Astronomy and Astrophysics Diffuse Ionized Gas toward Β Canis Majoris ⋆

This paper presents the study of the interstellar medium toward β CMa, a disk sight-line known for its low neutral gas density. This study uses high and medium resolution HST-GHRS spectra including lines from the following species : H i, D i, N i, O i, S ii, S iii, Si ii, Si iii, Si iv, Al ii, Al iii, Fe ii, Mg i, Mg ii, Mn ii, C ii and C iv. The line of sight to β CMa (153 pc) is dominated by two ionized regions with a velocity difference of 10 km s. The ionized regions account for most of the total hydrogen column density, around 2 10 cm, and the neutral gas represents only 10% of the total gas. The two ionized clouds display characteristics of the warm diffuse gas detected in the disk and the halo. Their gas-phase abundances indicate that their depletion is low, especially for the more ionized of the two clouds. Special models of photoionization by the two EUV-excess stars β CMa and ǫ CMa would be needed for a detailed discussion of the ionizing mechanisms of the clouds ; their ionization ratios are nevertheless roughly compatible with collisional ionization at temperatures around 20 000 K, substantially higher than the kinetic temperatures derived from the line widths. Their characteristics suggest that the clouds may be in the process of cooling down and recombining after having been shocked and ionized by some violent events, possibly related to the Local Bubble formation.