Observations of interstellar Na I and Ca II towards five southern stars and some comments on the location of the denser diffuse clouds in the southern Milky Way

High-resolution observations (3.6 km s–1 FWHM) of interstellar Na I and Ca II lines towards five southern early-type stars are presented. All five stars have strong absorption lines with low LSR radial velocities (|υlsr|≲5kms−1) which are interpreted as arising in material associated with the ridge of local molecular clouds identified by Dame et al. It is argued that the strong, low-velocity, atomic and molecular lines observed towards other southern stars close to the galactic plane also arise in this material, and special reference is made to the foreground interstellar spectrum of the Sco OB1 association. Four of the observed stars also have absorption components consistent with an origin in the Sagittarius–Carina spiral arm, and two (HD 111904 and 164402) have highly negative velocity components (υlsr≲−40kms−1) which are interpreted as arising in expanding shells centered on the Cen OB1 and Sgr OB1 associations, respectively