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OB stellar associations in the Large Magellanic Cloud: Survey of young stellar systems
The method developed by Gouliermis et al. (\cite{Gouliermis00}, PaperI), for the detection and classification of stellar systems in the LMC,was used for the identification of stellar associations and openclusters in the central area of the LMC. This method was applied on thestellar catalog produced from a scanned 1.2 m UK Schmidt Telescope Platein U with a field of view almost 6\fdg5 x 6\fdg5, centered on the Bar ofthis galaxy. The survey of the identified systems is presented herefollowed by the results of the investigation on their spatialdistribution and their structural parameters, as were estimatedaccording to our proposed methodology in Paper I. The detected openclusters and stellar associations show to form large filamentarystructures, which are often connected with the loci of HI shells. Thederived mean size of the stellar associations in this survey was foundto agree with the average size found previously by other authors, forstellar associations in different galaxies. This common size of about 80pc might represent a universal scale for the star formation process,whereas the parameter correlations of the detected loose systems supportthe distinction between open clusters and stellar associations.
| 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(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/547
| The Effects of Dust in Simple Environments: Large Magellanic Cloud H II Regions
We investigate the effects of dust on Large Magellanic Cloud (LMC)H II region spectral energy distributions usingarcminute-resolution far-ultraviolet (FUV), Hα, far-infrared(FIR), and radio images. Widely used indicators of the amount of lightlost to dust (attenuation) at Hα and in the FUV correlate witheach other, although often with substantial scatter. There are twointeresting systematic discrepancies: First, Hα attenuationsestimated from the Balmer decrement are lower than those estimated fromthe Hα-to-thermal radio luminosity ratio. Our data, at this stage,cannot unambiguously identify the source of this discrepancy. Second,the attenuation at 1500 Å and the UV spectral slope, β,correlate, although the slope and scatter are substantially differentfrom the correlation first derived for starbursting galaxies by Calzettiet al. Combining our result with those of Meurer et al. forultraluminous infrared galaxies and Calzetti et al. for starburstinggalaxies, we conclude that no single relation between β and 1500Å attenuation is applicable to all star-forming systems.
| The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. I. Results from 19 OB Associations in the Magellanic Clouds
We combine new CCD UBV photometry and spectroscopy with those from theliterature to investigate 19 Magellanic Cloud OB associations thatcontain Wolf-Rayet (W-R) and other types of evolved, massive stars. Ourspectroscopy reveals a wealth of newly identified interesting objects,including early O-type supergiants, a high-mass, double-lined binary inthe SMC, and, in the LMC, a newly confirmed luminous blue variable (LBV;R85), a newly discovered W-R star (Sk -69°194), and a newly foundluminous B[e] star (LH 85-10). We use these data to provide precisereddening determinations and construct physical H-R diagrams for theassociations. We find that about half of the associations may be highlycoeval, with the massive stars having formed over a short period(Δτ<1 Myr). The (initial) masses of the highest massunevolved stars in the coeval clusters may be used to estimate themasses of the progenitors of W-R and other evolved stars found in theseclusters. Similarly, the bolometric luminosities of the highest massunevolved stars can be used to determine the bolometric corrections(BCs) for the evolved stars, providing a valuable observational basisfor evaluating recent models of these complicated atmospheres. What wefind is the following: (1) Although their numbers is small, it appearsthat the W-R stars in the SMC come from only the highest mass (greaterthan 70 Msolar) stars. This is in accord with ourexpectations that at low metallicities only the most massive andluminous stars will have sufficient mass loss to become W-R stars. (2)In the LMC, the early-type WN (WNE) stars occur in clusters whoseturnoff masses range from 30 to 100 Msolar or more. Thissuggests that possibly all stars with mass greater than 30Msolar pass through a WNE stage at LMC metallicities. (3) Theone WC star in the SMC is found in a cluster with a turnoff mass of 70Msolar, the same as that for the SMC WN stars. In the LMC,the WC stars are found in clusters with turnoff masses of 45Msolar or higher, similar to what is found for the LMC WNstars. Thus we conclude that WC stars come from essentially the samemass range as do WN stars and indeed are often found in the sameclusters. This has important implications for interpreting therelationship between metallicity and the WC/WN ratio found in LocalGroup galaxies, which we discuss. (4) The LBVs in our sample come fromvery high mass stars (greater than 85 Msolar), similar towhat is known for the Galactic LBV η Car, suggesting that only themost massive stars go through an LBV phase. Recently, Ofpe/WN9 starshave been implicated as LBVs after one such star underwent an LBV-likeoutburst. However, our study includes two Ofpe/WN9 stars, BE 381 and Br18, which we find in clusters with much lower turnoff masses (25-35Msolar). We suggest that Ofpe/WN9 stars are unrelated to``true'' LBVs: not all ``LBV-like outbursts'' may have the same cause.Similarly, the B[e] stars have sometimes been described as LBV-like.Yet, the two stars in our sample appear to come from a large mass range(30-60 Msolar). This is consistent with other studies,suggesting that B[e] stars cover a large range in bolometricluminosities. (5) The bolometric corrections of early WN and WC starsare found to be extreme, with an average BC(WNE) of -6.0 mag and anaverage BC(WC4) of -5.5 mag. These values are considerably more negativethan those of even the hottest O-type stars. However, similar valueshave been found for WNE stars by applying Hillier's ``standard model''for W-R atmospheres. We find more modest BCs for the Ofpe/WN9 stars(BC=-2 to -4 mag), also consistent with recent analysis done with thestandard model. Extension of these studies to the Galactic clusters willprovide insight into how massive stars evolve at differentmetallicities.
| The fourth catalogue of Population I Wolf-Rayet stars in the Large Magellanic Cloud
The catalogue provides for each of the 134 W-R stars of Population Ipresently known in the Large Magellanic Cloud, accurate equatorialcoordinates, photometric data, spectral classification, binary status,correlation with OB associations and HII regions. The miscellaneousdesignations of the stars are also listed. Although completeness is notpretended, results published during the last decade are highlighted inthe notes given for each individual star. A uniform set of findingcharts is presented. Figures 2 to 12 only in the electronic version athttp://edpsciences.com
| Physical Structure of Small Wolf-Rayet Ring Nebulae
We have selected the seven most well-defined Wolf-Rayet (WR) ringnebulae in the Large Magellanic Cloud (LMC), Br 2, 10, 13, 40a, 48, 52,and 100, to study their physical nature and evolutionary stages. New CCDimaging and echelle observations have been obtained for five of thesenebulae; previous photographic imaging and echelle observations areavailable for the remaining two nebulae. Using the nebular dynamics andabundances, we find that the Br 13 nebula is a circumstellar bubble, andthat the Br 2 nebula may represent a circumstellar bubble merging with afossil main-sequence interstellar bubble. The nebulae around Br 10, 52,and 100 all show influence of the ambient interstellar medium. Theirregular expansion patterns suggest that they still contain significantamounts of circumstellar material. Their nebular abundances would beextremely interesting, as their central stars are WC5 and WN3-WN4 starswhose nebular abundances have not been derived previously. Intriguingand tantalizing implications are obtained from comparisons of the LMC WRring nebulae with ring nebulae around Galactic WR stars, Galactic LBVs,LMC LBVs, and LMC BSGs; however, these implications may be limited bysmall-number statistics. A SNR candidate close to Br 2 is diagnosed byits large expansion velocity and nonthermal radio emission. There is noindication that Br 2's ring nebula interacts dynamically with this SNRcandidate.
| 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.
| Young clusters in the Magellanics Clouds.
Not Available
| Extinction of H II regions in the Large Magellanic Cloud
The extinction properties of H II regions in the Large Magellanic Cloudare investigated using radio continuum data obtained from the MolongloObservatory Synthesis Telescope, digitized and calibrated H-alpha data,and published Balmer decrement measurements. The resultingextinction-color excess diagram suggests that (1) most H II regions inthe Magellanic Clouds have similar extinction properties to the Galacticones, (2) all imaginable gas/dust configurations are possible, and (3)the extinction of some highly reddened H II region cores originatesexternally in cocoon shells. The puzzle of different extinction-colorexcess ratios of Galactic and extragalactic H II regions is explained asbeing due to the different populations of observed samples rather thanany intrinsic differences. The extinction of the observed Galactic H IIregions produced by foreground dust overwhelms the internal extinction,while the situation in the observed extragalactic H II regions is justthe opposite.
| Evolutionary status and age spread in the young LMC cluster NGC 1850A
IUE spectra have been obtained for 15 members of the very young LMCcluster NGC 1850A. The extinction law toward the cluster is similar tothe one generally accepted for the 30 Dor region. We derive theeffective temperatures and bolometric luminosities to estimate the ageand mass for each star. An age spread of 2 to 10 Myr is estimated,suggesting a more or less continuous star formation process. The clustercontains at least four objects with mass >= 55 M_sun, still close tothe main sequence. The age of the main cluster NGC 1850 is estimated tobe about 50 Myr. Based on observations collected by the InternationalUltraviolet Explorer (IUE) at the Villafranca Satellite Tracking Stationof the European Space Agency.
| LH 72, the Center of Constellation III
Ultraviolet spectroscopy with IUE is combined with optical photometry tostudy the brightest blue members of the LMC OB association LH 72. LH 72lies near the geometric center of Shapley's Constellation III, a complexof gas and stars that makes up one of the largest known superbubbles.Ages for the stars range from 5 x 106 to 1.5 x 107 yr, with agesincreasing from the south to the north end of the association. Theyoungest stars are associated with the brightest H II emission and havethe largest reddening. This pattern of apparent sequential starformation is contrary to the expanding model proposed by Dopita,Mathewson, & Ford, in which the entire superbubble, more than anorder of magnitude larger in size than LH 72, evolves in about the sameamount of time. Our data, however, are consistent with an alternativemodel (Reid, Mould, & Thompson), in which star formation proceedslocally after the initial trigger from the superbubble shock.
| IUE Observations of Young Clusters in the Large Magellanic Cloud
Abstract image available at:http://adsabs.harvard.edu/abs/1996ApJ...458..580W
| 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.
| Extinction characteristics of giant HII regions - Star-forming complexes in the galaxies M33, LMC, and NGC 2403
The discrepancies between the extinction of gas emission and that of thestarlight in giant HII regions, star-forming complexes in the galaxiesM33, LMC, and NGC 2403, were empirically investigated. The extinctionvalues were determined for 30 stars in eight associations in M33. Anempirical relation between the extinction of starlight and that of thegas emission in giant HII regions, star-forming complexes in thegalaxies under study, was obtained.
| 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.
| Ultraviolet interstellar absorption lines in the LMC: Searching for hidden SNRs
Strong x-ray emission detected in Large Magellanic Cloud (LMC)superbubbles has been explained as the result of interior supernovaremnants (SNRs) hitting the dense superbubble shell. Such SNRs cannot befound using conventional criteria. We thus investigate the possibilityof using the interstellar absorption properties in the ultraviolet (UV)as a diagnostic of hidden SNR shocks. The International UltravioletExplorer (IUE) archives provide the database for this pilot study. Theycontain high-dispersion spectra of several stars in x-ray brightsuperbubbles. To distinguish the effects of SNR shocks from those oflocal stellar winds and a global hot halo around the LMC, we includedcontrol objects in different environments. We find that almost allinterstellar absorption properties can be explained by the interstellarenvironment associated with the objects. Summarizing the two mostimportant results of this study: (1) a large velocity shift between thehigh-ionization species (C IV and Si IV) and the low-ionization species(S II, Si II, and C II*) is a diagnostic of hidden SNR shocks; however,the absence of a velocity shift does not preclude the existence of SNRshocks; (2) there is no evidence that the LMC is uniformly surrounded byhot gas; hot gas is preferentially found associated with largeinterstellar structures like superbubbles and supergiant shells, whichmay extend to large distances from the plane.
| A Search for Methanol Masers in the Magellanic Clouds
We report the discovery of a second methanol maser in the LargeMagellanic Cloud and we present the results of synthesis observations ofthis and the methanol maser detected previously. The second discoverywas made using the Australia Telescope National Facility's 64-m Parkesradio telescope during an extensive maser search for 6.6-GHz maseremission from the 5_1_-6_0_ A^+^ transition in both Magellanic Clouds.Spectra were obtained towards 35 HII regions in the Large MagellanicCloud and 13 regions in the Small Magellanic Cloud, and also on a3-arcmin grid over an area 0.3^deg^ square, south of the 30 Doradusnebula. Parkes observations at 12.2 GHz towards the two maser sitesyielded no detectable emission from the 2_0_-3_-1_, E methanoltransition. The results suggest that methanol masers are less abundantin the Magellanic Clouds than in our Galaxy. Observations of the twomasers with the Australia Telescope Compact Array showed one to belocated near the continuum emission peak of the H II region MC18 (N11),while the other appeared to be centred near OH emission on thesouth-eastern boundary of MC23 (N105a).
| An atlas of the interstellar environment of Wolf-Rayet stars in the Magellanic clouds
We have made a complete study of the interstellar environment around theWolf-Rayet stars in the Magellanic Clouds. We present, in the form of anatlas, the results of a complete imaging survey in Hα and of anextensive survey in the (O III) alpha 5007 emission line. As a result ofthis survey, we have more than doubled the total number of ring nebulaeknown. These include cases of both rings of stellar ejecta and ringnebulae resulting from the sweeping up of the surrounding interstellarmedium. We find that 34% of WN3-WN4 stars, 36% of late WN types, and 26%of WC4-WC5 stars are associated with a ring nebula of some kind. Thesefigures are very similar to the percentage of Wolf-Rayet stars havingring nebulae in the solar neighborhood. The size distribution of ringnebulae is also similar. From the fact that the majority of Wolf-Rayetstars do not show ring nebulae, it is clear that mass loss in earlierphases of evolution, and the collective effects of the energy input fromthe clusters of OB stars with which the Wolf-Rayet stars are frequentlyassociated, has profoundly modified the preexisting circumstellarenvironment. However, on the basis of statistics, we cannot exclude thepossibility that all Wolf-Rayet stars have possessed a ring nebula atsome stage in their evolution.
| Kinematic structure of the 30 Doradus giant H II region
We have used the echelle CCD spectrograph on the Cerro TololoInter-American Observatory (CTIO) 4 m telescope to map the nebularvelocity field in the 30 Doradus giant H II region. The kinematics of 30Dor are very complex. The outer regions are charaterized by a smoothvelocity field, but its turbulent velocity, 30-40 km/s Full Width HalfMaximum (FWHM), is considerably higher than those in most smaller H IIregions. In the central 9 min core, multiple velocity components areobserved at most positions. The velocity field is dominated by a largenumber of expanding structures, ranging in size from 1 to 100 pc andexpansion velocities of 20-200 km/s, and often organized into largehierarchical networks. The integral of these complex expandingstructures in 30 Dor produces a surprisingly simple profile with a broadGaussian core and faint extended wings. Several fast-expanding shells,with diameters of 2-20 pc, expansion velocities of 100-300 km/s, andkinetic energies of 0.5-10 x 1050 ergs have been identified.The large fast-expanding shells and networks are coincident withextended X-ray sources and are probably associated with supernovaremnants embedded in supershells produced by the combined effects ofstellar winds and supernovae from OB associations. We have used theintensity-calibrated echelle spectra to determine the basic physical anddynamical properties of the kinematic features in 30 Dor. The expandingshells contain roughly half of the kinetic energy in the 30 Dor complex,and this energy is several times higher than the gravitational bindingenergy of the region. The energetic requirements of the gas areconsistent with the observed stellar content of 30 Dor, if the gas isaccelerated by a combination of stellar winds and supernovae.Extrapolating the current energy injection rate in the nebula over thelifetime of the OB complex suggest that 30 Dor and its vicinity willevolve into a supergiant shell as seen in the LMC and other nearbygalaxies.
| Results of the ESO / SEST Key Programme on Co/ in the Magellanic Clouds - Part One - a Survey of Co/ in the Large Magellanic Cloud and the Small Magellanic Cloud
As the first part of the ESO-Swedish SEST Key Programme on CO in theMagellanic Clouds, we have observed ^12^CO J = 1-0 towards 92 positionsin the LMC and 42 positions in the SMC. In the SMC we searched foremission from H II regions, dark clouds and IRAS infrared sources. Thegenerally negative detection rate of non-IRAS sources in the SMC led toan LMC source selection based on the IRAS results. In both galaxies, COwas detected towards the majority of sources observed. We also observed^13^C0 J = 1-0 towards the brighter ^12^CO sources in the LMC (37) andSMC (9). Compared to the strength of CO lines observed in the Milky WayGalaxy with identical linear resolutions, velocity-integrated COemission is weaker by at least a factor of three in the LMC sources andan order of magnitude in the SMC sources. The mean velocity-integratedisotopic intensity ratio I_12_/I_13_ is 12.5 in the LMC and about 15 inthe SMC. Individual ratios range from 8.5 to 20. These isotopicintensity ratios are two to three times higher than those found inGalactic molecular clouds.
| A comparison of far infrared and H-alpha emission of H-II regions in the Magellanic Clouds
From a comparison of the IRAS and smoothed H-alpha maps of theMagellanic Clouds, it was found that H-II regions with core-halostructure usually have higher F(60 microns)/F(H-alpha) ratios andprobably emit more in the far infrared than do extended low-density H-IIregions. This is consistent with the idea that the far infrared emissionis mainly produced by dust within H-II regions.
| A Comparison of Far Infrared and Hα Emission of HII Regions in the Magellanic Clouds
Not Available
| 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.
| 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.
| Uniformity of the WC stars in the Large Magellanic Cloud
Results on the uniformity of the WC stars in the LMC are presented. Itis shown to be a tenable hypothesis that (b-v)sub 0 is the same for allobserved stars, thus enabling estimates of absorption in the 30 Dorregion where local irregularities are large. The flux in the C IV lambda5808 line is 3 x 10 to the 36th ergs/sec and is constant withinobservational errors for all but one of the 10 WC4 stars with availableflux data. The flux ratio (lamda 5808/lamda 4650) = - 0.22 + or - 0.1dex and can be used to determine reddening. The width of the lambda 5808line is less than that of the lambda 4650 line in WC4 stars and greaterin the WO stars. The use of Wolf-Rayet star emission-line fluxes forreddening and distance determination has a great advantage over othertechniques: the broad lines stand out clearly even at low spectralresolution.
| A new radio continuum survey of the Magellanic Clouds at 1.4 GHz. II - The radio morphology, and thermal and nonthermal emission of the LMC
The distribution of the radio continuum emission of the LMC is examined.The distribution of the spectral index and a reliable spectrum of theradio emission of the LMC are derived using radio continuum surveys ofthe LMC at 408, 1400, and 2300 MHz. The distribution of the spectralindex across the LMC shows a close positional agreement between flatspectrum and H-alpha emitting regions. The total nonthermal radioluminosity is derived, giving a total equipartition magnetic fieldstrength of about 6 microG. Accurate thermal radio flux densities of themost prominent LMC H II complexes are determined.
| H II regions and star formation in the Magellanic Clouds
Photoelectrically calibrated maps of the H-alpha emission in theMagellanic Clouds have been used to measure integrated fluxes forseveral hundred H II regions and to study the properties of the H IIregion populations in the galaxies. The H II regions span a range of10,000 in luminosity, from objects on the scale of the Orion Nebula tothe 30 Doradus complex. The H-alpha luminosity function is wellrepresented over this entire range by a power law function, indicatingthat there is no characteristic luminosity scale for the H II regions.The distributions of nebular diameters, on the other hand, are fittedwell by exponential functions, with a scale length of 80 pc. Approximatefluxes for several of the extended filamentary networks in the galaxieshave also been measured. This extended component probability contributes15-25 percent of the total H-alpha luminosity of the galaxies.
| A new radio continuum survey of the Magellanic Clouds at 1.4 GHz. I - Observations and data analysis
New radio continuum surveys of the Large and Small Magellanic Clouds at1.4 GHz have been made with the Parkes 64-m radio telescope with abeamwidth of 15.0 arcmin and an rms noise of about 30 mJy per beam areain the final maps. The observations are described and results presented.Their radio continuum morphology is discussed, and sources contained inboth fields are listed. The bulk of the sources resident within theradio boundaries of the Magellanic Clouds have flat spectra and can beassociated with bright H II regions. There is observational evidence inthe data to support tidal interactions between the Clouds, andlarge-scale structures are clearly observed in both galaxies.
| Extinction and reddening of H II regions in the Large Magellanic Cloud
Absolute H-alpha and H-beta of H II regions in the Large MagellanicCloud are used in conjunction with radio continuum, 21-cm line, andstellar UBV data to study the dust associated with these regions.Various hypotheses concerning the optical properties and the spatialdistribution of the dust are examined, and the predicted characteristicsare compared with the observations. Uniform interstellar extinctioncannot explain the data. The following picture which is consistent withthe observations is suggested: some of the extinction is due to clumpeddust well outside of the emission zone, and the rest is caused by dustlocated closer to the ionized gas where scattering effects areimportant. This latter dust probably lies mostly just outside theboundary of the H II region, but some of the dust may be internal.
| Age determination of extragalactic H II regions
The H II region evolution models of Copetti et al. (1984) were comparedwith observational data of H II regions in the Magellanic Clouds, M 33,M 101 and of 'isolated extragalactic H II regions'. IMF with chi = 3 or2.5 are inconsistent with a large number of H II regions. The moreuniform age distribution of isolated extragalactic H II regions obtainedthrough an IMF with chi = 2 suggests that this value is more realisticthan chi = 1 or 1.5. The H II region age estimates indicate a burst ofstar formation about 5.5 + or - 1.0 10 to the -6th yr ago in the LMC andabout 2.3 + or - 0.9 x 10 to the 6th yr ago in the SMC. The observedforbidden O III/H-beta gradient in M 33 and M 101 must be caused bycolor temperature variation of the radiation ionizing the H II regions.
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