Local interstellar medium kinematics towards the Southern Coalsack and Chamaeleon–Musca dark clouds

We present the results of a spectroscopic programme aiming to investigate the kinematics of the local interstellar medium components towards the Southern Coalsack and Chamaeleon–Musca dark clouds. The analysis is based upon high-resolution (R≈ 60 000) spectra of the interstellar Na i D absorption lines towards 63 B-type stars (d⩽ 500 pc) selected to cover these clouds and the connecting area defined by the Galactic coordinates: 308°⩾l⩾ 294° and −22°⩽b⩽ 5°. The radial velocities, column densities, velocity dispersions, colour excess and photometric distances to the stars are used to understand the kinematics and distribution of the interstellar cloud components. The analysis indicates that the interstellar gas is distributed in two extended sheet-like structures permeating the whole area, one at d⩽ 60 pc and another around 120–150 pc from the Sun. The nearby feature is approaching the local standard of rest with an average radial velocity of −7 km s−1, has low average column density log NNa i≈ 11.2 cm−2 and velocity dispersion b≈ 5 km s−1. The more distant feature has column densities between 12.3 ⩽ log NNa i⩽ 13.2, average velocity dispersion b≈ 2.5 km s−1 and seems associated with the dust sheet observed towards the Coalsack, Musca and Chamaeleon direction. Its velocity is centred around 0 km s−1, but there is a trend for increasing from −3 km s−1 near b = 1° to 3 km s−1 near b = −18°. The nearby low column density feature indicates a general outflow from the Sco-Cen association, in agreement with several independent lines of data in the general searched direction. The dust and gas feature around 120–150 pc seem to be part of an extended large-scale feature of similar kinematic properties, supposedly identified with the interaction zone of the Local and Loop I Bubbles. Assuming that the interface and the ring-like volume of dense neutral matter that would have been formed during the collision of the two bubbles have similar properties, our results suggest that the interaction zone between the bubbles is twisted and folded.