A hot gaseous corona was postulated by Spitzer (1956) for the Milky Way as a consequence of the detection of cool interstellar clouds (seen in Call absorption) on paths to stars in the Milky Way halo. Halos around extra galactic systems were proposed by Bahcall and Spitzer (1969) as a possible explanation for the wealth of high redshift absorption line systems in the spectra of quasars. The recording with the International Ultraviolet Explorer (Boggess et al 1978) in 1978 of echelle spectra of R136 and HD38282 in the LMC showed strong absorption lines of the high ionization stage ions CIV and SiIV, due to material with velocities clearly pertaining to locations outside the Milky Way disk (Savage and de Boer 1979). Thus the reality of the galactic corona became established (review de Boer 1984). There is confusion with the words corona (gas – massive component) and halo (location – stars) and I (1984) proposed for the large mass from dynamics MASsive DArk Component, MASDAC.

HII region models were constructed which take into account: ) A burst for the formation of the ionizing association; ) Different Salpeter's initial mass function () and upper stellar mass limit Models of stellar evolution with and without mass loss (Maeder, 1980, Hellings et al. 1981).

Empirical calibrations based on prior studies of Galactic OB Stars are used to determine the integrated stellar wind mechanical luminosity (Lw) and the integrated Lyman continuum photon luminosity (S*) for 10 OB clusters in the L.M.C. These values of Lw and S*, together with narrow band H-α surface photometry of an overlapping sample of 15 L.M.C. HII regions, are used to show that the large shell-like HII regions, in the L.M.C. are stellar wind bubbles 3 to 5 million years old. In order to reproduce the general properties of these HII shells, a thick cushion of shocked stellar wind gas must have been present on the inside of the shell for most of the life time of the nebula, and the shell itself must be ram pressure confined by the HI/molecular cloud out of which it formed.

A nebular photometer equipped with a scanning Fabry-Perot interferometer has been used at the ESO 50-cm telescope to observe about 50 HII regions of the LMC (Caplan and Deharveng, 1983). This instrumentation gives the profiles of the observed lines, thus allowing discrimination against unwanted continuum and line emission. The integrated fluxes were measured with a 4.9′ diaphragm, to facilitate comparison with the 6-cm radio continuum fluxes obtained by McGee et al. (1972) with a similar resolution. Nebular reddening, from the Hα/Hβ ratio, is greatest for the 30 Doradus Nebula. Relatively high reddening was also observed for the following zones: N159 (at the border of the largest HI-molecular complex of the LMC) and N160; N79, N81 and N83; N48; N64. Reddening is rather low towards other HII regions, especially for those in the bar and for large bubble-shaped regions.

Four mechanisms have been proposed to explain the structures and large scale mass motions of HII regions: the Champagne model, the ‘classical’ picture of an expanding Strömgren sphere, interaction of stellar winds and supernova blast waves with the surrounding medium. Especially the last three models have been used to explain the LMC shell structures. Braunsfurth et al. (1983) showed that it is not possible to distinguish the driving mechanisms for individual regions from their global appearance (dimension). Detailed studies of individual shells however, have been limited to the largest and brightest (30 Dor) or the more symmetric ones (e.g. N70, rings around WR stars).

Physical properties of the ISM near SNRs in the LMC can be deduced from their very high-resolution, two-dimensional spectra. The ISM around the N49 and N63A SNRs apparently has as many as three different phases of varying density, dumpiness, and spatial distribution.

The measurements were made in Feb. 1982 with the Parkes 64 m telescope using a corrugated waveguide horn with total half-power beam width of 15′, the first sidelobes being 19 dB down, resulting in an aperture efficiency ηA=0.53±0.007, a main beam efficiency of ηmb=0.80±0.005 and a ratio of source flux to antenna temperature of Γ=0.62±0.1 K/Jy (Murray, priv. comm.). A cooled two channel FET frontend used in the frequency switching mode with Δν = 2 MHz resulted in a system noise temperature at zenith of Tsyst = 40 K for one channel and Tsyst = 50 K for the other. Each frontend channel received a single polarization mode, and this radiation was then further analysed in a 2 × 512 channel autocorrelation spectrometer set at a channel separation of 3.906 KHz corresponding to a velocity resolution of V = 0.824 km s−1. Hanning smoothed this resulted in a σT = 0.05 K for the average of both polarization.

This is a preliminary report on on-going observing programs in the LMC using CSIRO facilities. Molecular-line observations made with the 64-m Parkes telescope include OH at 18 cm (Whiteoak & Gardner 1976b; Caswell & Haynes 1981), H2CO at 6 cm (Whiteoak & Gardner 1976a) CH at 9 cm (Whiteoak et al. 1980) and H2O at 1.3 cm with beamwidths between 13′ and 1′.7 arc; observations of HCO+ at 3.4 mm (Batchelor et al. 1981) and CO at 2.6 mm have been made with the 4-m Epping telescope.

The dwarf Magellanic irregular galaxies apparently have a very low molecular content compared to the Milky Way. In the LMC, molecular clouds are fairly common, but the ratio of molecular to atomic gas is at least 5 times lower than in the Galaxy (Cohen et al. 1984). Elmegreen et al. (1980) searched for CO in 6 dwarf galaxies and failed to detect any emission even though their sensitivity was adequate to detect galactic giant molecular clouds placed at the distance of these galaxies. Israel (1984) observed the J=2→1 transition of CO at 15 points in the Small Magellanic Cloud and detected CO emission from five of them, but at a level two to six times lower than typical galactic values.

In our galaxy molecular clouds account for about half the total interstellar gas and are probably the sites of all star formation. The high gas content and widespread star formation in the Large Magellanic Cloud would therefore suggest a high molecular content. Very little however is actually known about molecules in the LMC. The most extensive previous survey (see Israel in this volume) found CO in half of the 22 points observed but covered less than 10−4 of the LMC area.

New IUE observations confirm the differences between the Galactic and LMC ultraviolet extinction curves and show some evidence for variations within the LMC. Visual and infrared photometry and polarimetry show that the anomalous properties of the LMC dust do not extend to longer wavelengths. Despite the much different dust to gas ratios in the Galaxy and the Magellanic clouds, the dust formation efficiency is similar when the abundances are included.

We have made an extensive study of the far-UV extinction in SMC stars using IUE spectra mostly obtained by us. The studied stars are Sk 7, 13, 18, 32, 76, 82, 85, 103, 119, 124, 142, 143, 159, 191 and AV 398. A few other stars have also been observed but are unsuitable for extinction studies. We have obtained at ESO new photometry and spectral classifications for all the studied stars. The far-UV extinction curves derived from IUE spectra of matched star pairs are all similar, except for Sk 143 (Lequeux et al., 1982), Sk 124 and 191. The mean curve, which supersedes that of Rocca-Volmerange et al. (1981) is plotted Fig. 1 with the label SMC “normal”, as well as the curves for the three “anomalous” stars (full lines) For comparison, we also plot the LMC “normal” curve and that for the anomalous star Sk-69°108 (dotted-dashed curves), the “normal” galactic curve and the curves for two stars in Scorpius-Ophiuchus.

Multiple high-dispersion IUE spectra of SK159 in the Small Magellanic Cloud (SMC) have been obtained and averaged in order to study interstellar gas associated with the SMC and, in particular, to search for evidence of SMC halo gas. A single high-dispersion IUE spectrum of SK159 was analyzed by Prévot et al. (1980). They reported the absence of C IV and only the marginal detection of Si IV at SMC velocities. The higher signal-to-noise ratio in our data allow a more definitive investigation of the absorption characteristics of these ions. SK159 is relatively cool, spectral type BO.5 Iaw (Walborn 1983) and is located in a region free of strong nebulosity. Thus the interstellar line spectrum will be less contaminated by H II region absorption than in previous studies of SMC stars. This favorable circumstance is somewhat offset by the great complexity of the underlying stellar spectrum and the resultant blending of stellar and interstellar lines.