Intermediate scale structure of the interstellar medium towards NGC 6231 in Sco OB1 with FUSE

Abstract

The FUSE far-ultraviolet interstellar spectra toward seven targets in NGC 6231 show that the molecules H_2, HD, and CO as well as various atomic species are distributed in several clouds. The main absorption component found on the sight lines lies in the Lupus cloud region at a distance of about 150 pc, and there is a weaker second one, presumably in the Sco OB1 shell surrounding NGC 6231 (d~1.8 kpc). H_2 excitation modelling is used to constrain the radiation field and the density in the gas; HD is used to estimate the abundance of H+. The small angular separation of the target stars allows column density variations to be probed over the field of view, on scales of 0.05 pc in the case of the Lupus cloud distance. They are 40% for H_2 and 60% for H I. A rather strong radiation field inside the molecular clouds suggests a separation of the gas into smaller cloudlets also along the line of sight.