surface of its larger neighbour. In the case of VFTS 352, however, both stars in the system are of almost identical size. Material is, therefore, not sucked from one to another, but instead may be shared. The component stars are estimated to be sharing about 30 per cent of their material. Such a system is very rare, because this phase in the life of the stars is short, making it difficult to catch them in the act. Because the stars are so close together, astronomers think that strong tidal forces lead to enhanced mixing of the material in the stellar interiors. "VFTS 352 is the best case yet found for a hot and massive double star that may show this kind of internal mixing," says Leonardo Almeida, lead author of the study, which appears in the Astrophysical Journal. "As such, it's a fascinating and important discovery." Astronomers predict that in the future, VFTS 352 will face a cataclysmic fate in one of two ways. The first potential outcome is a merging of the two stars, producing a rapidly rotating, and possibly magnetic, gigantic single star: "If it keeps spinning rapidly, it might end its life in one of the most energetic explosions in the Universe – known as a long-duration gamma-ray burst," explains co-author Hugues Sana, from the University of Leuven in Belgium. The second possibility is explained by the lead theoretical astrophysicist in the team, Selma de Mink from the University of Amsterdam: "If the stars are mixed well enough, they both remain compact and the VFTS 352 system may avoid merging. This would lead the objects down a new evolutionary path that is completely different from classic stellar evolution predictions. In the case of VFTS 352, the components would likely end their lives in supernova explosions, forming a close binary system of black holes. Such a remarkable object would be an intense source of gravitational waves." Proving the existence of this second evolutionary path would be an observational breakthrough in the field of stellar astrophysics. But, regardless of how VFTS 352 meets its demise, this system has already provided astronomers with valuable new insights into the poorly understood evolutionary processes of massive overcontact binary stars.
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