O meio interestelar local na direção das nuvens escuras Saco de Carvão e Chamaeleon-Musca

Abstract

Recent investigations of the colour excess distribution towards the geometric center of the Chamaeleon-Musca dark clouds complex and the Southern Coalsack dark cloud have suggested that, although being apart by more than 15, these clouds might be physically associated. In order to sustain such hypothesis a new photometric programme has been carried out to trace the extinction between the features. We used the Str¨omgren Automatic Telescope at the European Southern Observatory (ESO), in La Silla (Chile), to obtain uvby_ photometry for all stars, earlier than G0 in the catalog of the Smithsonian Astronomical Observatory, covering the area defined by the Galactic coordinates 308 l 294 and 20 b 5. The programme has resulted on very precise uvby_ data for 1017 stars. Analysis of the colour excess E(by) vs. distance diagrams for these stars has indicated the presence of a local low absorption volume limited at 150 ?30 pc from the Sun by an extended interstellar dust sheet-like structure, that is followed by a region where almost no additional reddening is measured for another 350 pc. The existence of such absorbing feature, at a distance identical to the molecular clouds, has suggested that Coalsack, Chamaeleon and Musca might be dense condensations embbeded in the diffuse medium composing the feature. Furthermore, the minimum column density of the absorbing structure seemed to increase with the Galactic latitude, but without clear dependence on the Galactic longitude. Such effect suggested either an inhomogeneous sheet-like structure, that could be perpendicular to the Galactic plane, or a sheet-like structure of same minimum column density, curved away from the Sun. When viewed in connection to the other data on the local ISM, the existence of these two low-reddening volumes has led to the idea that the dust sheet could be part of a large scale structure, probably related to the interface of the Local and Loop I Bubbles. To investigate the local interstellar medium (ISM) components towards the Southern Coalsack and Chamaeleon-Musca dark clouds we have used the 1.4m Coud´e Auxiliary Telescope at ESO to obtain high-resolution (R 60 000) spectra of the interstellar NaI D absorption lines towards 64 B-type stars (d 500 pc) selected to cover these clouds. The radial velocities, column densities, velocity dispersions, colour excess and distances to the stars have been 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, that corresponds to the dust sheet observed photometrically, around 120-150 pc from the Sun. The nearby feature is approaching the Sun with average radial velocity of -7 kms1, has low average column density logNNa i 11.2 cm2 and is subject to supersonic turbulent motion as indicated by the velocity dispersion b 5 kms1. The more distant feature has column densities between 12.3 logNNa i 13.2 cm2, average velocity dispersion b 2.5 kms1. Its velocity is centered around 0 kms1, but there is a trend for increasing from -3 kms1 near b = 1 to 3 kms1 near b = 18. The nearby low column density feature indicates a general outflow from the Sco-Cen association, in aggreement with several independent lines of data in the general searched direction. From the radial velocities point of view, the component around 120 150 pc has column densities that suggest neutral gas associated to the dust sheet-like feature observed towards the studied clouds. The fact that the velocities observed in this work (4 km s1 VLSR +3 km s1) are also observed in adjacent directions, supposedly related to the interface of the Local and Loop I bubbles, suggest that Coalsack, Chamaeleon and Musca seem to be part of such interaction zone.