Shapley constellation III: a region of self-propagating star formation

The H I dynamics and stellar content of the Shapley III/supergiant loop LMC 4 region are studied. There are three components, one associated with the disk H I with a mean velocity of 285 km/s with respect to the local standard of rest, and the other two with shells of gas ejected above and below the plane of the LMC with an expansion velocity of 36 km/s. The disk H I shows a ringlike structure, 1.8 kpc in diameter with a very pronounced central hole, and the H II regions form a ring 1.4 kpc in diameter. Star formation appears to have started approximately 15 Myr ago near the center and has propagated outwards at a constant speed of 36 km/s. Newly formed stars generate a local overpressure in the ISM which drives a shock in the intercloud medium, sweeping over and around dense H I clouds and so initiating new episodes of star formation. 54 references.