Collisional dynamics of stellar systems in the Northern and Southern Coalsack regions

The visual interstellar extinction in the Northern and Southern Coalsack regions is estimated from a photographic survey of proper motion and B, V photometry of 6 × 10⁴ stars as faint as apparent magnitude 14 over a field of view of 6° × 6° in each region. Each cloud is located at a distance of about 160 ± 50 pc from the Sun and exceeds the mean interstellar extinction by more than 0.7 ± 0.1 mag. Galactic plane orbits of 10³ stars located at the same distance of 160 ± 50 pc from the Sun in these regions show a semimajor axis of 8.45 ± 0.15 kpc and a mean inclination of 0°.3 ± 0°.2 over the Galactic plane, suggesting a mean impact parameter of 9 ± 3 pc with diffuse clouds of relative velocity on the order of 4 ± 1 km s^-1. The weak distant encounter of stars with the cold neutral medium can be explained by a mean mass of about 2500 ± 1250 M_⊙ for the gas and dust content in order to account for the diffusion of stellar orbits. A mean displacement of 0″.012 ± 0″.001 yr^-1 of the western limb of the Southern Coalsack is estimated in the northwest direction and permits a search for strong close stellar encounters. The transfer of angular momentum between the interstellar medium and the scattering of stellar orbits resulting from the encounters suggests a dissipation rate of (2.0 ± 0.7) × 10^-4 M_⊙ yr^-1, leading to an equipartition time of 12 ± 4 Myr. A spin rate of 0.25 ± 0.05 km s^-1 pc^-1 of the diffuse clouds in the Southern Coalsack is measured and interpreted as the result of the perturbation induced by stellar systems during flyby encounters with a crossing time of 3 Myr, suggesting a cloud core radius of 1 pc.