Far to near UV spectroscopy of the interstellar medium at very high
  resolution and very high signal-to-noise ratio
  A comprehensive study of interstellar medium phases and the nature of their
boundaries or connections requires comparing abundances and velocity profiles
from tracers of the different phases. In the UV, studies of a wealth of
absorption features appearing in the spectra of hot stars yield fundamental
insights into the composition and physical characteristics of all phases of the
ISM along with the processes that influence them. They also inform us on the
nature of boundaries between them. However no single instrument has as yet
given access to species in all ISM phases at the same high spectral resolution:
from the molecular bands of CO and H2 in the far-UV, to the cold neutral medium
tracers C I and S I and the warm medium tracers like C II, NI, OI, Mg II, Fe
II, Si II etc, and to the high ions of the hot ionized medium C IV, Si IV in
the UV, as well as O VI in the far UV. We have yet to design the spectrometer
that will enable observing the full UV domain at resolving power R>200 000 and
signal-to-noise ratio SNR>500. The line FWHM being governed by turbulence,
temperature, and species mass, such a resolution is necessary to resolve lines
from both the cold molecular hydrogen and the warm metal ions with a turbulence
of ~1 km/s, and to differentiate distinct velocity components, typically
separated by less than 2 km/s.
