February 22, 2007
Viewing the Sky Through Three Giant Eyes
AMBER Instrument on VLT Delivers a Wealth of Results
The ESO Very Large Telescope Interferometer, which allows astronomers to scrutinise objects with a precision equivalent to that of a 130-m telescope, is proving itself an unequalled success every day. One of the latest instruments installed, AMBER, has led to a flurry of scientific results, an anthology of which is being published this week as special features in the research journal Astronomy & Astrophysics.
"With the capability of AMBER to combine up to three of the 8.2-m VLT Unit Telescopes, we can really achieve what nobody else can do," added Fabien Malbet, from the LAOG (France) and the AMBER Project Scientist.
Eleven articles will appear this week in Astronomy & Astrophysics' special AMBER section. Three of them describe the unique instrument, while the other eight reveal completely new results about the early and late stages in the life of stars.
The first results presented in this issue cover various fields of stellar and circumstellar physics. Two papers deal with very young solar-like stars, offering new information about the geometry of the surrounding discs and associated outflowing winds. Other articles are devoted to the study of hot active stars of particular interest: Alpha Arae, Kappa Canis Majoris, and CPD -57o2874. They provide new, precise information about their rotating gas envelopes.
An important new result concerns the enigmatic object Eta Carinae. Using AMBER with its high spatial and spectral resolution, it was possible to zoom into the very heart of this very massive star. In this innermost region, the observations are dominated by the extremely dense stellar wind that totally obscures the underlying central star. The AMBER observations show that this dense stellar wind is not spherically symmetric, but exhibits a clearly elongated structure. Overall, the AMBER observations confirm that the extremely high mass loss of Eta Carinae's massive central star is non-spherical and much stronger along the poles than in the equatorial plane. This is in agreement with theoretical models that predict such an enhanced polar mass-loss in the case of rapidly rotating stars.
Several papers from this special feature focus on the later stages in a star's life. One looks at the binary system Gamma 2 Velorum, which contains the closest example of a star known as a Wolf-Rayet. A single AMBER observation allowed the astronomers to separate the spectra of the two components, offering new insights in the modeling of Wolf-Rayet stars, but made it also possible to measure the separation between the two stars. This led to a new determination of the distance of the system, showing that previous estimates were incorrect. The observations also revealed information on the region where the winds from the two stars collide.
The famous binary system RS Ophiuchi, an example of a recurrent nova, was observed just 5 days after it was discovered to be in outburst on 12 February 2006, an event that has been expected for 21 years. AMBER was able to detect the extension of the expanding nova emission. These observations show a complex geometry and kinematics, far from the simple interpretation of a spherical fireball in extension. AMBER has detected a high velocity jet probably perpendicular to the orbital plane of the binary system, and allowed a precise and careful study of the wind and the shockwave coming from the nova.
The stream of results from the VLTI and AMBER is no doubt going to increase in the coming years with the availability of new functionalities.
"In addition to the 8.2-m Unit Telescopes, the VLTI can also combine the light from up to 4 movable 1.8-m Auxiliary Telescopes. AMBER fed by three of these AT's will be offered to the user community as of April this year, and from October we will also make FINITO available," said Melnick. "This 'fringe-tracking' device allows us to stabilise changes in the atmospheric conditions and thus to substantially improve the efficiency of the observations. By effectively 'freezing' the interferometric fringes, FINITO allows astronomers to significantly increase the exposure times."
The Astronomy & Astrophysics special feature (volume 464 - March II 2007) on AMBER first results includes 11 articles. They are freely available on the A&A web site.
The AMBER consortium, led by Romain Petrov (Nice, France), includes researchers from the Laboratoire d'Astrophysique de Grenoble (France), Laboratoire d'Astrophysique Universitaire de Nice (France), Max-Planck Institut fr Radioastronomie (Bonn, Germany), INAF-Osservatorio Astrofisico di Arcetri (Italy), and the Observatoire de la CÃ´te d'Azur (Nice, France).
In March 2004, the first on-line tests of AMBER (Astronomical Multiple BEam Recombiner) were completed, when astronomers combined the two beams of light from the southern star Theta Centauri from two test 40-cm aperture telescopes (ESO 07/04). It was later used to combine light from two, then three Unit Telescopes of ESO's VLT and light from the Auxiliary Telescopes.
AMBER is part of the VLT Interferometer (VLTI) and completes the planned set of first-generation instruments for this facility. It continues the success story of the interferometric mode of the VLT, following the unique initial scientific results obtained by the VINCI and MIDI instruments, the installation of the four MACAO adaptive optics systems and the recent arrival of the last of the four 1.8-m Auxiliary Telescopes at Paranal.
The principle of the interferometric technique is to combine the light collected by two or more telescopes. The greater the distance between the telescopes, the more details one can detect. For the VLTI, this distance can be up to 200 metres, providing observers with milli-arcsecond spatial resolution. With such a high spatial resolution, one would be able to distinguish between the headlights of a car located on the Moon. In addition, AMBER also provides astronomers with spectroscopic measurements, allowing the structure and the physics of the source to be constrained by comparing the measures at different wavelengths.
AMBER combines the light beams from three telescopes - this is a world first for large telescopes such as the VLT. The ability to combine three beams, rather than just two as in a conventional interferometer, provides a substantial increase in the efficiency of observations, permitting astronomers to obtain three baselines simultaneously instead of one. The combination of these three baselines also permits the computation of the so-called closure phase, an important mathematical quantity that can be used in imaging applications.
The AMBER instrument is mounted on a 4.2 x 1.5-m precision optical table, placed in the VLT Interferometric Laboratory at the top of the Paranal mountain. The total shipping weight of the instrument and its extensive associated electronics was almost 4 tons.
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