G. Duvert

AMBER, the near-infrared spectro-interferometric three-telescope VLTI instrument

Context: Optical long-baseline interferometry is moving a crucial step forward with the advent of general-user scientific instruments that equip large aperture and hectometric baseline facilities, such as the Very Large Telescope Interferometer (VLTI). Aims: AMBER is one of the VLTI instruments that combines up to three beams with low, moderate and high spectral resolutions in order to provide milli-arcsecond spatial resolution for compact astrophysical sources in the near-infrared wavelength domain. Its main specifications are based on three key programs on young stellar objects, active galactic nuclei central regions, masses, and spectra of hot extra-solar planets. Methods: These key science goals led to scientific specifications, which were used to propose and then validate the instrument concept. AMBER uses single-mode fibers to filter the entrance signal and to reach highly accurate, multiaxial three-beam combination, yielding three baselines and a closure phase, three spectral dispersive elements, and specific self-calibration procedures. Results: The AMBER measurements yield spectrally dispersed calibrated visibilities, color-differential complex visibilities, and a closure phase allows astronomers to contemplate rudimentary imaging and highly accurate visibility and phase differential measurements. AMBER was installed in 2004 at the Paranal Observatory. We describe here the present implementation of the instrument in the configuration with which the astronomical community can access it. Conclusions: .After two years of commissioning tests and preliminary observations, AMBER has produced its first refereed publications, allowing assessment of its scientific potential.

Interferometric data reduction with AMBER/VLTI. Principle, estimators, and illustration

Aims. In this paper, we present an innovative data reduction method for single-mode interferometry. It has been specifically developed for the AMBER instrument, the three-beam combiner of the Very Large Telescope Interferometer, but it can be derived for any single-mode interferometer. Methods. The algorithm is based on a direct modelling of the fringes in the detector plane. As such, it requires a preliminary calibration of the instrument in order to obtain the calibration matrix that builds the linear relationship between the interferogram and the interferometric observable, which is the complex visibility. Once the calibration procedure has been performed, the signal processing appears to be a classical least-square determination of a linear inverse problem. From the estimated complex visibility, we derive the squared visibility, the closure phase, and the spectral differential phase. Results. The data reduction procedures have been gathered into the so-called amdlib software, now available for the community, and are presented in this paper. Furthermore, each step in this original algorithm is illustrated and discussed from various on-sky observations conducted with the VLTI, with a focus on the control of the data quality and the effective execution of the data reduction procedures. We point out the present limited performances of the instrument due to VLTI instrumental vibrations which are difficult to calibrate.

Mass-radius relation of low and very low-mass stars revisited with the VLTI

We measured the radii of 7 low and very low-mass stars using long baseline interferometry with the VLTI interferometer and its VINCI and AMBER near-infrared recombiners. We use these new data, together with literature measurements, to examine the luminosityradius and mass-radius relations for K and M dwarfs. The precision of the new interferometric radii now competes with what can be obtained for double-lined eclipsing binaries. Interferometry provides access to much less active stars, as well as to stars with much better measured distances and luminosities, and therefore complements the information obtained from eclipsing systems. The radii of magnetically quiet late-K to M dwarfs match the predictions of stellar evolution models very well, providing direct confirmation that magnetic activity explains the discrepancy that was recently found for magnetically active eclipsing systems. The radii of the early K dwarfs are reproduced well for a mixing length parameter that approaches the solar value, as qualitatively expected.

Discovery of a moon orbiting the asteroid 45 Eugenia

Evidence for asteroidal satellites (moons) has been sought for decades, because the relative frequency of such satellites will bear on the collisional history of the asteroid belt and the Solar System, yet only one has been detected unambiguously. Here we report the discovery of a satellite of the asteroid 45 Eugenia, using an adaptive optics system on a ground-based telescope. The satellite has a diameter of about 13u2009km, and an orbital period of about 4.7 days with a separation of 1,190u2009km from Eugenia. Using a previously determined diameter for Eugenia, we estimate that its bulk density is about 1.2u2009gu2009cm-3, which is similar to that of the C-type asteroid Mathilde. This implies that Eugenia, also a low-albedo C-type asteroid, may be a rubble pile, or composed of primitive, icy materials of low bulk density.

Disk and wind interaction in the young stellar object MWC 297 spatially resolved with VLTI/AMBER

The young stellar object MWC 297 is an embedded B1.5Ve star exhibiting strong hydrogen emission lines and a strong near-infrared continuum excess. This object has been observed with the VLT interferometer equipped with the AMBER instrument during its first commissioning run. VLTI/AMBER is currently the only near infrared interferometer which can observe spectrally dispersed visibilities. MWC 297 has been spatially resolved in the continuum with a visibility of

Constraining the wind launching region in Herbig Ae stars: AMBER/VLTI spectroscopy of HD 104237

0.50^{+0.08}_{-0.10}

Direct constraint on the distance of Gamma-2 Velorum from AMBER/VLTI observations

as well as in the Brgamma emission line where the visibility decrease to a lower value of

Panchromatic Observations and Modeling of the HV Tau C Edge-on Disk

0.33pm0.06

Structural and compositional properties of brown dwarf disks: the case of 2MASS J04442713+2512164

. This change in the visibility with the wavelength can be interpreted by the presence of an optically thick disk responsible for the visibility in the continuum and of a stellar wind traced by the Brgamma emission line and whose apparent size is 40% larger. We validate this interpretation by building a model of the stellar environment that combines a geometrically thin, optically thick accretion disk model consisting of gas and dust, and a latitude-dependent stellar wind outflowing above the disk surface. The continuum emission and visibilities obtained from this model are fully consistent with the interferometric AMBER data. They agree also with existing optical, near-infrared spectra and other broad-band near-infrared interferometric visibilities. We also reproduce the shape of the visibilities in the Brgamma line as well as the profile of this line obtained at an higher spectral resolution with the VLT/ISAAC spectrograph, and those of the Halpha and Hbeta lines. The disk and wind models yield a consistent inclination of the system of approximately 20 degrees. A picture emerges in which MWC 297 is surrounded by an equatorial flat disk that is possibly still accreting and an outflowing wind which has a much higher velocity in the polar region than at the equator. The VLTI/AMBER unique capability to measure spectral visibilities therefore allows us for the first time to compare the apparent geometry of a wind with the disk structure in a young stellar system.

An asymmetry detected in the disk of Kappa CMa with the AMBER/VLTI

This work has been partly supported by the nMIUR COFIN grant 2003/027003-001 and 025227/2004 to the INAFOsservatorio nAstrofisico di Arcetri. This project has benefited from nfunding from the French Centre National de la Recherche Scientifique n(CNRS) through the Institut National des Sciences de l’Univers n(INSU) and its Programmes Nationaux (ASHRA, PNPS). The authors nfrom the French laboratories would like to thank the successive ndirectors of the INSU/CNRS directors. C. Gil work was supported nin part by the Fundac¸˜ao para a Ciˆencia e a Tecnologia through nproject POCTI/CTE-AST/55691/2004 from POCTI,with funds from nthe European program FEDER.