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Astron. Astrophys. 326, 1215-1227


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

W.J.B. Corradi1,2, G.A.P. Franco1, and J. Knude2

1Departamento de Física - ICEx - UFMG, Caixa Postal 702, 30161-970, Belo Horizonte - MG, Brazil
2Niels Bohr Institute for Astronomy, Physics and Geophysics, Juliane Maries Vej 30, DK-2100, Copenhagen Ø, Denmark
(corradi@astro.ku.dk, franco@fisica.ufmg.br, indus@astro.ku.dk)

Received 20 August 1996 / Accepted 5 June 1997

Abstract

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 FORMULA 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: FORMULA and FORMULA . To ensure a more complete sample the data were complemented by FORMULA 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 FORMULA 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 FORMULA . The quoted increase suggests either the approaching of the tangential point of a warped sheet-like structure of same column density and curved away from the Sun, or an inhomogeneous sheet-like structure roughly perpendicular to the galactic plane.

Key words: stars: distances - ISM: clouds - ISM: individual objects: Southern Coalsack - Chamaeleon-Musca - Loop I - Local Bubble

*Based on data collected with the Strömgren Automatic Telescope - SAT - Danish 50 cm installed at the European Southern Observatory (ESO) in La Silla, Chile

Send offprint requests to: W.J.B. Corradi (Brazilian Address)


© European Southern Observatory (ESO) 1997

Online publication: October 1, 1997
Last change: April 8, 1998
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