Using ESO's Very Large Telescope, an international team of astronomers have found the hottest and most massive double star with components so close that they touch each other. The two stars in the extreme system VFTS 352 could be heading for a dramatic end, during which the two stars either coalesce to create a single giant star, or form a binary black hole.
The double star system VFTS 352 is located about 160 000 light-years away in the Tarantula Nebula . This remarkable region is the most active nursery of new stars in the nearby Universe and new observations from ESO's VLT  have revealed 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 day. The centres of the stars are separated by just 12 million kilometres . In fact, the stars 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 tiny class of "overcontact binaries" -- it has a combined mass of about 57 times that of the Sun -- but it also contains the hottest components -- with surface temperatures above 40 000 degrees Celsius.
Extreme stars like the two components of VFTS 352, 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 star sucks matter from the surface of its larger neighbour (eso1230).
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 of VFTS 352 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.
"The VFTS 352 is the best case yet found for a hot and massive double star that may show this kind of internal mixing," explains lead author Leonardo A. Almeida of the University of São Paulo, Brazil. "As such it's a fascinating and important discovery."
Astronomers predict that VFTS 352 will face a cataclysmic fate in one of two ways. The first potential outcome is the merging of the two stars, which would likely produce 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," says the lead scientist of the project, Hugues Sana, of the University of Leuven in Belgium .
The second possibility is explained by the lead theoretical astrophysicist in the team, Selma de Mink of 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 star systems.
 This star's name indicates that it was observed as part of the VLT FLAMES Tarantula Survey, which utilised the FLAMES and GIRAFFE instruments on ESO's Very Large Telescope (VLT) to study over 900 stars in the 30 Doradus region of the Large Magellanic Cloud (LMC). The survey has already led to many exciting and important findings including the fastest rotating star (eso1147 - http://www.
 This study also used brightness measurements of VFTS 352 over a period of twelve years made as part of the OGLE survey.
 Both components are classed as O-type stars. Such stars are typically between 15 and 80 times more massive than the Sun and can be up to a million times brighter. They are so hot that they shine with a brilliant blue-white light and have surface temperatures over 30 000 degrees Celsius.
 These regions around the stars are known as Roche lobes. In an overcontact binary such as VFTS 352 both stars overfill their Roche lobes.
 Gamma-ray Bursts (GRBs) are bursts of highly energetic gamma rays that are detected by orbiting satellites. They come in two types -- short duration (shorter than a few seconds), and long duration (longer than a few seconds). Long-duration GRBs are more common and are thought to mark the deaths of massive stars and be associated with a class of very energetic supernova explosions.
 Predicted by Einstein's theory of general relativity, gravitational waves are ripples in the fabric of space and time. Significant gravitational waves are generated whenever there are extreme variations of strong gravitational fields with time, such as during the merger of two black holes.
This research was presented in a paper in entitled "Discovery of the massive overcontact binary VFTS 352: Evidence for enhanced internal mixing", by L. Almeida et al., in the Astrophysical Journal.
The team is composed of L.A. Almeida (Johns Hopkins University, Baltimore, Maryland, USA; Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Brazil), H. Sana (STScI, Baltimore, Maryland, USA; KU Leuven, Belgium), S.E. de Mink (University of Amsterdam, Netherlands), F. Tramper (University of Amsterdam, Netherlands), I. Soszynski (Warsaw University Observatory, Poland), N. Langer (Universität Bonn, Germany), R.H. Barba (Universidad de La Serena, Chile), M. Cantiello (University of California, Santa Barbara, USA), A. Damineli (Universidade de São Paulo, Brazil), A. de Koter (University of Amsterdam, Netherlands; Universiteit Leuven, Belgium), M. Garcia (Centro de Astrobiologa (INTA-CSIC), Spain), G. Gräfener (Armagh Observatory, UK), A. Herrero (Instituto de Astrofísica de Canarias, Spain; Universidad de La Laguna, Spain), I. Howarth (University College London, UK), J. Maz Apellaniz (Centro de Astrobiologa (INTA-CSIC), Spain), C. Norman (Johns Hopkins University, USA), O.H. Ramrez-Agudelo (University of Amsterdam, Netherlands) and J.S. Vink (Armagh Observatory, UK).
ESO is the foremost intergovernmental astronomy organisation in Europe and the world's most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world's most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world's largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become "the world's biggest eye on the sky".
- Research paper published in the Astrophysical Journal - http://iopscience.
iop. org/ article/ 10. 1088/ 0004-637X/ 812/ 2/ 102
- Freely accessible preprint of the research paper - http://arxiv.
org/ abs/ 1509. 08940
- Photos of the VLT - http://www.
eso. org/ public/ images/ archive/ category/ paranal/
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