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Revealing New Physical Structures in the Supernova Remnant N63A through Chandra Imaging Spectroscopy
We present Chandra X-ray observations of the supernova remnant (SNR)N63A in the Large Magellanic Cloud (LMC). N63A, one of the brightest LMCremnants, is embedded in an H II region and is probably associated withan OB association. The optical remnant consists of three lobes ofemission contained within the approximately 3 times larger X-rayremnant. Our Chandra data reveal a number of new physical structures inN63A. The most striking of these are the several ``crescent''-shapedstructures located beyond the main shell that resemble similar featuresseen in the Vela SNR. In Vela, these have been interpreted as arisingfrom high-speed clumps of supernova ejecta interacting with the ambientmedium. Another distinct feature of the remnant is a roughly triangular``hole'' in the X-ray emission near the location of the optical lobesand the brightest radio emission. X-ray spectral analysis shows thatthis deficit of emission is the result of absorption by an interveningdense cloud with a mass of ~450 Msolar that is currentlybeing engulfed by the remnant's blast wave. We also find that the rim ofthe remnant, as well as the crescent-shaped features, has considerablysofter X-ray spectra than the interior. Limits on hard X-ray emissionrule out a young, energetic pulsar in N63A, but the presence of an olderor less active one, powering a wind nebula with a luminosity less than~4×1034 ergs s-1, is allowed.

A statistical study of binary and multiple clusters in the LMC
Based on the Bica et al. (\cite{bica}) catalogue, we studied the starcluster system of the LMC and provide a new catalogue of all binary andmultiple cluster candidates found. As a selection criterion we used amaximum separation of 1farcm4 corresponding to 20 pc (assuming adistance modulus of 18.5 mag). We performed Monte Carlo simulations andproduced artificial cluster distributions that we compared with the realone in order to check how many of the found cluster pairs and groups canbe expected statistically due to chance superposition on the plane ofthe sky. We found that, depending on the cluster density, between 56%(bar region) and 12% (outer LMC) of the detected pairs can be explainedstatistically. We studied in detail the properties of the multiplecluster candidates. The binary cluster candidates seem to show atendency to form with components of similar size. When possible, westudied the age structure of the cluster groups and found that themultiple clusters are predominantly young with only a few cluster groupsolder than 300 Myr. The spatial distribution of the cluster pairs andgroups coincides with the distribution of clusters in general; however,old groups or groups with large internal age differences are mainlylocated in the densely populated bar region. Thus, they can easily beexplained as chance superpositions. Our findings show that a formationscenario through tidal capture is not only unlikely due to the lowprobability of close encounters of star clusters, and thus the evenlower probability of tidal capture, but the few groups with largeinternal age differences can easily be explained with projectioneffects. We favour a formation scenario as suggested by Fujimoto &Kumai (\cite{fk}) in which the components of a binary cluster formedtogether and thus should be coeval or have small age differencescompatible with cluster formation time scales. Table 6 is only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/547

A photoionization model of the compact H II region G29.96-0.02
We present a detailed photoionization model of G29.96-0.02 (hereafterG29.96), one of the brightest Galactic Ultra Compact H Ii (UCHII)regions in the Galaxy. This source has been observed extensively atradio and infrared wavelengths. The most recent data include a completeISO (SWS and LWS) spectrum, which displays a remarkable richness inatomic fine-structure lines. The number of observables is twice as greatas the number available in previous studies. In addition, most atomicspecies are now observed in two ionization stages. The radio andinfrared data on G29.96 are best reproduced using a nebular model withtwo density components: a diffuse (ne ~ 680 cm-3)extended (~ 1 pc) component surrounding a compact ( ~ 0.1 pc) dense(ne ~ 57 000 cm-3) core. The properties of theionizing star were derived using state-of-the-art stellar atmospheremodels. CoStar models yield an effective temperature of ~30+2-1 kK whereas more recent non-LTE lineblanketed atmospheres with stellar winds indicate somewhat highervalues, Teff ~ 32-38 kK. This range in Teff iscompatible with all observational constraints, including near-infraredphotometry and bolometric luminosity. The range 33-36 kK is alsocompatible with the spectral type O5-O8 determined by \citet{WH97} whenrecent downward revisions of the effective temperature scale of O starsare taken into account. The age of the ionizing star of G29.96 is foundto be a few 106 yr, much older than the expected lifetime ofUCHII regions. Accurate gas phase abundances are derived with the mostrobust results being Ne/S = 7.5 and N/O = 0.43 (1.3 and 3.5 times thesolar values, respectively). Based on observations with ISO, an ESAproject with instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the United Kingdom) andwith the participation of ISAS and NASA.

Calibrating Nebular Diagnostics of T[ scriptstyle star ]and Abundance
We obtained nebular spectroscopy of LMC H II regions having classifiedstellar populations, thereby strongly constraining the ionization inputparameters. Using photoionization models, we then evaluate theperformance of nebular diagnostics of T[ scriptstyle star ]andabundance. We introduce [Ne III]/H beta as a nebular diagnostic of theionizing stellar T[ scriptstyle star ]. In contrast to the widely-usedensuremath {eta ^prime } parameter, [Ne III]/H beta has greatersensitivity to mid and early O-stars, and is robust to nebularmorphology and the presence of shocks. We present a preliminarycalibration of both T[ scriptstyle star ]diagnostics for LMCmetallicity. We also introduce S[234] equiv ([S II] + [S III] + [SIV])/H beta as a diagnostic of S abundance. S[234] is much lesssensitive to the nebular ionization parameter than is S[23] or R[23].The intensity of [S IV]10.5 mu m is easily estimated from the opticaland near-IR line ratios. We present calibrations of S[23] and S[234]that are reliable at metallicities Z ≲ 0.5Z[ scriptstyle sun ].

Calibration of Nebular Emission-Line Diagnostics. II. Abundances
We examine standard methods of measuring nebular chemical abundances,including estimates based on direct Te measurements and alsoemission-line diagnostics. We use observations of the LMC H II regionsDEM L199, DEM L243, DEM L301, and DEM L323, the ionizing stars of whichhave classifications ranging from O7 to WN3. Following common practice,we assume a two-zone Te structure given by T(O++)and T(O+) to compute ionic abundances. We compare withphotoionization models tailored to the observed properties of theindividual objects, and we emphasize the importance of correctlyrelating Te in the two zones, which can otherwise causeerrors of ~0.2 dex in abundance estimates. The data show no spatialvariations or local metallicity enhancements to within 0.1-0.15 dex inany of the objects, notably including DEM L199, which hosts threeWolf-Rayet stars. Our data agree well with both the modeled R23 and S23abundance diagnostics for O and S. We present the first theoreticaltracks for S23, which are in excellent agreement with a larger availabledata set. However, contrary to earlier suggestions, S23 is much moresensitive to the ionization parameter (U) than is R23. This occursbecause S23 does not sample S IV, which is often a significantpopulation. We therefore introduce S234≡([S II]+[S III]+[SIV])/Hβ and demonstrate that it is virtually independent of U.Predicted and observed spatial variations in S234 are thus dramaticallydecreased in contrast to S23. The intensity of [S IV] 10.5 μm can beeasily estimated from the simple correspondence between [S IV]/[S III]and [O III]/[O II]. Using this method to estimate S234 for data in theliterature yields excellent agreement with our model tracks, hence wegive a theoretical calibration for S234. Our models show that thedouble-valued structure of S23 and S234 remains an important problem asfor R23, and, at present, we consider calibrations of these Sdiagnostics reliable only at Z<~0.5Zsolar. However, theslightly larger dynamic range and excellent compatibility withtheoretical predictions suggest the S parameters to be more effectiveabundance diagnostics than R23.

Calibration of Nebular Emission-Line Diagnostics. I. Stellar Effective Temperatures
We present a detailed comparison of optical H II region spectra tophotoionization models based on modern stellar atmosphere models. Weexamine both spatially resolved and integrated emission-line spectra ofthe H II regions DEM L323, DEM L243, DEM L199, and DEM L301 in the LargeMagellanic Cloud. The published spectral classifications of the dominantstars range from O7 to WN3, and morphologies range from Strömgrensphere to shell structure. Two of the objects include SNR contamination.The overall agreement with the predictions is generally within 0.2 dexfor major diagnostic line ratios. An apparent pattern in the remainingdiscrepancies is that the predicted electron temperature is ~1000 Khotter than observed. [Ne III] intensities are also slightlyoverpredicted, which may or may not be related. We model the shockemission for the SNR-contaminated objects and find excellent agreementwith the observations for composite shock and photoionized spectra. DEML301's emission apparently results from both shocks and density-boundedphotoionization. The existence of contaminating shocks can be difficultto ascertain in the spatially integrated spectra. Our analysis of thecomplex DEM L199 allows a nebular emission-line test of unprecedenteddetail for WR atmospheres. Surprisingly, we find no nebular He IIλ4686 emission, despite the fact that both of the dominant WN3stars should be hot enough to fully ionize He I in their atmospheres.The nebular diagnostics are again in excellent agreement with the data,for stellar models not producing He+-ionizing photons. Theoptical diagnostics are furthermore quite insensitive to the ionizingenergy distribution for these early WR stars. We confirm that the η'emission-line parameter is not as useful as hoped for determining theionizing stellar effective temperature, T*. Both empiricallyand theoretically, we find that it is insensitive forT*>~40 kK and that it also varies spatially. Theshock-contaminated objects show that η' will also yield a spuriouslyhigh T* in the presence of shocks. It is furthermoresensitive to shell morphology. We suggest [Ne III]/Hβ as anadditional probe of T*. Although it is abundance dependent,[Ne III]/Hβ has higher sensitivity to T*, is independentof morphology, and is insensitive to shocks in our objects. Theseobservations should be useful data points for a first empiricalcalibration of nebular diagnostics of T*, which we attemptfor LMC metallicity.

H I Shells in the Large Magellanic Cloud
A recent high-resolution H I survey of the Large Magellanic Cloud (LMC)shows that the structure of the neutral atomic interstellar gas isdominated by numerous shells and holes, as well as complex filamentaryand spiral-type structure. We present an up-to-date catalog of candidateH I supergiant and giant shells in the LMC. The candidates are visuallyselected from the H I data cube using selection and classificationcriteria that are described. Twenty-three supergiant shells, defined asthose regions whose extent is much larger than the H I scale height, arecataloged; 103 giant shells (radii less than the scale height of the H Igas) are cataloged. We further classify the H I shells into fivedifferent types, based on the comparison of the H I with theirassociated Hα emission. For this purpose, we obtained newwide-field Hα images of the LMC with a CCD camera mounted on a 16inch (0.41 m) telescope at Siding Spring Observatory. The pixel size of20" and the field of view of 12 deg are well matched to the H I survey.The size distribution of H I shells follows a crude power law,N(logR)~R-1.5. For constant energy input to the H I shellsand a constant shell creation rate, a shell luminosity spectrum of theform φ(L)~L-β, where β=1.75+/-0.2, isobtained. This agrees well with the observed H II region luminosityspectrum for the LMC of Kennicut, Edgar, & Hodge, which hasβ=1.75+/-0.15. H I shells containing H II regions and OBassociations seem to expand more rapidly than those without, providingdirect evidence for substantial input of mechanical energy from regionsof star formation.

The Supergiant Shell LMC 2. I. The Kinematics and Physical Structure
LMC 2 has the brightest, most coherent filamentary structure of allknown supergiant shells in the Large Magellanic Cloud. The opticalemission-line images show active star formation regions along thewestern edge and long filaments to the east. ROSAT PSPC and HRI imagesshow bright X-ray emission from within the shell boundary, indicatingthe presence of hot gas. Counterintuitively, neither high-resolutionechelle spectra in the Hα line nor aperture synthesis H I 21 cmemission-line observations show LMC 2 to have the kinematics expected ofan expanding shell. Rather, LMC 2 appears to consist of hot gas confinedbetween H I sheets. The interior surfaces of these sheets are ionized bythe UV flux of massive stars in the star formation regions along theperiphery of LMC 2, while the heating is provided by outflows of hot gasfrom the star formation regions and by SNRs interior to LMC 2. We havecompared LMC 2 to other supergiant shells in the LMC and in more distantgalaxies. When the spatial resolution of our data are degraded, we findthat LMC 2 resembles supergiant shells observed at a distance of 4 Mpcthat have previously been interpreted as expanding shells. Therefore,great caution should be exercised in the analysis and interpretation ofthe kinematics of distant supergiant shells to prevent overestimates oftheir velocities and total kinetic energies.

A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. II. The Large Magellanic Cloud
A survey of extended objects in the Large Magellanic Cloud was carriedout on the ESO/SERC R and J Sky Survey Atlases, checking entries inprevious catalogs and searching for new objects. The census provided6659 objects including star clusters, emission-free associations, andobjects related to emission nebulae. Each of these classes containsthree subclasses with intermediate properties, which are used to infertotal populations. The survey includes cross identifications amongcatalogs, and we present 3246 new objects. We provide accuratepositions, classification, and homogeneous measurements of sizes andposition angles, as well as information on cluster pairs andhierarchical relation for superimposed objects. This unification andenlargement of catalogs is important for future searches of fainter andsmaller new objects. We discuss the angular and size distributions ofthe objects of the different classes. The angular distributions show twooff-centered systems with different inclinations, suggesting that theLMC disk is warped. The present catalog together with its previouscounterpart for the SMC and the inter-Cloud region provide a totalpopulation of 7847 extended objects in the Magellanic System. Theangular distribution of the ensemble reveals important clues on theinteraction between the LMC and SMC.

N63A: A Supernova Remnant in a Cloudy Medium
N63A is a supernova remnant (SNR) associated with the OB associationLH83 in the Large Magellanic Cloud. The progenitor of the supernovaprobably had a mass >= 30 Mo, the inferred mass of the most luminousstar in LH83 (van den Bergh & Dufour 1980, PASP, 92, 32). Theoptical image of N63A shows a three-lobed, clover-shaped nebulosityabout 20" (~ 5 pc) across. The two eastern lobes exhibit a high [S II]/Halpha ratio, indicating a shock excitation, while the western lobe showsspectra characteristic of photoionization. It has been puzzling that theoptical size of the SNR N63A is much smaller than ~ 70", the extent ofradio and X-ray emission. Our recent HST WFPC2 images of N63A resolvedthe filamentary structure in the two eastern lobes and diffuse emissionin the western lobe, consistent with the excitation mechanisms diagnosedfrom their spectral properties. Additionally, the WFPC2 images reveal anumber of cloudlets as small as 0.1 pc across, within the X-ray-emittingregions of the SNR. The [S II]/H alpha ratio and location of thesecloudlets suggest that these are shocked cloudlets lagging behind theshock front. The morphology of the cloudlets indicates that they areevaporating isotropically; therefore, the magnetic field is either tooweak to significantly prohibit evaporation across the field lines, ornot organized enough to produce a noticeable directional dependence inthe evaporation. The evaporating cloudlets are probably responsible forinjecting mass into the hot SNR interior to produce the high X-raysurface brightness. Thus N63A provides a clear example of a SNR evolvingin a cloudy medium, such as investigated by White and Long (1991, ApJ,373, 543). This is also the first clear evidence of engulfed cloudletswithin a SNR.

ASCA X-Ray Spectroscopy of Large Magellanic Cloud Supernova Remnants and the Metal Abundances of the Large Magellanic Cloud
We present the results of X-ray spectroscopy of a flux-limited sample ofseven middle-aged supernova remnants (SNRs) in the Large MagellanicCloud (LMC): N23, N49, N63A, DEM 71, N132D, 0453-68.5, and N49B. Weconstructed self-consistent nonequilibrium ionization SNR modelsassuming a Sedov solution for the dynamical evolution, and then appliedthe resulting spectral models to the data obtained by the Solid StateImaging Spectrometer on board the Advanced Satellite for Cosmology andAstrophysics. All the remnants were reasonably well described by themodel, which allowed us to derive accurate values for their physicalparameters, i.e., ages, densities, initial explosion energies, and metalabundances. The derived explosion energies vary from 5 x 10^50 to 6 x10^51 ergs. A restricted subset of the sample exists for which theionization and Sedov dynamical ages agree quite well under theassumption that the electron and ion temperatures are not fullyequilibrated at the shock front; for these four SNRs, the mean value ofthe initial explosion energy is (1.1 +/- 0.5) x 10^51 ergs. We show thatit is likely that the other three remnants, all of which have inferredexplosion energies >~3 x 10^51 ergs, exploded within preexistingcavities in the interstellar medium. The limits on high-energy X-rayemission (>~3 keV) that we present indicate that these SNRs do notcontain very luminous pulsar-powered synchrotron nebulae, in generalagreement with our picture of them as evolved, middle-aged remnants. Wefind statistical evidence for enrichment by supernova ejecta in thesense that smaller remnants show a somewhat higher mean metallicity thanthe larger ones. In the case of DEM 71, the putative remnant of a TypeIa supernova, the derived abundance of iron is about a factor of 2larger than the other remnants in the sample. These things being said,however, the derived abundances are in general dominated by swept-upinterstellar material, and so we use the SNR sample to estimate the meanLMC gas-phase abundances. We find that the astrophysically commonelements from oxygen to iron are less abundant than the solar values byfactors of 2-4. Overall, these results are consistent with previous onesbased on optical and UV data but do not show the anomalous overabundanceof magnesium and silicon seen by others.

LMC HII region luminosities versus observed ionizing stars
We use the stellar census of OB associations in the Large MagellanicCloud (LMC) to predict the H-alpha luminosities of the host HII regions,based on results from stellar atmosphere models. These values arecompared to the observed HII region luminosities, yielding an estimatefor the mean fraction of H-ionizing photons that escape the localnebulae in this sample. We formally estimate that, overall, 0% to 51% ofthe ionizing radiation escapes the local HII regions and is available toionize the warm, ionized medium in the LMC. We find both nebulae thatappear to be density-bounded, and ones that appear to beradiation-bounded.

Comparison of H II region luminosities with observed stellar ionizing sources in the Large Magellanic Cloud
We estimate the total predicted Lyman continuum emission rates of OBassociations for which the complete census of O star spectral typesexists. The results are compared to the observed H-alpha luminosities ofthe host H II regions. We find evidence for substantial leakage ofionizing photons from some H II regions, while others appear to beradiation-bounded. We estimate that overall for the LMC, 0-51 percent ofthe ionizing radiation escapes the local nebulae, and would be availableto ionize the diffuse, warm, ionized medium (WIM) in that galaxy. Thisrange of values is consistent with the observed 35 percent fraction ofH-alpha luminosity emitted by the WIM in the LMC, as well as thecorresponding fractions observed in other nearby galaxies. It istherefore possible that photoionization by O stars is indeed thedominant ionization mechanism for the WIM.

Supernova Remnants in OB Associations
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....113.1815C

High velocity motions inside the HII region N 103 of the Large Magellanic Cloud.
We have observed the HII region N 103 of the Large Magellanic Cloud witha scanning Fabry-Perot interferometer at Hα and [OIII]5007wavelengths. The kinematics of this field shows high velocity motions.We discuss their origin: Supernova explosion or particularly strongstellar winds. By calculating the energy input inside the gas, we showthat it is unlikely that the high velocity motions are due to thestellar winds of the embbeded stars. Then the nebula N 103 is linked totwo supernova remnants of different ages. The oldest one can berepresented by a bubble, 152pc wide, seen projected against the HIIregion, and probably lying at the edge of the HII region. The excitingstars of the nebula are actually members of the LMC cluster NGC 1850B;they provide a photon flux large enough to ionize the quiet part of thegas.

Stellar Content of Superbubble H II Regions in the Large Magellanic Cloud
I examine the stellar population enclosed within a sample of 6 LMCsuperbubbles and compare these clusters with previously studied OBassociations in classical H II regions. The H-R diagrams, constructedwith spectral classifications of the most massive stars, do not revealany systematic differences between OB associations resident withinsuperbubbles and classical nebulae: the main-sequence turnoffs showstars as massive and luminous as those in classical H II regions.Assuming the superbubble structures result from the stellar winds and/orsupernovae of the associations, the similarity of the stellarpopulations to those of classical H II regions implies that the shellformation timescale is somewhat shorter than the cluster evolutionarytimescale for these objects. The stellar winds and/or supernovae of theone or two most massive stars must therefore dominate the formation ofthe superbubbles. The star-forming events for the superbubbleassociations are also no more extended in duration than those of otherOB associations. Finally, the initial mass function slopes are notsystematically different from those previously found. Since the OBassociations within superbubbles appear normal, the shell structuresmust be the result of normal OB stellar influences. I also present a fewspectrograms of interesting massive stars, including S149, a probablenew B[e] supergiant.

UBV Photometry of OB Associations within Superbubbles of the Large Magellanic Cloud
This work presents UBV photometry of the stellar populations associatedwith seven superbubble nebulae and five classical H II regions in theLarge Magellanic Cloud. Although the nebular morphology of thesuperbubbles appears to be substantially evolved compared to theclassical nebulae, the color-magnitude diagrams do not reveal anynoticeable correlation between the resident stellar population andnebular morphology. The photometry presented here will be used in aforthcoming paper to examine further the stellar content and dynamics ofthese superbubbles.

Integrated UBV Photometry of 624 Star Clusters and Associations in the Large Magellanic Cloud
We present a catalog of integrated UBV photometry of 504 star clustersand 120 stellar associations in the LMC, part of them still embedded inemitting gas. We study age groups in terms of equivalent SWB typesderived from the (U-B) X (B-V) diagram. The size of the spatialdistributions increases steadily with age (SWB types), whereas adifference of axial ratio exists between the groups younger than 30 Myrand those older, which implies a nearly face-on orientation for theformer and a tilt of ~45^deg^ for the latter groups. Asymmetries arepresent in the spatial distributions, which, together with thenoncoincidence of the centroids for different age groups, suggest thatthe LMC disk was severely perturbed in the past.

Blue-violet spectral evolution of young Magellanic Cloud clusters
We study the integrated spectral evolution in the blue-violet range of97 blue star clusters in the Magellanic Clouds, from those associatedwith gas emission to those as old as a few hundred Myr. Some clustersare dominated by the flux of those massive stars that pass throughevolutionary stages such as Wolf-Rayet, Luminous Blue Variable, Be, andsupergiant stars of different temperatures. The relationships amongspectral features such as absorption and emission lines, Balmerdiscontinuity and Balmer continuum are used to study the spectralevolution of the clusters. Finally, we sort into groups spectra ofsimilar evolutionary stages, creating a template spectral library withpossible applications in stellar populations syntheses of star-forminggalaxies and in the spectral simulation of bursts of star formation withdifferent mean ages and durations.

On the Propinquity of Shock-Excited and Photoionized Plasma:The Supernova Remnant and the H II Region of N63A
We present spectra covering the range λ3400-λ10100 ofnebula N63A in the Large Magellanic Cloud. This object contains asupernova remnant adjacent to an H II region and thus illustratesvividly the optical and near-infrared differences between shock-ionizedand photoionized emission nebulae. Species of low ionization potential,such as [S II] and [C I], predominate and emit strongly in theshock-heated region, while these elements are more highly ionized in theH II region. The gas-phase abundance of elements also affects theiremission line strengths, and this depends on the particular excitationmechanism. A supernova shock can liberate calcium that is bound tointerstellar grains, while photoionization does not reduce gas-phasedepletion. In addition to providing a laboratory for comparison of theeffects of shocks and photons on the interstellar medium, N63A is abright object, which enables the measurement of lines that are typicallytoo weak to observe elsewhere. In the shocked region we identify fivelines of the 1F multiplet of Cr II and measure lines of [Cl II] and [CI].

Ultraviolet spectral evolution of star clusters in the IUE library.
The ultraviolet integrated spectra of star clusters and H II regions inthe IUE library have been classified into groups based on their spectralappearance, as well as on age and metallicity information from otherstudies. We have coadded the spectra in these groups according to theirS/N ratio, creating a library of template spectra for futureapplications in population syntheses in galaxies. We define spectralwindows for equivalent width measurements and for continuum tracings.These measurements in the spectra of the templates are studied as afunction of age and metallicity. We indicate the windows with a strongmetallicity dependence, at different age stages.

A radio continuum study of the Magellanic Clouds. IV. Catalogues of radio sources in the Large Magellanic Cloud at 1.40, 2.45, 4.75, 4.85 and 8.55 GHz.
From observations with the Parkes radio telescope, we present cataloguesof radio sources in the Large Magellanic Cloud at four frequencies:1.40, 2.45, 4.75 and 8.55GHz, and an additional catalogue from a sourceanalysis of the Parkes-MIT-NRAO survey at 4.85GHz. A total of 469sources have been detected at least one of these frequencies, 132 ofwhich are reported here for the first time as radio sources.

Classical H II regions in the Magellanic Clouds. 2: Stellar content
In this second in a series of papers on the nature of classical H IIregions in the Magellanic Clouds I investigate the properties of theunderlying stellar content of the nebulae. Particular emphasis is placedon identifying and classifying the ionizing source(s) for each H IIregion. With the exception of the LMC H II regions DEM 20 and DEM 8c, Ifind that all of the objects in this sample are ionized by more than oneO or B star. Even the faintest H II regions reflect the formation of ahandful of massive, albeit early B type, stars. Typically, one staraccounts for 60% - 70% of the ionizing photons and 2 - 5 less massivestars provide the remaining 30% - 40%. From the statistics of thehottest stars in these H II regions, and from considering all the bluestars contained within each region, the distribution of massive starswith spectral type is consistent with results found in similar galacticH II regions.

The initial mass function for massive stars in the Magellanic Clouds. 1: UBV photometry and color-magnitude diagrams for 14 OB associations
UBV charge coupled device (CCD) photometry has been obtained for 14 OBassociations in the Magellanic Clouds using the University of Toronto's0.6 m telescope and the Carnegie Institution of Washington's 1.0 mreflector, both on Las Campanas, Chile. The data are presented and usedto construct color-magnitude diagrams for the purposes of investigatingthe massive-star content of the associations.

Classical H II regions in the Magellanic Clouds. 1: Physical properties
In the first in a series of papers I present a detailed examination ofthe basic physical properties of a sample of classical H II regions inthe Magellanic Clouds. Narrow band H alpha imaging is used to show thatsuch objects are representative of the most common H II regions in theClouds and that they display a wide range of morphologicalcharacteristics. Spectroscopic data show that, as a result of the lowmetal content, classical H II regions in the Clouds are systematicallyhotter than their Galactic counterparts. The data also suggest theexistence of a correlation between the excitation of an H II region andits emission measure. The spectra of one object, DEM 243, indicates thepresence of a low velocity shock passing through this H II region. Ialso discuss the characteristics of the extinction towards these objectsand speculate that it arises from dust local to the H II regionsthemselves.

Supernova remnants in plane-stratified media - Predictions for H-alpha-emitting regions
Numerical models for the time-dependent evolution of adiabatic SNRs inplane-stratified media are presented. These models approximate SNRevolution in the density distributions found near the edges of densemolecular clouds or diffuse stellar wind bubbles. Use of a simple T exp-1/2 cooling law enables cooling surfaces within the remnants to beidentified and their H-alpha emission estimated. Projection of thisemission onto the plane of the sky at a variety of angles ofinclination, together with a consideration of the kinematics, allowscomparisons with observed remnants to be made. It is found that thephenomenon of fast H-alpha emission, as observed in the Cygnus Loop andN49 and N63 in the LMC, can be easily explained as a consequence of SNRevolution in regions having high density contrasts.

X-rays from superbubbles in the Large Magellanic Cloud
Diffuse X-ray emission not associated with known supernova remnants(SNRs) are found in seven Large Magellanic Cloud H II complexesencompassing 10 OB associations: N44, N51D, N57A, N70, N154, N157 (30Dor), and N158. Their X-ray luminosities range from 7 x 10 to the 34thergs/s in N57A to 7 x 10 to the 36th ergs/s in 30 Dor. All, except 30Dor, have simple ring morphologies, indicating shell structures.Modeling these as superbubbles, it is found that the X-ray luminositiesexpected from their hot interiors fall an order of magnitude below theobserved values. SNRs close to the center of a superbubble add verylittle emission, but it is calculated that off-center SNRs hitting theionized shell could explain the observed emission.

Dust in emission nebulae of the LMC derived from photometric reddening of stars
VBLUW photometric observations of stars in emission nebulae of the LMCare reported. The luminosities and extinctions of the stars are derived.Agreement is found between the average photometric extinctions of thenebulae and the extinctions derived from the Balmer line decrementmeasured by Caplan and Deharveng (1985 and 1986). The photometricextinctions are shown in the CO map of the LMC (Cohen et al., 1988).

A Morphological Criterion for Distinguishing Between Supernova Remnants and HII Regions
A variant of the vector discriminant pattern recognition technique isapplied to UK Schmidt telescope images of nebulae in the LargeMagellanic Cloud. A criterion is thereby obtained which may be appliedgenerally to distinguish between supernova remnants (SNRs) and H IIregions from their optical images. Furthermore, two nebulae previouslyidentified as H II regions are now interpreted as SNRs.

The cluster system of the Large Magellanic Cloud
A new catalog of clusters in the Large Magellanic Cloud has beenconstructed from searches of the IIIa-J component of the ESO/SERCSouthern Sky Atlas. The catalog contains coordinate and diametermeasurements of 1762 clusters in a 25 deg x 25 deg area of sky centeredon the LMC, but excluding the very crowded 3.5 sq deg region around theBar. The distribution of these clusters appears as two superimposedelliptical systems. The higher density inner system extends over about 8deg; the lower density outer system can be represented by a 13 deg x 10deg disk inclined at 42 deg to the line of sight. There are suggestionsof two weak 'arms' in the latter.

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Right ascension:05h35m31.10s
Apparent magnitude:99.9

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