Context :

Aims : Our aim is to characterize the conditions in the closest interstellar cloud .

Methods : 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 .

Results : 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 ) 10 ^ { 18 } cm ^ { -3 } , 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 .

Conclusions :