D. Mourard

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.

Darwin - A Mission to Detect and Search for Life on Extrasolar Planets

The discovery of extrasolar planets is one of the greatest achievements of modern astronomy. The detection of planets that vary widely in mass demonstrates that extrasolar planets of low mass exist. In this paper, we describe a mission, called Darwin, whose primary goal is the search for, and characterization of, terrestrial extrasolar planets and the search for life. Accomplishing the mission objectives will require collaborative science across disciplines, including astrophysics, planetary sciences, chemistry, and microbiology. Darwin is designed to detect rocky planets similar to Earth and perform spectroscopic analysis at mid-infrared wavelengths (6-20 mum), where an advantageous contrast ratio between star and planet occurs. The baseline mission is projected to last 5 years and consists of approximately 200 individual target stars. Among these, 25-50 planetary systems can be studied spectroscopically, which will include the search for gases such as CO(2), H(2)O, CH(4), and O(3). Many of the key technologies required for the construction of Darwin have already been demonstrated, and the remainder are estimated to be mature in the near future. Darwin is a mission that will ignite intense interest in both the research community and the wider public.

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

Cepheid distances from infrared long-baseline interferometry III. Calibration of the surface brightness-color relations

0.50^{+0.08}_{-0.10}

Cepheid distances from infrared long-baseline interferometry I. VINCI/VLTI observations of seven Galactic Cepheids

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

Imaging the spinning gas and dust in the disc around the supergiant A[e] star HD 62623

0.33\pm0.06

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

. 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.

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

The recent VINCI/VLTI observations presented in Paper I have nearly doubled the total number of available angular diameter measurements of Cepheids. Taking advantage of the significantly larger color range covered by these ob- servations, we derive in the present paper high precision calibrations of the surface brightness-color relations using exclu- sively Cepheid observations. These empirical laws make it possible to determine the distance to Cepheids through a Baade- Wesselink type technique. The least dispersed relations are based on visible-infrared colors, for instance FV (V − K) = −0.1336±0.0008 (V − K) + 3.9530±0.0006. The convergence of the Cepheid (this work) and dwarf star (Kervella et al. 2004c) visible-infrared surface brightness-color relations is strikingly good. The astrophysical dispersion of these relations appears to be very small, and below the present detection sensitivity.

Self consistent modelling of the projection factor for interferometric distance determination

We report the angular diameter measurements of seven classical Cepheids, X Sgr, η Aql, W Sgr, ζ Gem, β Dor, Y Oph andCar that we have obtained with the VINCI instrument, installed at ESOs VLT Interferometer (VLTI). We also present reprocessed archive data obtained with the FLUOR/IOTA instrument on ζ Gem, in order to improve the phase coverage of our observations. We obtain average limb darkened angular diameter values of θLD(X Sgr) = 1.471 ± 0.033 mas, θLD(η Aql) = 1.839 ± 0.028 mas, θLD(W Sgr) = 1.312 ± 0.029 mas, θLD(β Dor) = 1.891 ± 0.024 mas, θLD(ζ Gem) = 1.747 ± 0.061 mas, θLD(Y Oph) = 1.437 ± 0.040 mas, and θLD(� Car) = 2.988 ± 0.012 mas. For four of these stars, η Aql, W Sgr, β Dor, andCar, we detect the pulsational variation of their angular diameter. This enables us to compute directly their distances, using a modified version of the Baade-Wesselink method: d(η Aql) = 276 +55