A 3-DIMENSIONAL STUDY USING LIGHT ECHOES OF THE STRUCTURE OF THE INTERSTELLAR-MEDIUM IN FRONT OF SN-1987A

A thorough study of the interstellar light echo images from SN 1987A o ver more than 5 years reveals a complicated three-dimensional structur e in front of the supernova, ranging from 310 to 3240 It-yr. A three-d imensional map was constructed based on a long procedure of image proc essing and an artificial intelligence analysis. The three-dimensional properties of these structures are carefully measured and are presente d in this paper. The map shows that several dust clouds form a bubble shape of 700-800 It-yr in diameter. By comparing it with an H alpha im age and by computing its geometric configuration, we identified this s tructure as superbubble N157C, centered on star cluster LH 90. Based o n our fitting result of the curvature center of N157C, we propose that LH 90 is located about 1600 It-yr in front of SN 1987A and hence that the supernova is a runaway star behind the disk of the Large Magellan ic Cloud. This map also suggests that N157C and two dense dust clouds about 3000 It-yr in the supernova foreground embrace a giant superbubb le of 2000 It-yr in diameter. The southeastern one of the two faraway objects correlates with a prominent H alpha filament reaching out to t he southwest from its origin over N157C. We suspect that this giant bu bble is the merger of two or more smaller superbubbles and is filled w ith hot X-ray-emitting gas. The possibility that this region is a remn ant of ancient, active star formation is advocated. Our ongoing light echo monitoring program will further reveal this structure.