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Near-Infrared Observations of N11 in the Large Magellanic Cloud: Triggered Star Formation around the Periphery of LH 9
Near-infrared observations have been carried out to survey young stellarobjects in the second-largest H II region in the Large Magellanic Cloud,N11. A total area of about 700 arcmin2 is covered in the J,H, and KS bands. We selected a total of 559 OB and 127 HerbigAe/Be star candidates out of the detected sources based on theirnear-infrared colors and magnitudes. The existence of these youngstellar objects indicates that star formation activity is underway inthe whole N11 region. Many Herbig Ae/Be star candidates are distributedaround the periphery of the OB association LH 9. Spatial correlations ofthe OB and Herbig Ae/Be star candidates with the objects observed atother wavelengths (optical, radio continuum, Hα, CO, and X-ray)suggest that the birth of the young stellar populations in peripheralmolecular clouds was triggered originally by LH 9. It is likely that thetrigger for this star formation was an expanding supershell blown by theOB association. In N11 a new generation of stars would have been formedin the clouds developed from swept-up interstellar medium.
| High spatial resolution radio continuum observations of compact H {II} regions in the Magellanic Clouds
We present high spatial resolution observations of the 6 cm continuumemission of compact H II regions in well-known sites of massive starformation located in the Small and Large Magellanic Clouds. Theobservations include N81 in the SMC, and N4A, N83B, N11A, N160A andN159-5 in the LMC. Some of the compact H II regions are isolated, whileothers are embedded in more diffuse ionised regions. A description ofthe radio morphology of the sources, together with comparisons withother observations, is given in detail. The regions cover a wide rangein size (from ˜ 0.1 to 7 pc), rms electron density (from ˜200 to 6500 cm-3), emission measure (from~3×105 to 2×107 pc cm-6),ionised gas mass (from ˜ 0.2 to 750 Mȯ) and rateof Lyman continuum photons (from ~ 3× 1047 to5×1049 s-1). The spectral types determinedfrom the Lyman continuum fluxes are consistent with opticaldeterminations. We have compared these Magellanic Cloud H II regionswith their Galactic counterparts in terms of size, rms electron densityand Lyman continuum flux. This comparison shows that their propertiesrelate to each other in the same way as those in Galactic H II regions.
| XMM-Newton observations of the giant H II region N 11 in the LMC
Using the sensitive XMM-Newton observatory, we have observed the giant HII region N 11 in the LMC for ˜30 ks. We have detected severallarge areas of soft diffuse X-ray emission along with 37 point sources.One of the most interesting results is the possible association of afaint X-ray source with BSDL 188, a small extended object of uncertainnature.The OB associations in the field-of-view (LH9, LH10 and LH13) are alldetected with XMM-Newton, but they appear very different from oneanother. The diffuse soft X-ray emission associated with LH9 peaks nearHD 32228, a dense cluster of massive stars. The combined emission of allindividual massive stars of LH9 and of the superbubble they have createdis not sufficient to explain the high level of emission observed: hiddenSNRs, colliding-wind binaries and the numerous pre-main sequence starsof the cluster are most likely the cause of this discrepancy. Thesuperbubble may also be leaking some hot gas in the ISM since faint,soft emission can be observed to the south of the cluster. The X-rayemission from LH10 consists of three pointlike sources and a softextended emission of low intensity. The two brightest point sources areclearly associated with the fastest expanding bubbles blown by hot starsin the SW part of the cluster. The total X-ray emission from LH10 israther soft, although it presents a higher temperature than the othersoft emissions of the field. The discrepancy between the combinedemission of the stars and the observed luminosity is here less severethan for LH9 and could be explained in terms of hot gas filling thewind-blown bubbles. On the other hand, the case of LH13 is different: itdoes not harbour any extended emission and its X-ray emission could mostprobably be explained by the Sk -66°41 cluster alone.Finally, our XMM-Newton observation included simultaneous observationswith the OM camera that provide us with unique UV photometry of morethan 6000 sources and enable the discovery of the UV emission from theSNR N11L.Based on observations collected with XMM-Newton, an ESA Science Missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA).Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr /cgi-bin/qcat?J/A+A/418/841
| Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. X. CO emission from star formation regions in LMC and SMC
We present J=1-0 and J=2-1 12CO maps of several star-formingregions in both the Large and the Small Magellanic Cloud, and brieflydiscuss their structure. Many of the detected molecular clouds arerelatively isolated and quite small with dimensions of typically 20 pc.Some larger complexes have been detected, but in all cases the extent ofthe molecular clouds sampled by CO emission is significantly less thanthe extent of the ionized gas of the star-formation region. Very littlediffuse extended CO emission was seen; diffuse CO in between orsurrounding the detected discrete clouds is either very weak or absent.The majority of all LMC lines of sight detected in 13CO hasan isotopic emission ratio I( 12CO)/I( 13CO) ofabout 10, i.e. twice higher than found in Galactic star-formingcomplexes. At the lowest 12CO intensities, the spread ofisotopic emission ratios rapidly increases, low ratios representingrelatively dense and cold molecular gas and high ratios marking COphoto-dissociation at cloud edges.
| Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. IX. The giant LMC HII region complex N 11
The second-brightest star formation complex in the Large MagellanicCloud, N 11, was surveyed extensively in the J = 1-0 transition of12CO. In this paper we present maps and a cataloguecontaining the parameters of 29 individual molecular clouds in thecomplex, although more may be present. The distribution of molecular gasin the N 11 complex is highly structured. In the southwestern part of N11, molecular clouds occur in a ring or shell surrounding the major OBstar association LH 9. In the northeastern part, a chain of molecularclouds delineates the rim of one of the so-called supergiant shells inthe LMC. There appears to be very little diffuse molecular gasin-between the individual well-defined clouds, especially in thesouthwestern ring. Most of the clouds have dimensions only slightlylarger than those of the survey beam, i.e. diameters of 25 pc or less. Asubset of the clouds mapped in J= 1-0 12CO transition wasalso observed in the J= 2-1 12CO transition, and in thecorresponding transitions of 13CO. Clouds mapped in J= 2-112CO with a two times higher angular resolution show further,clear substructure. The elements of this substructure, however, havedimensions once again comparable to those of the mapping beam. For a fewclouds, sufficient information was available to warrant an attempt atmodelling their physical parameters. They contain fairly warm(Tkin = 60-150 K) and moderately dense (nH_2 =3000 cm-3) gas. The northeastern chain of CO clouds, althoughlacking in diffuse intercloud emission, is characteristic of the morequiescent regions of the LMC, and appears to have been subject torelatively little photo-processing. The clouds forming part of thesouthwestern shell or ring, however, are almost devoid of diffuseintercloud emission, and also exhibit other characteristics of anextreme photon-dominated region (PDR).
| The relation between radio flux density and ionising ultra-violet flux for HII regions and supernova remnants in the Large Magellanic Cloud
We present a comparison between the Parkes radio surveys (Filipovic etal. 1995) and Vacuum Ultra-Violet (VUV) surveys (Smith et al. 1987) ofthe Large Magellanic Clouds (LMC). We have found 72 sources in common inthe LMC which are known HII regions (52) and supernova remnants (SNRs)(19). Some of these radio sources are associated with two or more UVstellar associations. A comparison of the radio flux densities andionising UV flux for HII regions shows a very good correlation, asexpected from theory. Many of the Magellanic Clouds (MCs) SNRs areembedded in HII regions, so there is also a relation between radio andUV which we attribute to the surrounding HII regions.
| The physical structure of Magellanic Cloud H II regions. I. Dataset
We present infrared and optical spectroscopic data for 11 H Ii regionsand one Supernova Remnant in the Large and Small Magellanic Cloud. Theinfrared data have been obtained with the Short Wavelength Spectrometerand Long Wavelength Spectrometer on board the Infrared Space Observatoryas part of a Guaranteed Time Program on H Ii regions in Local GroupGalaxies. Aim of this project is to give a new and improved analysis ofthe physical structure of the sample H Ii regions by combining as muchspectral data as possible. A detailed account is given here of thereduction process, and the quality and reliability of the presentedfluxes are discussed. Based on observations with ISO, an ESA projectwith instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA.
| 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.
| A CO Survey of the LMC with NANTEN: III. Formation of Stellar Clusters and Evolution of Molecular Clouds
In order to elucidate star formation in the LMC, we made a completestudy of CO clouds with NANTEN. In the present paper, we compare 55giant molecular clouds (GMCs), whose physical quantities were welldetermined, with young objects, such as young stellar clusters and HIIregions. We find that the GMCs are actively forming stars and clusters;23 and 40 are found to be associated with the clusters and the HIIregions, respectively. The clusters associated with the GMCs aresignificantly young; ~ 85% of them are younger than ~ 10 Myr. Inaddition, compact groups of the young clusters are often found at thepeak position of the GMCs, e.g., N 159 and N 44, while much loosergroups are away from the GMCs. This suggests that the clusters areformed in groups and disperse as they become old. The distributions ofthe CO, [CII], and UV indicate that the GMCs are likely to be rapidlydissipated within several Myr due to UV photons from the clusters. Wealso estimate the evolutionary time scale of the GMCs; they form starsin a few Myr after their birth, and form clusters during the next fewMyr, and are dissipated in the subsequent few Myr.
| HST observations of the LMC compact \ion{H}{ II} region N 11A
We present a study of the LMC compact H ii region N 11A using HubbleSpace Telescope imaging observations which resolve N 11A and reveal itsunknown nebular and stellar features. The presence of a sharp ionizationfront extending over more than 4'' (1 pc) and fine structure filamentsas well as larger loops indicate an environment typical of massive starformation regions, in agreement with high [O iii]/Hβ line ratios. N11A is a young region, as deduced from its morphology, reddening, andespecially high local concentration of dust, as indicated by the Balmerdecrement map. Our observations also reveal a cluster of stars lyingtowards the central part of N 11A. Five of the stars are packed in anarea less than 2'' (0.5 pc), with the most luminous one being a mid Otype star. N 11A appears to be the most evolved compact H ii region inthe Magellanic Clouds so far studied. Based on observations with theNASA/ESA Hubble Space Telescope obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| Ultraviolet Imaging Polarimetry of the Large Magellanic Cloud. II. Models
Motivated by new sounding-rocket wide-field polarimetric images of theLarge Magellanic Cloud (reported simultaneously by Cole et al.), we haveused a three-dimensional Monte Carlo radiation transfer code toinvestigate the escape of near-ultraviolet photons from young stellarassociations embedded within a disk of dusty material (i.e., a galaxy).As photons propagate through the disk, they may be scattered or absorbedby dust. Scattered photons are polarized and tracked until they escapethe dust layer, allowing them to be observed; absorbed photons heat thedust, which radiates isotropically in the far-infrared where the galaxyis optically thin. The code produces four output images: near-UV andfar-IR flux, and near-UV images in the linear Stokes parameters Q and U.From these images we construct simulated UV polarization maps of theLMC. We use these maps to place constraints on the star+dust geometry ofthe LMC and the optical properties of its dust grains. By tuning themodel input parameters to produce maps that match the observedpolarization maps, we derive information about the inclination of theLMC disk to the plane of the sky and about the scattering phase functiong. We compute a grid of models with i=28 deg, 36 deg, and 45 deg, andg=0.64, 0.70, 0.77, 0.83, and 0.90. The model that best reproduces theobserved polarization maps has i=36 deg+2-5 andg~0.7. Because of the low signal-to-noise in the data, we cannot placefirm constraints on the value of g. The highly inclined models do notmatch the observed centrosymmetric polarization patterns around brightOB associations or the distribution of polarization values. Our modelsapproximately reproduce the observed ultraviolet photopolarimetry of thewestern side of the LMC; however, the output images depend on many inputparameters and are nonunique. We discuss some of the limitations of themodels and outline future steps to be taken; our models make somepredictions regarding the polarization properties of diffuse lightacross the rest of the LMC.
| Ultraviolet Imaging Polarimetry of the Large Magellanic Cloud. I. Observations
We have used the rocketborne Wide-Field Imaging Survey Polarimeter(WISP) to image a 1.5dx4.8d area of the western side of the LargeMagellanic Cloud (LMC) at a wavelength of λ=2150 Å and aresolution of 1'x1.5′. These are the first wide-field ultravioletpolarimetric images in astronomy. We find the UV background light of theLMC to be linearly polarized at levels ranging from our sensitivitylimit of 4% to as high as ~40%. In general, the polarization in a pixelincreases as the flux decreases; the weighted mean value of polarizationacross the WISP field is 12.6%+/-2.3%. The LMC's diffuse UV background,in uncrowded areas, rises from a minimum of (5.6+/-3.1)x10-8ergs s-1 cm-2 Å-1 sr-1(23.6+/-0.5 mag arcsec-2) to (9.3+/-1.1)x10-8 ergss-1 cm-2 Å-1 sr-1(23.1+/-0.2 mag arcsec-2) in regions near the brightassociations. We use our polarization maps to investigate the geometryof the interstellar medium in the LMC and to search for evidence of asignificant contribution of scattered light from OB associations to thediffuse galactic light of the LMC. Through a statistical analysis of ourpolarization map, we identify nine regions of intense UV emission whichmay be giving rise to scattering halos in our image. We find thatstarlight from the N11 complex and the LH 15 association are thestrongest contributors to the scattered light component of the LMC'sdiffuse galactic light. This region of the northwestern LMC can bethought of as a kiloparsec-scale reflection nebula in which OB starsilluminate distant dust grains that scatter the light into our sightline. In contrast, the polarization map does not support the scatteringof light from the large B2 complex in the southern WISP field; thiseffect may be astrophysical, or it may be the result of bias in ouranalysis.
| 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.
| Ultraviolet Imaging Telescope Observations of the Magellanic Clouds
We present wide-field far-ultraviolet (FUV; 1300-1800 Å) images ofthe Large and Small Magellanic Clouds (LMC, SMC). These data wereobtained by the Ultraviolet Imaging Telescope (UIT) during the Astro-1(1990 December 1-10) and Astro-2 (1995 March 2-18) missions; the imagesprovide an extensive FUV mosaic of the SMC and contain numerous regionsin the LMC, covering a wide range of stellar densities and current starformation activity. A total of 47 LMC/Lucke-Hodge and 37 SMC/Hodge OBassociations are completely or partially included in the observedfields. FUV data can identify the hottest OB stars more easily than canoptical photometry, and these stars dominate the ionizing flux, which iscorrelated to the observed Hα flux of the associated H ii regions.Of the H ii regions in the catalog of Davies, Elliott, & Meaburn(DEM), the UIT fields completely or partially include 102 DEM regions inthe LMC and 74 DEM regions in the SMC. We present a catalog of FUVmagnitudes derived from point-spread function photometry for 37,333stars in the LMC (the UIT FUV magnitudes for 11,306 stars in the SMCwere presented recently by Cornett et al.), with a completeness limit ofm_UV ~ 15 mag and a detection limit of m_UV ~ 17.5. The averageuncertainty in the photometry is ~0.1 mag. The full catalog withastrometric positions, photometry, and other information is alsoavailable from publicly accessible astronomical data archives. We dividethe catalog into field stars and stars that are in DEM regions. Weanalyze each of these two sets of stars independently, comparing thecomposite UV luminosity function of our data with UV magnitudes derivedfrom stellar evolution and atmosphere models in order to derive theunderlying stellar formation parameters. We find a most probable initialmass function (IMF) slope for the LMC field stars of Gamma = -1.80 +/-0.09. The statistical significance of this single slope for the LMCfield stars is extremely high, though we also find some evidence for afield star IMF slope of Gamma ~ -1.4, roughly equal to the Salpeterslope. However, in the case of the stars in the DEM regions (the starsin all the regions were analyzed together as a single group), we findthree IMF slopes of roughly equal likelihood: Gamma = -1.0, -1.6, and-2.0. No typical age for the field stars is found in our data for timeperiods up to a continuous star formation age of 500 Myr, which is themaximum age consistent with the completeness limit magnitude of thecatalog's luminosity function. The best age for the collection ofcluster stars was found to be t_0 = 3.4 +/- 1.9 Myr; this is consistentwith the age expected for a collection of OB stars from many differentclusters.
| A radio continuum study of the Magellanic Clouds. VIII. Discrete sources common to radio and infrared surveys of the Magellanic Clouds
We compare Parkes Telescope radio surveys with the IRAS Infrared (IR)surveys of the Magellanic Clouds (MCs). We find 130 discrete sources incommon towards the Large Magellanic Cloud (LMC) with both radio and IRemission. These 130 sources are mainly H Ii regions (89) and supernovaremnants (21). For 12 of the sources we have no identification and eightare background objects. We find 38 sources in common for the SmallMagellanic Cloud (SMC). Most of these sources are intrinsic (31) to theSMC, five sources are previously known background galaxies and twosources remain ambiguous. A flux density comparison of the radio and IRsources shows very good correlation and we note that the strongestsources at both radio and IR frequencies are H Ii regions. From theradio-IR comparison we propose that some 40 new sources in the LMC and10 in the SMC are H Ii regions or SNRs. All these new sources are alsoidentified in optical surveys.
| A radio continuum study of the Magellanic Clouds. VII. Discrete radio sources in the Magellanic Clouds
We present a study of discrete radio sources in the Magellanic Clouds(MCs) using the latest large-scale radio surveys made with the Parkesradio telescope between 1.4 and 8.55 GHz. These surveys achieved highersensitivity then previous surveys done with the Parkes telescope and sothe number of discrete radio sources detected towards the MCs hasincreased by factor of five. Also, we have obtained improved positions,flux densities and radio spectral indices for all of these sources. Atotal of 483 sources towards the Large Magellanic Cloud (LMC) and 224towards the Small Magellanic Cloud (SMC) have been detected at at leastone radio frequency. Most of the MC's sources have been classified inone of three groups: SNRs, H Ii regions or background sources accordingto classification criteria established here. In total, 209 discreteradio sources in the LMC and the 37 sources in the SMC are classifiedhere to be either H Ii regions or SNRs. We investigate their luminosityfunctions as well as the statistics of background sources behind theMCs. Also, we examine the distribution of SNRs and H Ii regions in theMCs. Tables 5 and 6 are only available electronically at the CDS via ftp130.79.128.5 or via http://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| X-Rays from Superbubbles in the Large Magellanic Cloud. V. The H II Complex N11
The giant H II complex N11 in the Large Magellanic Cloud contains OBassociations at several different stages in their life histories. Wehave obtained ROSAT PSPC and HRI X-ray observations, Curtis Schmidt CCDimages, echelle spectra in H alpha and [N II] lines, and IUEinterstellar absorption line observations of this region. The centralbubble of N11 has an X-ray luminosity a factor of only 3-7 brighter thanthat predicted for an energy-conserving superbubble, making this thefirst detection of X-ray emission from a superbubble without a strongX-ray excess. The region N11B contains an extremely young OB associationanalogous to the central association of the Carina Nebula, apparentlystill embedded in its natal molecular cloud. We find that N11B emitsdiffuse X-ray emission, probably powered by stellar winds. Finally, wecompare the tight cluster HD 32228 in N11 to R136 in 30 Dor. The latteris a strong X-ray source, while the former is not detected, showing thatstrong X-ray emission from compact objects is not a universal propertyof such tight clusters.
| Ultraviolet Imaging Telescope Observations of OB Stars in th N 11 Region of the Large Magellanic Cloud
We present an analysis of far-ultraviolet (FUV; 1300--1800 A) andoptical (U, B, and V) data of the stellar and nebular content of the OBassociations LH 9, 10, and 13 in the Large Magellanic Cloud region N 11.The FUV images from the Ultraviolet Imaging Telescope strongly selectthe hot O and B stars; over 1900 stars were detected in the FUV to alimiting magnitude of m152 = 17 mag. The resulting FUV photometrycombined with optical ground-based data indicate there are approximately88 confirmed or candidate O stars in the LH 9, 10, and 13 fields alone(in an area of ~41 arcmin2) and possibly as many as 170--240 O-typestars within the entire 40' diameter field of view.
| Formation of the nebular complex N11 in the Large Magellanic Cloud.
N 11, the second largest nebula of the LMC, is formed of a large bubblesurrounded by 9 bright nebulae and filaments. We have observed thisnebular complex with a scanning Fabry-Perot interferometer at Hαand [OIII] 5007 wavelengths. The kinematics of this field agrees withthe results of the stellar content and of the molecular studies, andshows that such a structure can be the consequence of a sequential starformation. The elements of this result are deduced from the energeticinput inside the ionized gas by the stellar winds of the associationsLH9 and LH10. The evaluated dynamical lifespan of small nebular entitiesassociated with the more massive stars of LH10, are found significantlyshorter than for the gas ionized by the association LH9. Thereforesequential star formation can be triggered at the shock boundaries ofthe nebula excited by an OB association in a time scale shorter thanpreviously found, and of at most a few 10^6^yrs. It thus leads to theevidence for a new class of giant bubble, designated as "ring of HIIregions" bubble with several distinctive characteristics. It isdiscussed that the bubbles of this type are formed by sequential starformation over time scales shorter than previously found for somesuperbubbles.
| 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.
| 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.
| Two-stage starbursts in the Large Magellanic Cloud - N11 as a once and future 30 Doradus
A recent, detailed study of the stellar content in the LMC giant shell HII region N11 has revealed a distinct, dual structural morphology, whichis remarkably analogous to that emerging from current IR imaging of 30Doradus. In both regions, the energetic stellar activity from aninitial, massive, centrally condensed starburst has apparently triggereda secondary burst around its periphery about 2 x 10 exp 6 years later,but in N11 the entire process is more advanced than in 30 Dor by 2 x 10exp 6 years. That is, N11 appears to be an evolved 30 Dor. There isevidence that the two sequential star-formation mechanisms may producedifferent IMFs. In both regions, very early stages of massive stellarevolution are evident, including IR protostars, O stars embedded indense nebular knots, candidate ZAMS O stars, and the O3 stars. Spectraof several recently discovered examples of the latter three categoriesin N11 are illustrated, and previously unpublished 4 m photographs ofthe region demonstrate their spectacular interactions with thesurrounding interstellar medium.
| 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).
| The expansive motions around the central hole of the complex giant filamentary shell DEM 34 (N 11) in the Large Magellanic Cloud
DEM 34 (N 11) is a 350-pc-diameter giant interstellar shell which has anOB association in its central hole and OB associations within H IIregions in its irregular filamentary perimeter. Spatially resolvedprofiles of the H-alpha emission line have been obtained with theManchester echelle spectrometer on the Anglo-Australian telescope alonga 232-pc-long NS line of measurements which passes over the center ofDEM 34. In the faint emission over the central hole of DEM 34 whichcontains the OB association LH 9, up to 5 separate velocity componentswithin a range of heliocentric radial velocities of 210-370 km/sec arefound. Some evidence of systematic radial expansions within the ionizedgas of this hole are found; however, expansions more complex than for asingle radially expanding spherical shell are indicated.
| A catalog of 255 new clusters in the Large Magellanic Cloud
A catalog of 255 newly discovered clusters has resulted from a search often fields in the LMC. Most of the clusters are faint, small, and old.The equatorial coordinates, sizes, and estimates of the B magnitudes ofthe brightest stars are given for all of the clusters. The mean diameterfor all LMC clusters in the fields is shown to be 7.7 pc. A correlationis found between size and distance from the center of the LMC, with morelarge clusters in the remote outer parts of the LMC. The total clusterpopulation of the LMC is estimated at about 4200.
| Photometric reddenings of stars in emission nebulae of the Magellanic Clouds
VBLUW observations of stars in emission nebulae of the SMC and the LMCare discussed. The color indices, luminosities, and extinction in thecentral regions of the nebulae have been obtained. Stars such as N 159and N 160A are found to have extinctions AV of greater than1.5 mag. It is suggested that more luminous (and thus more massive)stars or groups of stars are located in the regions of highestextinction.
| The LMC H II regions N11C and E and their stellar contents
The LMC H II regions N11C and N11E and the stars associated with theseregions are studied. Spectral types are given for nine stars, and B andV photometry for 58 stars. The star Sk-6641 is shown to be a multiplesystem and not to be the main exciting source of N11C. The main excitingstar of N11E, Sk-6643, is classified as O4-5V. CCD long-slit spectra areused to investigate the excitation and extinction along severaldirections in N11C and E, and indicate that gas may be coupled with dustalong several directions. It is suggested that the dust may beassociated with the molecular cloud observed near N11.
| Vacuum ultraviolet images of the Large Magellanic Cloud
Linearized, absolutely calibrated VUV images of the LMC with aresolution of about 50 arcsec are presented. The images were made by asounding rocket payload in two bandpasses with effective wavelengths forhot stars near 1500 A and 1930 A. The flux in each bandpass is measuredfor the associations in the list of Lucke and Hodge (1970). The resultsare discussed and their relationship to the overall characteristics ofstar formation in the LMC are discussed. A simple model for propagatingstar formation in the LMC is presented whose results closely resemblethe distribution of associations revealed by the VUV images.
| 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.
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