We describe the use of interferometry to make images of features on the surfaces of cool stars. We describe observations of Betelgeuse (α Ori) and use this example to illustrate the requirements for astrophysical imaging with interferometers. We emphasise the importance of obtaining closure phase measurements and good Fourier plane coverage when imaging complex objects.

Interferometry in the visible now provides milliarcsecond spatial resolution and thus has been intensely used for studying the circumstellar environment of evolved and pre-sequence stars. Up to now, most of the scientific results were obtained using only the modulus of the visibility as a function of baseline, time or projected baseline (the so-called "super-synthesis" effect). In this paper we will try to show that even without images, the phase of the visibility is able to put very strong constraints on stellar modeling since it is possible to achieve μarsecond measurements using this technique. We will illustrate through two examples from the GI2T interferometer how the modulus and the phase of the visibility were used to understand the physics of Be disks. We will stress on the importance and the potential of coupling high angular resolution with high spectral resolution for the study of stellar physics. Finally, We will present a possible study on the phase signatures due to stellar rapid rotation that can be done using the VLTI interferometer.

We present here a proposal for the mid-infrared study of the circumstellar material around Z CMa. The suggested configuration of the interferometer and the observational strategy are based on model simulations performed using the software ASPRO. We find that the UT1-UT3 telescope pair (the first mode to be offered) is able to constrain the flux ratio of the binary but not the extension of possible circumbinary disks. We suggest to use an AT unit to increase the baseline and thus resolve the material orbiting one or two components.

W Hydrae is a prototype AGB star with a dusty circumstellar shell. The shell is marginally resolved at ground-based sub-arcsecond resolution; the unresolved dust condensation radius is expected to be of order 30-50 mas. Since the dusty circumstellar shell is brightest in the mid-IR, the high spatial resolution required at mid-infrared wavelengths can currently only be achieved by using MIDI on the VLTI. We therefore outline a possible observing programme for the VLTI, using MIDI at 10 μm. In a single night of short-baseline VLTI observations with two Auxiliary Telescopes, we can achieve sufficient (u,v) plane coverage to resolve the shell geometry of W Hya, which will provide strong constraints on the mass loss processes in late-type stars. We expect to either obtain an estimate of the dust condensation temperature, or evidence for interrupted mass loss.

We propose interferometric observations of the Be star 28 CMa. Constraints on the geometrical and physical parameters of the disk could be obtained by combining three VLTI auxiliary telescopes.

To understand star formation and its evolution, one needs to solve the problem of ejection and collimation mechanisms in jets from young stars. By measuring the jet opening angle at the ejection region we can test models for jet origin. However this very small region cannot be investigated with current monolithic telescopes. Only the AMBER near-IR instrument coupled with the VLTI provides enough resolution to reach the ejection region and enough spectral resolution/coverage to properly separate the emission line jet from the continuum. In this proposal we show that expected visibilities from jets can constrain the jet opening angle.

The VLTI is the ideal instrument for measuring the distances of nearby Cepheids with the Baade-Wesselink method, allowing an accurate recalibration of the Cepheid Period-Luminosity relation. The high accuracy required by such measurement, however, can only be reached taking into account the effects of limb darkening, and its dependence on the Cepheid pulsations. We present here our new method to compute phase- and wavelength-dependent limb darkening profiles, based on hydrodynamic simulation of Classical Cepheid atmospheres.

We made detailed abundance analysis for ultimative test of membership of twenty λ Bootis type stars candidates. Atmosphere parameters, LTE abundances for ten elements (including C and O) and NLTE values for Na were determined. We are able to confirm or establish the membership for nine objects (HD 23258, HD 36726, HD 40588, HD 74911, HD 84123, HD 91130, HD 106223, HD 111604 and HD 290799). Six stars (HD 90821, HD 98772, HD 103483, HD 108765, HD 201184 and HD 261904) can be definitely ruled out as being member of the λ Bootis group whereas no ambiguous decision can be drawn for another five stars (HD 66684, HD 105058, HD 120500, HD 141851 and HD 294253). We also tryed to find apparent spectroscopic binaries which have two solar abundant components mimic one metal-weak star as proposed by Faraggiana & Bonifacio (1999).

Optical Interferometry can play an important role in modern astronomy. Extrasolar planet and binary star research, are some of the fields, where we will face important discoveries in the future. For the VLTI this will be a real challenge, since it will be the pioneer in angular resolution. In addition, high flux sensitivity will help detecting not only Jupiter-sized planets, but Earth-sized planets too. The ability of the VLTI in detecting new planets, measuring stellar diameters and stellar proper motions, with the use of such techniques, is presented.

We are studying the two unique radio galaxies Hercules A and 3C310. We are trying to determine whether the unusual and similar structure and behaviour of these two radio galaxies originate in a similar fashion or not. We try to find why these sources are different from double lobed AGNs. This research is being made by using multiwavelength observations across the electromagnetic spectrum.

We investigated the equation describing the evolution of the kinetic momentum vector of planets by the action of gravitational and magnetic interaction. The obtained gallery of portraits of the kinetic momentum evolution illustrates the various regimes of planetary system evolution. We are going to investigate a fine structure of protoplanetary disk around the pulsar by VLTI for detection of extrasolar planets on early time of formation.

We carried out Radiative Transfer (RT) calculations to investigate the properties of disks around young, Very Low Mass Objects (VLMOs) and Brown Dwarfs (BDs). We explore the possibility of resolving such disks in the mid-infrared with MIDI depending on the parameters: source distance and declination, disk inclination and extension.

Subdwarf B stars (sdB) dominate the populations of faint blue stars and are found in both the old disk (field sdBs) and halo populations (globular cluster members) of our own Galaxy. The origin of sdB stars remains largely a puzzle, but evidence is accumulating that close binary evolution is fundamental to the problem. With interferometric methods, we expect to detect and learn about close cool companions to derive further insight in the evolution of sdB stars.

VLTI observations of individual, massive objects in dense star forming regions limited by crowding in conventional astronomy, can have impacts on the determination of the stellar mass spectrum, and test classical methods to derive the mass function.

Fluxes in Ly-alpha and H-alpha lines from Proto Globular Clusters (PGCs) at redshifts z>1 are estimated. Two possible sources are considered: HII regions and supernovae remnats in PGCs. Intensities in Lyα and Hα lines are calculated. It is shown that such PGCs can be detectable for the ongoing and future projects (VLTI, NGST).

The few Vega-type stars whose dusty debris disks have been resolved show this dust to lie in cool Kuiper belt-like rings. However, roughly half of all debris disk candidates exhibit little or no cool dust, since their dust emission peaks at about 25 μm. By analogy with the solar system, these warm disks would lie mid-way between the asteroid and Kuiper belt regions in their systems. Are these disks the Kuiper belt-like rings of a truncated planetary system? Or do they represent the destruction of massive interplanetary asteroid/comet belts? Or maybe these systems are in a transitional stage and have yet to evolve into classically cool debris disks? To answer these questions we need to know where the dust lies, and for that we require the resolving power of mid-IR interferometry with the VLTI.