Stars and exoplanets in Stokes IQUV: a decadal opportunity for HIRES at the ELT

We proposed that the European ELT will be equipped with an ultra-high-precision polarimetric light feed as part of its high resolution optical and near-IR spectrograph HIRES. Such a feed is unique among the new ELTs and only possible in a rotationally symmetric focus of the telescope. The ELT's f/4.4 intermediate focus near M4 could provide such a capability with a polarimetric sensitivity of down to 10^(-5) for the brightest targets. Among the new science steps forward with HIRES-Pol at the 39m ELT would be the full characterization of solar-like stellar magnetospheres by means of Zeeman-Doppler Imaging. In particular for planet-hosting stars it could constrain the habitability of a planet based on its particle-emission geometry from the host star. Besides, any stellar linear-polarization spectrum is new territory for astrophysics and I refresh reasons why this can be also important for exoplanet atmospheres.