Physical association between the Southern Coalsack and the Chamaeleon - Musca dark clouds ?

The results of a photometric programme aiming to investigate the hypothesis of a physical association between the Southern Coalsack and the Chamaeleon-Musca dark clouds are presented. The analysis is based upon uvbyβ photometry for 1017 stars selected from the Smithsonian Astrophysical Observatory star catalog to cover these clouds and the connecting area defined by the galactic coordinates: 308 • ≥ l ≥ 294 • and −20 • ≤ b ≤ 5 •. To ensure a more complete sample the data were complemented by uvbyβ photometry for 213 stars of Kapteyn's Selected Area 203, that lies at the center of the Chamaeleon-Musca dark clouds complex. The distribution of the colour excesses E(b − y) for stars with line-of-sight inside and outside the dark clouds' contours indicates the presence of a local low absorption volume that is limited at 150 ± 30 pc from the Sun by an extended interstellar dust feature, and is followed by another region with almost no additional reddening for another 350 pc. Combined with other data on the local ISM, the existence of the dust feature permeating the whole connecting area and the identical distance of the Southern Coalsack and Chamaeleon-Musca dark clouds suggest that these clouds could be dense condensations in the diffuse medium composing the interface of the Local and Loop I Bubbles. Apparently, the minimum column density of the dust feature does not show a clear dependence with the galactic longitude , but may increase with the galactic latitude in the sense that [E(b − y) min , b] = [0.