GAS MOTIONS IN AND ORIGIN OF THE SUPERGIANT SHELL LMC-4

IUE high-dispersion spectra of stars in the supergiant shell LMC 4 hav e been used to derive velocities and column densities of absorbing gas clouds. The HI 21-cm profiles for the LMC 4 region from the LMC data of Rohlfs et al. (1984) have been analysed and interpreted in cloud co mponents. Combining the absorption and emission line data, a detailed picture emerges of the location along the line of sight of the various gas components and of the stars. LMC 4 has a systemic velocity of 285 km s(-1). A pocket of gas in the center of LMC 4 is found to be reced ing at similar to 325 km s(-1) while over the entire area we find gas near 260 km s(-1). These two components of gas differ in various ways. We argue that the gas at 325 km s(-1) most likely belongs to a shell breakup. The gas at 260 km s(-1) probably does not belong to the LMC 4 but lies in front of the supershell. There is no evidence for a syste matic radial expansion of the gas from the geometric center of LMC 4. Also, there is gas near 220 km s(-1) found in several of the IUE spect ra. Given that the velocity differs by 65 km s(-1) from that of the ma in body of LMC 4 and the large spatial extent we propose this gas clou d to be in the Milky Way halo. Our results are compared with several t heoretical scenarios for superbubble structures. We conclude that the origin of LMC 4 is best explained by a process of self-propagating sta r formation. Our data are in agreement with expansion velocities aroun d 10 km s(-1).