Optical interferometry, the only means to reach angular resolutions an order of magnitude larger than the largest single optical telescopes available, will play a central role in:
- Understanding the lifecycles of stars in the Milky Way
The last decade witnessed the discovery of disks around the analogs of the progenitor of our Sun in nearby star-forming regions. Current technologies only reach spatial regions larger than the orbit of Saturn. However the interesting science is taking place on scales smaller than the orbit of the Earth. The European Very Large Telescope Interferometer (VLTI), the top optical interferometric facility in the world , can probe regions down to one third of the orbit of Mercury. Questions to be addressed are: the structure (thermal, atomic/molecular, mineralogical) of accretion disks; the mechanisms and initial conditions of planet formation; the instabilities responsible for angularmomentum transport; the magneto hydrodynamics of outflows; stellar structure evolution in the first few million years. Evolved stars (and in particular asymptotic giant branch stars) are the molecular and dust factories of the universe providing the placental material of stellar nurseries. Some of them pulsate as they cross the instability strip in the Hertzsprung-Russell diagram. The pulsations generate shock waves propagating through the weakly gravitationally bound atmospheres being therefore responsible for considerable mass loss. Observationally, very little is known as current imaging telescopes, which only marginally resolve them, reveal puzzling asymmetric surfaces sometimes presenting bright spots.
- The discovery and characterization of giant planets orbiting nearby stars
The classical technique to detect these objects is through radial velocity measurements of special late-type stars, from which mass and orbital parameters projected along the line of sight can be derived. When combined with astrometric information the true parameters can be recovered. Astrometric planet hunting with PRIMA will use considerable amounts of VLTI time and include early-type and active young stars unfeasible with radial velocity measurements. Debris disks sometimes surround these early-type stars - what is the planet formation efficiency as a function of parent star mass - VLTI planet searches for active young stars will reveal planet formation properties during the first few million years of stellar lifetime.
- Probing the energy conversion mechanisms in Active Galactic Nuclei
Optical and near infrared interferometry has so far played a marginal role in extragalactic astronomy. Being bright and compact, Active Galactic Nuclei are ideal targets for interferometry provided the interferometer is sensitive enough � this is the case of the VLTI. The energy liberated by infall (accretion) of matter into the central supermassive black hole is the driving energy source of these active galaxies. The central engine and the region emitting the broad lines are encircled by a dust torus and relativistic jets are observed. The non-thermal emission from the relativistic jet is probed by radio interferometric arrays. The spatial scales of the dust engines of active galaxies are of the order of milli-arcseconds, the broad line emission regions emit in the optical. Both thegeometry and dust type content of AGN tori are poorly known as current observations are mainly based on highly degenerate spectral energy distribution information. The broad line emission regions can only be reached through differential interferometry (differential displacement of the photocenter along the emission line profile). The mass of the central black hole in local galaxies is found to be correlated with certain host properties like the random motion of stars. The causes of this correlation are unknown. The low spatial resolution of current observations translates in upper limits to the central densities and therefore in uncertainties in mass distribution between the central massive black hole and dark objects (e.g. neutron stars and other stella r remnants).
- Measuring space-time in a few Schwarzschild radius at the galactic centre super massive black-hole
Finally let us recall the vigorous evolution of astrophysical interferometry across wavelength: a) initially maturing at radio wavelengths; b) ALMA at the (sub)-millimetre regime is the first truly global astronomical project; c) the VLTI is the premier world facility operating in the optical and infrared; d) the microarcsecond X-ray imaging mission (MAXIM), under study, will use X-ray interferometry to directly image the Schwarzschild radius in the nearby super massive black holes.High-precision astrometry of stars in the cluster surrounding the central black hole in our own Galaxy will test the black hole gravitational field at a few Schwarzschild radii. Further astrometric studies of the flaring activity offer the exciting possibility of measuring the central black-hole angular momentum.
@