THE 3-DIMENSIONAL STRUCTURE OF THE ISM IN FRONT OF SUPERNOVA 1987A

The expanding light echoes surrounding SN 1987A have been studied phot ographically over a period exceeding 3 yr, sufficiently long that a vi ew of the three-dimensional nature of the reflecting material can now be derived. We present two different projections of the dust distribut ion. We find a number of discrete clouds, a linear formation and two p rominent sheets of material, one of which is clearly split while the o ther has significant depth. We find an association with the HII region N157C, but not with the supergiant shell that forms part of the N157C complex, which we show to lie behind the supernova. Although we still prefer to assign the major echoes to the front and rear faces of a si ngle, uncatalogued supergiant shell, we cannot preclude the existence of two foreground supergiant shells, one inside the other and both con taining the supernova. We discuss the association of the echoes with a bsorption features and recommend further observations that can clarify the structure of this region of the LMC.