A Closer Look at Betelgeuse
CERRO PARANAL, Chile, Oct. 15, 2009 – Two independent teams of astronomers have obtained the sharpest-ever views of one of the biggest stars in the sky – a red supergiant called Betelgeuse. One of the most luminous stars known, Betelgeuse emits more light than 100,000 suns. But it is nearing the end of its life and is doomed to explode as a supernova, a sight that should be seen easily from Earth, even in broad daylight.
Red supergiants still hold several unsolved mysteries, including how such gigantic stellar objects shed enormous quantities of material. Two groups, one from the Paris Observatory in France and the other from Max Planck Institute for Radio Astronomy in Bonn, Germany, hope that their data will help shed light on what happens when stars die.
Model images are shown in the (a) continuum, (b) blue wing, (c) line core and (d) red wing. (e) This graph shows normalized flux (left ordinate) and visibility on the shortest baseline (right ordinate). The filled circles and solid line represent the observed and model flux, respectively. The observed visibility and model predictions are shown by the filled diamonds and dashed line, respectively. The line positions are marked with vertical ticks. Courtesy of Keiichi Ohnaka.
Although astronomers know from spectroscopy that Betelgeuse is losing a lot of mass, the exact mechanism is still unclear. Now, using various state-of-the-art instruments on the Very Large Telescope (VLT) in Chile, the two groups believe they may have found the answer.
“We expected a more or less spherical geometry of mass loss, but what we found instead were a few plumes extending into space from the surface of the star,” said Pierre Kervella from the Paris Observatory. “We believe that this mass loss is probably linked to powerful convection occurring at the surface of the star.”
Classic imaging (top) is contrasted with “lucky” imaging (bottom). Lucky imaging allows for improved resolution. Courtesy of ESO/P. Kervella.
Kervella’s team used the VLT’s Nasmyth Adaptive Optics System, which, coupled with the CONICA infrared camera, is known as NACO. NACO is the main adaptive optics instrument installed at the VLT and is optimized to observe in the infrared. It is equipped with a tip-tilt mirror, a deformable mirror controlled by 185 actuators and two wavefront sensors, one for visible light and one for infrared. NACO corrects the turbulent blur caused by the Earth’s atmosphere and provides near-perfect images.
The Paris team didn’t stop there, however: It applied a technique known as “lucky imaging” to boost the resolution to a record 37 milliarcseconds (mas). In the study, due to be published in Astronomy & Astrophysics, Kervella and colleagues obtained about 1 million individual images of Betelgeuse using NACO, with an exposure time of just 7.2 ms. By carefully recentering a selection of the individual images, the team effectively “freezes” the atmospheric turbulence.
“Using the lucky imaging technique, we improved the resolution by a factor of approximately two at a wavelength of one micron,” Kervella noted. “This is the best resolution obtained from a ground-based telescope using direct imaging and is comparable to or slightly better than the resolution obtained by the Hubble Space Telescope in the ultraviolet.”
Although the Paris group imaged the plumes extending out to space, Keiichi Ohnaka at Max Planck Institute for Radio Astronomy and colleagues in France, Italy and Germany examined the star itself. They discovered a peculiar thing happening on the surface of the supergiant: By spatially resolving the gas motion on the surface of Betelgeuse, they found gas clumps, as large as the star itself, moving up and down vigorously.
“This motion may be the cause of the large plumes found by Kervella’s team,” Ohnaka said. “We think that the material is not spilling out from the star in an ordered manner, but flung out dynamically in arcs or clumps.”
The three telescopes that make up the AMBER instrument are prepared for observations. Courtesy of Keiichi Ohnaka.
In Ohnaka’s study, which was published in the August issue of Astronomy & Astrophysics, the VLT’s Astronomical Multiple Beam Recombiner (AMBER) instrument was used. AMBER, an interferometer system, comprises three movable 1.8-m telescopes and achieves higher angular resolution than is possible with a single telescope alone. Combining light from the three enables astronomers to obtain a resolution equivalent to that of a single 50-m telescope.
“Our AMBER observations at a wavelength of two microns are the sharpest of any kind ever made of Betelgeuse,” Ohnaka said. “The resolution is four times higher than today’s largest [8- to 10-m] telescopes and twice the resolution achieved by the Hubble Space Telescope in the ultraviolet. Moreover, we detected how the gas is moving in different areas of Betelgeuse’s surface – the first time this has ever been done for a star other than the sun.”
Both groups plan to continue their observations of the supergiant to better establish the link between convection and mass loss.
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