2001AJ....121.3075O

Stellar spectroscopy, UBV photometry, Hα imaging, and analysis of data from the Australia Telescope Compact Array H I survey of the LMC are combined in a study of the LMC OB association LH 72 and its surroundings. LH 72 lies on the rim of a previously identified H I shell, SGS 14, and in the interior of LMC 4, one of the LMC's largest known supergiant shells. Our analysis of the H I data finds that SGS 14 is expanding with a velocity of v_exp∼15 km.s^–1, giving it an expansion age of ∼15 Myr. Through the stellar spectroscopy and photometry, we find similar ages for the oldest stars of LH 72, ∼15-30 Myr. We confirm that LH 72 contains an age spread of ∼15-30 Myr, similar to the range in ages of stars derived for the entire surrounding supergiant shell. Combining analysis of the O and B stars with Hα imaging of the H II region DEM 228, we find that DEM 228 accounts for only 60% of the available ionizing Lyman continuum photons. Comparing the distribution of ionized gas with that of the H I, we find that DEM 228 and LH 72 are offset by ∼1'-2' from the peak 21 cm emission toward the interior of SGS 14. Taken together, these results imply that SGS 14 has cleared its interior of gas and triggered the formation of LH 72. On the basis of our results, we suggest that LMC 4 was not formed as a unit but by overlapping shells, such as SGS 14, and that LH 72 will evolve to produce a stellar arc similar to others seen within LMC 4.