Interstellar NaI absorption towards the stellar association ARA OB-1.

Observations have been made of the interstellar D-line absorption of Na I towards a loose grouping of OB stars (containing the association Ara OB1 and probably the H-alpha emission regions RCW 107 and 108) centered at 1 = 337.5 deg and b = - 0.5 deg. The individual absorption spectra contain several velocity components which show well-defined groupings at - 37, -18 and 0 km/s. The corresponding median optical depths are 2, 7 and 6. The evidence indicates that both the -37 and -18 km/s Na I clouds are located in the Carina-Sagittarius spiral arm. Several H II regions are located at similar longitudes, but most are close to zero latitude and are more distant than the arm. The association contains the galactic cluster NGC 61993. The results suggest that the turbulence seen optically towards this cluster has been caused by the interaction between two cloud groups moving towards each other at velocities of 10 to 15 km/s.