VLTI Spectro Imager
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3. Science cases

Friday 18 January 2008

The high level specifications of the instrument are derived from science cases based on the capability to reconstruct for the milli-arcsecond-resolution images of a wide range of targets. These science cases are detailed below.

Formation of stars and planets

PNG - 62 kb
Disks around Herbig Ae/Be stars
Image reconstruction performed with 6 ATs on a model disk around an Herbig Ae star. Left: model image; middle: coverage of the spatial frequencies; right: reconstructed image. The dust structure, the inner dust radius and the asymmetry (vertical structure) are well retrieved. Relative photometry is reliable (17% vs 19% of flux in the central star)

The early evolution of stars and the initial conditions for planet formation are determined by the interplay between accretion and outflow processes. Due the small spatial scales where these processes take place, very little is known about the actual physical and chemical mechanisms at work. Interferometric imaging at 1 milli-arcsecond spatial resolution will directly probe the regions responsible for the bulk of excess continuum emission from these objects, therefore constraining the currently highly degenerate models for the spectral energy distribution

In the emission lines a variety of processes will be probed, in particular outflow and accretion magnetospheres. The inner few AUs of planetary systems will also be studied, providing additional information on their formation and evolution processes, as well as on the physics of extrasolar planets.


Imaging stellar surfaces

Optical and near-infared imaging instruments provide a powerful means to resolve stellar features of the generally patchy surfaces of stars throughout the Hertzsprung-Russell diagram. Optical/infrared interferometry has already proved its ability to derive surface structure parameters such as limb darkening or other atmospheric parameters. VSI, as an imaging device, is of strong interest to study various specific features such as vertical and horizontal temperature profiles and abundance inhomogeneities, and to detect their variability as the star rotates. This will provide important keys to address stellar activity processes, mass-loss events, magneto-hydrodynamic mechanisms, pulsation and stellar evolution.

Evolved stars, stellar remnants & stellar winds

HST and ground-based observations revealed that the geometry of young and evolved planetary nebulae and related objects (e.g., nebulae around symbiotic stars) show an incredible variety of elliptical, bi-polar, multi-polar, point-symmetrical, and highly collimated (including jets) structures. The proposed mechanisms explaining the observed geometries (disks, magnetohydrodynamics collimation and binarity) are within the grasp of interferometric imaging at 1 mas resolution. Extreme cases of evolved stars are stellar black holes. In microquasars, the stellar black-hole accretes mass from a donor. The interest of these systems lies in the small spatial scales and high multi-wavelength variability. Milliarcsecond imaging in the near-infrared will allow disentangling between dust and jet synchrotron emission, comparison of the observed morphology with radio maps and correlation of the morphology with the variable X-ray spectral states.


Active Galactic Nuclei & Supermassive Black Holes

PNG - 76.2 kb
AGN torus
LTI/VSI image reconstruction simulation performed with 4 UTs on a model of a clumpy torus at the center of an AGN. Left: model image; middle left: coverage of the spatial frequencies; middle right: model image convolved with a perfect beam corresponding to the maximum spatial resolution; right: image reconstructed from simulated VSI data using the Building Block method.

AGN consist of complex systems composed of different interacting parts powered by accretion onto the central supermassive black hole. The imaging capability will permit the study of the geometry and dust composition of the obscuring torus and the testing of radiative transfer models

Milli-arcsecond resolution imaging will allow us to probe the collimation at the base of the jet and the energy distribution of the emitted radiation. Supermassive black hole masses in nearby (active) galaxies can be measured and it will be possible to detect general relativistic effects for the stellar orbits closer to the galactic center black hole. The wavelength-dependent differential-phase variation of broad emission lines will provide strong constraints on the size and geometry of the Broad Line Region (BLR). It will then be possible to establish a secure size-luminosity relation for the BLR, a fundamental ingredient to measure supermassive black hole masses at high redshift.

Science context

We have shown that this astrophysical program could provide the premises for a legacy program at the VLTI. For this goal, the number of telescopes to be combined should be at least 4, or better 6 to 8 at the VLTI at the time when the James Webb Space Telescope will hammer faint infrared science ( 2013), when HAWK-I, KMOS, will have hopefully delivered most of their science, and ALMA will be fully operational. The competitiveness and uniqueness of the VLT will remain on the high angular (AO/VLTI) and the high spectral resolution domains. In a context where the European Extremely Large Telescope (ELT) will start being constructed, then have first light, and, where Paranal science operations will probably be simplified with less VLT instruments and an emphasis on survey programs, VSI will take all its meaning by bringing the VLTI to a legacy mode.

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