LH 72, the Center of Constellation III

Abstract

Ultraviolet spectroscopy with IUE is combined with optical photometry to study the brightest blue members of the LMC OB association LH 72. LH 72 lies near the geometric center of Shapley's Constellation III, a complex of gas and stars that makes up one of the largest known superbubbles. Ages for the stars range from 5 × 10⁶ to 1.5 × 10⁷ yr, with ages increasing from the south to the north end of the association. The youngest stars are associated with the brightest H II emission and have the largest reddening. This pattern of apparent sequential star formation is contrary to the expanding model proposed by Dopita, Mathewson, & Ford, in which the entire superbubble, more than an order of magnitude larger in size than LH 72, evolves in about the same amount of time. Our data, however, are consistent with an alternative model (Reid, Mould, & Thompson), in which star formation proceeds locally after the initial trigger from the superbubble shock.