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We analyze interstellar absorption features in the full UV spectrum of thenearby (d = 24 pc) B8 IVn star alpha Leo (Regulus) obtained at high resolutionand high S/N by the HST ASTRAL Treasury program. We derive column densities formany key atomic species and interpret their partial ionizations. The gas infront of alpha Leo exhibits two absorption components, one of which coincidesin velocity with the local interstellar cloud (LIC) that surrounds the Sun. Thesecond, smaller, component is shifted by +5.6 km/s relative to the maincomponent, in agreement with results for other lines of sight in this region ofthe sky. The excitation of the C II fine-structure levels and the ratio of Mg Ito 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 allthe available species indicates that about 1/3 of the hydrogen atoms areionized and that metals are significantly depleted onto grains. We infer thatN(H I) = 1.9 (+0.9,-0.6) X 10^{18} cm^-2, which indicates that this partlyneutral 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 emitssoft X-rays. We do not detect any absorption features from the highly ionizedspecies that could be produced in an interface between the warm medium and thesurrounding hot gas. Finally, the radial velocity of the LIC agrees with thatof the Local Leo Cold Cloud, indicating that they may be physically related.

The nearby interstellar medium towards alpha Leo -- UV observations and modeling of a warm cloud within hot gas