1995A&A...296..523D

IUE high-dispersion spectra of stars in the supergiant shell LMC4have been used to derive velocities and column densities of absorbing gas clouds. The HI 21-cm profiles for the LMC4region from the LMC data of Rohlfs et al. (1984A&A...137..343R) have been analysed and interpreted in cloud components. 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. LMC4 has a systemic velocity of 285km/s. A pocket of gas in the center of LMC4is found to be receding at ∼325km/s while over the entire area we find gas near 260km/s. These two components of gas differ in various ways. We argue that the gas at 325km/s most likely belongs to a shell breakup. The gas at 260km/s probably does not belong to the LMC4but lies in front of the supershell. There is no evidence for a systematic radial expansion of the gas from the geometric center of LMC4. Also, there is gas near 220km/s found in several of the IUE spectra. Given that the velocity differs by 65km/s from that of the main body of LMC4and the large spatial extent we propose this gas cloud to be in the Milky Way halo. Our results are compared with several theoretical scenarios for superbubble structures. We conclude that the origin of LMC4 is best explained by a process of self-propagating star formation. Our data are in agreement with expansion velocities around 10km/s.