Triggered star formation in the LMC4/Constellation III region of the Large Magellanic Cloud

ABSTRACT The origin of a regular, 600 pc-long arc of young stars and clusters in the ConstellationIII region of the Large Magellanic Cloud is considered. The circular form of this arcsuggests that the prestellar gas was uniformly swept up by a central source of pressure.In the center of the arc are six ∼ 30 My old A-type supergiant stars and a Cepheidvariableofsimilarage,which maybe relatedto the sourceofthis pressure.We calculatethe expansion of a bubble around a cluster of this age, and show that it could havetriggered the formation of the arc at the right time and place. Surrounding the centralold stars and extending well outside the young arc is the LMC4 superbubble andgiant HI shell. We show how this superbubble and shell could have formed by thecontinued expansion of the 15 My old cavity, following star formation in the arc andthe associated new pressures. The age sequence proposed here was not evident in therecent observations by Olsen et al. and Braun et al. because the first generation starsin the center of the LMC superbubble are relatively faint and scarce compared tothe more substantial population of stars less than 15 My old that formed throughoutthe region in a second generation. These considerations lead to an examination of theorigin of the LMC4/Constellation III region and other large rings in the LMC andother galaxies. Their size and circularity could be the result of low galactic shear anda thick disk, with several generations of star formation in their interiors now too faintto see.Key words: Magellanic Clouds — stars: formation — ISM: bubbles — instabilities