The nearby interstellar medium towards alpha Leo -- UV observations and
  modeling of a warm cloud within hot gas
  We analyze interstellar absorption features in the full UV spectrum of the
nearby (d = 24 pc) B8 IVn star alpha Leo (Regulus) obtained at high resolution
and high S/N by the HST ASTRAL Treasury program. We derive column densities for
many key atomic species and interpret their partial ionizations. The gas in
front of alpha Leo exhibits two absorption components, one of which coincides
in velocity with the local interstellar cloud (LIC) that surrounds the Sun. The
second, smaller, component is shifted by +5.6 km/s relative to the main
component, in agreement with results for other lines of sight in this region of
the sky. The excitation of the C II fine-structure levels and the ratio of Mg I
to Mg II reveal a temperature T = 6500 (+750,-600)K and electron density n(e) =
0.11 (+0.025,-0.03) cm^-3. Our investigation of the ionization balance of all
the available species indicates that about 1/3 of the hydrogen atoms are
ionized and that metals are significantly depleted onto grains. We infer that
N(H I) = 1.9 (+0.9,-0.6) X 10^{18} cm^-2, which indicates that this partly
neutral gas occupies only 2 to 8 pc (about 13%) of the space toward the star,
with the remaining volume presumably being filled with a hot gas that emits
soft X-rays. We do not detect any absorption features from the highly ionized
species that could be produced in an interface between the warm medium and the
surrounding hot gas. Finally, the radial velocity of the LIC agrees with that
of the Local Leo Cold Cloud, indicating that they may be physically related.
