S. Kim , L. Staveley-Smith , R.J. Sault , M.J. Kesteven , D. McConnell , M.A. Dopita , M. Bessell, PASA, 15 (1), 132
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Results
The peak brightness temperature map (Kim et al. 1997) of the LMC shows many filamentary features, combined with HI holes and shells. In our preliminary analysis we have defined 32 HI supergiant shells and these shells are overlayed on the H
image in Figure 1. The selection criteria applied to the shells are: (1) they must be edge-brightened ring-like structures larger than 600 pc that are visible in at least three integrated channel maps (the spacing between each integrated channel map is 5 km 
); and (2) the approaching and receding hemispheres (or one of hemispheres) are visible in position-velocity cuts (P-V diagrams).
Figure: The position of the candidate supergiant shells are overlayed as ellipses on the SSO H image. The classification, based on the comparision between the HI and H properties is marked (see text). The numbers indicate the dynamical age of the shells in Myr.
Figure 2: The geometrical relationship between HI (grey scale) and H (contours) for the five categories of HI shells described in the text.
Many of the shells are not simple expanding shells in the P-V diagrams. The cause of this is a combination of confusion between neighbouring shells, lack of zero-spacing data and breakouts from the disk of the LMC. If we compare the distribution of HI Supergiant shells in each of the four quadrants of the LMC, we clearly see that the largest number of shells are located in the south-eastern quadrant, near the 30 Doradus nebula as seen in Fig. 1. Fig. 1 also shows the age of each shell, 
(in units of Myr), calculated from 
as given by the standard theory of wind-driven bubbles (Weaver et al. 1977), where 
is the shell radius and 
is the expansion velocity of the shell. The measured ages of the expanding supergiant shells are distributed between 3 and 25 Myr. The shells in Fig. 1 may also be classified morphologically by comparing their HI and H structures. We define five categories for the geometrical correlation between HI and H emission (Fig. 2). These categories should approximate an age sequence. In Type I shells, the HI shells are filled with ionized gas or else include discrete HII regions inside the HI shell. In Type II shells, the ionized gas forms a thin shell and has been trapped by the HI shell. Type III represents HI shells which have discrete HII regions on the wall of the shell. Type IV represents HI shells which are not associated with ionized gas. Type III and Type IV may have both have evolved from Type II with the difference between the two types probably dependent on the density of the interstellar medium. Type V are HI shells which have morphologically complex structure, which might be due to distortions caused by the secondary star formation on the surface of the shell. Age estimates for some of shells marked in Fig. 1 is due uncertain or unmeasurable expansion velocities.
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