VLTI: Amber penetrates to the heart of the stars

Numerous astronomical results have been obtained thanks to AMBER, the instrument which equips the Very Large Telescope Interferometer (VLTI) at the European Southern Observatory (ESO). AMBER (Astronomical Multi-BEam Recombiner) makes it possible to combine the beams of three of the four 8-metre VLT telescopes. Thanks to its very high angular resolution, AMBER enables the observation of practically all stages in the evolution of stars, from their infancy to their death. This equipment is unique in the world and was built by a consortium managed by CNRS-associated French teams with the support of the INSU/CNRS1. Astronomy & Astrophysics devotes a special issue in March 2007 to this remarkable facility.

Having recombined the beams from the different VLT telescopes, AMBER analyses the signal using a spectrograph. Working in the near infrared field at 1 to 2.5 microns, AMBER now renders the VLT the largest telescope ever available, with an equivalent mirror diameter of more than 130 metres, corresponding to a resolution 16 times better than that of a single telescope, and a collection area of more than 150m². Use of the interferometric mode can overcome some of the problems faced by astronomers when building extremely large telescopes, and constitutes an essential complement to very large surface instrument by specialising in enhancing the resolving power. It then becomes possible to probe the formative regions of planets, to observe the winds of very rapidly rotating stars, to study the different types of matter ejected by a massive star, to separate the two components of a tight binary star and directly visualise the evolution of a nova only a few days after its explosion.

One of the results presented in Astronomy & Astrophysics concerns the eruption on February 12, 2006, of the recurrent nova, RS Oph, only 25 years after a similar eruption which was actively observed in 1985. RS Oph then became visible to the naked eye and was the subject of intense observations not only by numerous amateur astronomers but also by most of the major observatories on Earth and in space. This extreme system was observed for the first time by optical interferometers – including AMBER on the VLTI – only 5 days after its discovery. These observations revealed a complex geometry and kinetics, far removed from the simple representation of an expanding, spherical ball of fire.

In particular, AMBER detected a high velocity jet, probably perpendicular to the orbital plan of the binary system, and enabled detailed study of the shock wave and wind generated by the nova. Another important observation by AMBER concerned the star η Carinae, one of the most massive and luminous stars in our galaxy. It showed that η Carinae is completely obscured by a oblong cloud of expanding gas, called a wind. Because the strength of this wind is latitude-dependent,

1 INSU/CNRS : Institut National des Sciences de l'Univers (the French National Institute for Sciences of the Universe)

2 the measurements obtained using AMBER have confirmed the theoretical prediction according to which stars rotating rapidly on their axis have a stellar wind which is aligned along their rotational axis. Other results published in this special issue of A&A have already appeared in the press since AMBER came into operation. They show that thanks to its very high angular resolution, the VLT equipped with AMBER can probe the heart of stars, both those in formation and those coming to the end of their existence. When the VLTI has reached its full capacity, AMBER should be capable of achieving its most ambitious objectives: observation of the dust torus within the active cores of galaxies, and that of extrasolar planets orbiting close to their stars.

General view of AMBER during its assembly at the LAOG.
The paths of the optical beams have been added to this photo.
© F. Millour. AMBER/VLTI. ESO. LAOG. OSUG. INSU. (this image can be obtained from the CNRS photo library, 01 45 07 57 90, phototheque@cnrs-bellevue.fr) Image of the remains after explosion of the nova RS Ophiuchus.

Dotted elipses represent the structures observed by AMBER.
© O. Chesneau AMBER/VLTI/ESO. OCA. Seven years were necessary to construct AMBER, and its overall cost reached €6 million, including €1.5 million for the equipment.

Of the Construction of AMBER is managed by:

• The University Astrophysics Laboratory in Nice (CNRS, University of Nice), where the principal investigator of the instrument, R. Petrov, is based.

• The Grenoble Astrophysics Laboratory (CNRS, Université Joseph Fourier, Grenoble Observatory for Sciences of the Universe), where the scientific manager of the project, F. Malbet, is based, and which has focused on integration of the instrument, its control software and data reduction.

• The Gemini Laboratory (CNRS, Côte d'Azur Observatory) which coordinated the project with P. Antonelli and is responsible for its optical, mechanical and electronic constructions.

• The Arcetri Astrophysics Observatory (OAA-INAF) in Florence, which constructed the cooled spectroph - The Max Planck Radioastronomy Institute (MPIfR) in Bonn which supplied the near infrared camera.

The National Institute for Sciences of the Universe, MPIfR, OAA-INAF, ESO and the Provence-Alpes-Côtes d'Azur region funded this instrument.

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REFERENCES

the AMBER site at the LAOG : http://amber.obs.ujf-grenoble.fr/
The press release concerning the launch of AMBER with a film about this instrument, on the INSU website: http://www.insu.cnrs.fr/web/article/art.php?art=695
press release on MWC 297 : http://www.insu.cnrs.fr/web/article/art.php?art=1617
press release on Alpha Arae : http://www.insu.cnrs.fr/web/article/art.php?art=1812
press releases by partners: A&A, ESO, MPG, INAF

CONTACTS

Researcher
Romain Petrov
T 04 92 07 63 47
romain.petrov@unice.fr

Researcher
Fabien Malbet
T 04 76 63 58 33
fabien.malbet@obs.ujf-grenoble.fr

INSU Communication Office
Philippe Chauvin
T 01 44 96 43 36
Philppe.chauvin@cnrs-dir.fr

Press
Cécile Pérol
T 01 44 96 43 09
Cecile.perol@cnrs-dir.fr

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