Thanks to the unique possibilities offered by ESO's Very Large Telescope Interferometer (VLTI), astronomers have solved a 140-year-old mystery concerning active hot stars. They indeed show that the star Alpha Arae is spinning almost on the verge of breaking and that its disc rotates the same way planets do around the Sun.
"This result could only be achieved because of the great details we could observe with the AMBER instrument combining three 8.2-m Unit Telescopes of ESO's VLT," said Philippe Stee, leader of the team that performed the study ".
With AMBER on the VLTI , the astronomers were able to see details on the scale of one milli-arcsecond, corresponding to being able to distinguish, from the Earth, the headlights of a car on the Moon.
Lying about 300 light-years away from the Sun, Alpha Arae is the closest member of the class of active stars known as 'Be stars'. Be stars  are very luminous, massive and hot stars that rotate rapidly. They are losing mass along the poles through a strong stellar wind and are surrounded at the equator by a disc of matter. Alpha Arae has ten times the mass of the Sun, is three times hotter and 6 000 times as luminous.
The question how the discs around active stars known as Be-stars rotate was posed since the discovery of the first one, Gamma Cassiopeiae, by Italian astronomer Father Angelo Secchi, exactly 140 years ago, on 23 August 1866 in Rome.
With AMBER, the team of astronomers could examine in details the structure of the disc surrounding Alpha Arae. Moreover, because AMBER also provides spectra, the astronomers could study the motion of the gas in the disc and so understand how it rotates.
"Although previous theoretical studies had already provided some indications, our result - the first to supply observational evidence - may be the final exclamation mark regarding this puzzle", said Stee.
The scientists found the material in the disc surrounding Alpha Arae to be in 'Keplerian rotation', that is, obeying the same rules as discovered by Johannes Kepler for the planets circling the Sun: the velocity of the material decreases with the square root of the distance from the star.
The new result rules out the possibility for the disc to rotate with a uniform velocity, as would be the case if a strong magnetic field were present that would oblige the matter to spin at the same rate as the star.
Combining the new data with previous studies, the astronomers also show that the star Alpha Arae, which is five times larger than the Sun, rotates around completely in about half a day, 50 times faster than our Sun. In fact, with a speed at the equator of 470 km/s, it is spinning so quickly that it is near its break-up velocity. Matter having such a critical velocity would be able to freely escape from the star, in the same way that we would be ejected from a 'gone crazy' merry-go-round.
"This nearly critical rotation may be the clue to the 'Be phenomenon'", said Stee. "It may bring sufficient energy to levitate material to create the circumstellar disc."
Finally, the astronomers were also able to show that the star loses mass through a stellar wind emerging predominantly from the poles and reaching velocities of the order of 2000 km/s.
These observations demonstrate once again the great potential of the ESO Very Large Telescope Interferometer that allows astronomers to combine 2 or 3 of the VLT Unit Telescopes or the associated moveable Auxiliary Telescopes, to obtain great details with spectroscopic information. The VLTI already provided useful information about other very fast rotating stars, such as Achernar (ESO PR 14/03) or Eta Carinae (ESO PR 31/03).
: The team is composed of A. Meilland, Ph. Stee, A. Spang (Observatoire de la Côte d'Azur, France). F. Millour, A. Domiciano de Souza, R. Petrov. (Université de Nice, France), M. Vannier, A. Richichi (ESO), C. Martayan (Observatoire de Paris, France), F. Malbet (Laboratoire d'Astrophysique de Grenoble, France), and F. Paresce (INAF, Itally). This result is presented in "First direct detection of a Keplerian rotating disk around the Be star Alpha Arae using the VLTI/AMBER instrument", by A. Meilland et al., in press in the research journal Astronomy and Astrophysics.
: The Astronomical Multiple BEam Recombiner (AMBER) is a near-infrared, multi-beam interferometric instrument, combining simultaneously 3 telescopes. It was built in collaboration with ESO by a consortium of French, German and Italian institutes. It is offered to the users since October 2005. For more information, see the AMBER homepage. A press release about the First Light is available as ESO Press Release 07/04.
: Be stars are stars of spectral type B, with emission lines in their spectra, hence the "e". Because they are an important source of ultraviolet photons, Be stars play an important role in the heating of galaxies. What causes B stars to become Be stars is not yet well understood.
Henri Boffin | alfa
One-way roads for spin currents
23.05.2018 | Singapore University of Technology and Design
Tunable diamond string may hold key to quantum memory
23.05.2018 | Harvard John A. Paulson School of Engineering and Applied Sciences
At the LASYS 2018, from June 5th to 7th, the Laser Zentrum Hannover e.V. (LZH) will be showcasing processes for the laser material processing of tomorrow in hall 4 at stand 4E75. With blown bomb shells the LZH will present first results of a research project on civil security.
At this year's LASYS, the LZH will exhibit light-based processes such as cutting, welding, ablation and structuring as well as additive manufacturing for...
There are videos on the internet that can make one marvel at technology. For example, a smartphone is casually bent around the arm or a thin-film display is rolled in all directions and with almost every diameter. From the user's point of view, this looks fantastic. From a professional point of view, however, the question arises: Is that already possible?
At Display Week 2018, scientists from the Fraunhofer Institute for Applied Polymer Research IAP will be demonstrating today’s technological possibilities and...
So-called quantum many-body scars allow quantum systems to stay out of equilibrium much longer, explaining experiment | Study published in Nature Physics
Recently, researchers from Harvard and MIT succeeded in trapping a record 53 atoms and individually controlling their quantum state, realizing what is called a...
The historic first detection of gravitational waves from colliding black holes far outside our galaxy opened a new window to understanding the universe. A...
A team led by Austrian experimental physicist Rainer Blatt has succeeded in characterizing the quantum entanglement of two spatially separated atoms by observing their light emission. This fundamental demonstration could lead to the development of highly sensitive optical gradiometers for the precise measurement of the gravitational field or the earth's magnetic field.
The age of quantum technology has long been heralded. Decades of research into the quantum world have led to the development of methods that make it possible...
02.05.2018 | Event News
13.04.2018 | Event News
12.04.2018 | Event News
23.05.2018 | Life Sciences
23.05.2018 | Life Sciences
23.05.2018 | Physics and Astronomy