  POLLUX is a high-resolution, UV spectropolarimeter proposed for the 15-meter
primary mirror option of LUVOIR. The instrument Phase 0 study is supported by
the French Space Agency (CNES) and performed by a consortium of European
scientists. POLLUX has been designed to deliver high-resolution spectroscopy
(R> 120,000) over a broad spectral range (90-390 nm). Its unique
spectropolarimetric capabilities will open-up a vast new parameter space, in
particular in the unexplored UV domain and in a regime where high-resolution
observations with current facilities in the visible domain are severely photon
starved. POLLUX will address a range of questions at the core of the LUVOIR
Science portfolio. The combination of high resolution and broad coverage of the
UV bandpass will resolve narrow UV emission and absorption lines originating in
diffuse media, thus permitting the study of the baryon cycle over cosmic time:
from galaxies forming stars out of interstellar gas and grains, and stars
forming planets, to the various forms of feedback into the interstellar and
intergalactic medium (ISM and IGM), and active galactic nuclei (AGN). UV
circular and linear polarimetry will reveal the magnetic fields for a wide
variety of objects for the first time, from AGN outflows to a diverse range of
stars, stellar explosions (both supernovae and their remnants), the ISM and
IGM. It will enable detection of polarized light reflected from exoplanets (or
their circumplanetary material and moons), characterization of the
magnetospheres of stars and planets (and their interactions), and measurements
of the influence of magnetic fields at the (inter)galactic scale. In this
paper, we outline the key science cases of POLLUX, together with its high-level
technical requirements. The instrument design, its estimated performances, and
the required technology development are presented in a separated paper.
