Scientific Drivers for a Future Polarimetric Mode for the VLTI: VISPER

The scientific drivers and the concept of a Polarimetric instrument for the VLTI, VISPER (Vlti Imaging Spectro-PolarimetER) will be presented. Many scientific programs in stellar physics as well as in extra-galactic astronomy would benefit from interferometric high-resolution imaging combined with polarimetry. For instance, we can study scattering phenomena in the extended atmospheres of hot stars, the circumstellar environment of young stellar objects, or AGN nuclei. In addition, Polarimetric observations allow getting information related to stellar magnetic fields through the detection of spatially resolved Zeeman effect. Indeed this kind of studies requires very accurate measurements of the interferometric fringe phase and visibility as well as good spectral resolution (> 10000) and a very stable instrument. As great care was taken of instrumental polarization effects during the design and construction of the VLTI, polarimetry could be straightforwardly implemented in the interferometric laboratory. Moreover, the phase-reference astrometric facility of the VLTI (PRIMA) will offer a very stable and accurate reference to measure small polarization effects on the visibility and astrometric position of the star at certain spectral lines.