TRIGGERED STAR-FORMATION IN THE LMC4 CONSTELLATION-III REGION OF THE LARGE-MAGELLANIC-CLOUD

The origin of a regular, 600-pc-long are of young stars and clusters i n the Constellation III region of the Large Magellanic Cloud is consid ered. The circular form of this are suggests that the pre-stellar gas was uniformly swept up by a central source of pressure. In the centre of the are are six similar to 30-Myr-old A-type supergiant stars and a Cepheid variable of similar age, which may be related to the source o f this pressure. We calculate the expansion of a bubble around a clust er of this age, and show that it could have triggered the formation of the are at the right time and place. Surrounding the central old star s and extending well outside the young are is the LMC4 superbubble and giant H I shell. We show how this superbubble and shell could have fo rmed by the continued expansion of the 15-Myr-old cavity, following st ar formation in the are and the associated new pressures. The age sequ ence proposed here was not evident in the recent observations by Olsen et al. and Braun et al, because the first generation stars in the cen tre of the LMC superbubble are relatively faint and scarce compared to the more substantial population of stars less than 15 Myr old that fo rmed throughout the region in a second generation. These consideration s lead to an examination of the origin of the LMC4/Constellation III r egion and other large rings in the LMC and other galaxies. Their size and circularity could be the result of low galactic shear and a thick disc, with several generations of star formation in their interiors no w too faint to be seen.