A Hot Envelope Around the Southern Coalsack: X-ray and Far-Ultraviolet Observations

We present Far Ultraviolet Spectroscopic Explorer and ROSAT X-ray observations toward the Southern Coalsack. An almost complete X-ray halo can be seen around the cloud in the 0.75 and 1.5 keV images, and most of the observed stars show O VI absorption. Both the cloud and the stars have highly accurate distance determinations, allowing us to reliably place the stars and the cloud relative to each other. Using these distance determinations, we find no O VI-bearing gas in the foreground of the Coalsack, while for stars in the background of the cloud, O VI absorption is the norm. The column density of O VI correlates with the 0.75 and 1.5 keV intensities. These results suggest that the X-ray-emitting hot plasma is associated with the dense cloud. We propose that the heating of the Coalsack envelope is due to the hot gas in the interior of the Upper Cen-Lup superbubble. The Coalsack interaction region provides a nearby example of the hot-cold gas interfaces thought to be responsible for the O VI absorptions seen on many sight lines throughout the Galaxy.