MATISSE is foreseen as a mid-infrared spectro-interferometer combining the beams of up to four UTs/ATs of the Very
Large Telescope Interferometer (VLTI) of the European Southern Observatory. The related science case study
demonstrates the enormous capability of a new generation mid-infrared beam combiner.
MATISSE will constitute an evolution of the two-beam interferometric instrument MIDI. MIDI is a very successful
instrument which offers a perfect combination of spectral and angular resolution. New characteristics present in
MATISSE will give access to the mapping and the distribution of the material (typically dust) in the circumstellar
environments by using a wide mid-infrared band coverage extended to L, M and N spectral bands. The four beam
combination of MATISSE provides an efficient UV-coverage : 6 visibility points are measured in one set and 4 closure
phase relations which can provide aperture synthesis images in the mid-infrared spectral regime.
Interferometry has been intensively done at long wavelengths, starting with the radio interferometers in the years 50 since it was easier to guide radio wavelengths in cable while keeping the phase information or using a local oscillator and a correlator to recombine "a posteriori" the beams over intercontinental distances. In the optical a lot of work as been done at IR and near-IR wavelengths since it was technically easier, or we must say, less difficult to recombine directly the optical beams since the coherence length is larger and the turbulence slower than at shorter wavelengths.
Therefore, the visible domain of the electromagnetic spectrum is not covered at the same level than near or mid infrared.
Some very nice and important results have been however obtained with the GI2T interferometer in south of France, the
Mark III interferometer on the Mount Wilson, USA, the NPOI array in Flagstaff, USA or the SUSI interferometer in Australia. We will present in this paper the science cases of a new but already existing and tested instrument: the
REGAIN focal instrument which was designed and built for the GI2T. This instrument, in his CHARA adaptation, called VEGA will open new fields in a wide range of Astrophysical topics only addressable in the visible domain. It will provide a spectral resolution up to 30000 within the spectral range 0.4-0.9 micron and a spatial resolution of less than 1mas for up to 4 telescopes in its X-lambda special configuration. A polarimetric device (SPIN) measuring simultaneously the polarization in 2 directions either circular or linear is also implemented in this instrument. Since VEGA was already tested on the sky on 1.5 m telescopes it is also very well suited for the 1m CHARA array and will only need minor adaptations for the injection of the CHARA beams. This paper will focus on some of the most promising science drivers only possible with this visible instrument.
Similarly as the technique of Doppler Imaging from spectroscopic observations, Differential Interferometry makes it possible to
measure the disturbances of photocentroid location of an unresolved star as a function of wavelength and to deduce the corresponding stellar map. We show the imaging potential of a tomographic technique which combines time-resolved spectroscopy and long baseline
interferometry, providing information that cannot be obtained
otherwise with each of these techniques taken at once. In particular, here we consider the example of mapping abundance inhomogeneities, performing numerical experiments with realistic spectral resolutions and signal-to-noise ratios expected for operating (VLTI, GI2T) or close-to-operating long baseline interferometers (Chara, Keck). We show that the accurate maps of stellar surface abundance distribution can be obtained using regularized inversion by Maximum Entropy method. The technique is also applicable to other classes of stellar surface imaging as magnetic field and temperature spots but within the classical instrumental context (without polarimetric device) it can hardly discriminate among different distributions. We discuss the importance of Spectro-Polarimetric Interferometry observations
(Rousselet-Perraut et al., this proceedings) in order to
discriminate and simultaneously map abundance/temperature
inhomogeneities and magnetic fields of chemically peculiar (CP)
Simultaneously mapping the abundance inhomogeneities and the magnetic fields of chemically peculiar (CP) stars is essential to improve our understanding of stellar magnetism and its key role in structuring stellar atmospheres, in particular relative to ion migration and chemical stratification. However, magnetic fields and chemical inhomogenities tend to have similar effects on classical observables. Magnetic and abundance maps have therefore to be reconstructed most often either independently or in making a priori assumptions. To overcome these difficulties, we propose to take benefit of optical aperture synthesis arrays to resolve local magnetic structures and patchy stellar surfaces. This requires ability to resolve polarimetrically magnetically-sensitive spectral lines, and thus to add a polarimetric device at the combined focus of an interferometric array. Within this instrumental context, it becomes possible to map magnetic fields with visibility and phase measurements in circularly polarized light and to map the chemical inhomogeneities thanks to "classical" interferometric measurements (i.e. without the polarimeter). In this paper, we show that the interference fringe phase is the suitable observable for polarimetric measurements and for mapping patchy surfaces (see also Jankov et al. in these proceedings). We present some illustrative cases of different magnetic topologies and abundance distributions. We focus on two well-known CP stars, βCrB and α2CVn, and we show observational predictions with different instruments currently in operation (GI2T, VLTI).