Science drivers for a shorter wavelength; high-sensitivity 3rd-generation VLTI instrument

We present the science case and design considerations for a 3rd generation VLTI instrument operating in the YJH bands. Extending the VLTI to shorter wavelengths will open a new observational window, providing complementary spectral diagnostics to current VLTI instruments. The expanded spectral coverage will give access to molecular tracers of exoplanet atmospheres and diagnostic lines of accretion and outflow processes, enabling mapping of magnetospheric regions, disk winds, and circumplanetary environments around young and forming planets. Shorter wavelengths, combined with vibration control at the VLTI, will provide improved astrometric precision enabling the detection of exomoons, searches for intermediate- mass black holes, and resolved microlensing studies probing the lower mass gap between neutron stars and black holes. High-contrast dual-field capability will allow spectroscopy of giant and rocky planets in the 30 mas–1� range, including systems from GAIA DR4 and RV surveys. Polarimetry will constrain cloud, haze, and surface properties, while spin-axis measurements of host stars will test spin–orbit alignments, providing key insights into the dynamical evolution of planetary systems. In the extragalactic domain, resolving the broad-line regions of AGNs for large samples will enable dynamical mass measurements and reverberation mapping campaigns. In synergy with the ELT, these will contribute to resolving the Hubble tension.