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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.
| The VLT-FLAMES survey of massive stars: observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region
We present new observations of 470 stars using the Fibre Large ArrayMulti-Element Spectrograph (FLAMES) instrument in fields centered on theclusters NGC 330 and NGC 346 in the Small Magellanic Cloud (SMC), andNGC 2004 and the N11 region in the Large Magellanic Cloud (LMC). Afurther 14 stars were observed in the N11 and NGC 330 fields using theUltraviolet and Visual Echelle Spectrograph (UVES) for a separateprogramme. Spectral classifications and stellar radial velocities aregiven for each target, with careful attention to checks for binarity. Inparticular, we have investigated previously unexplored regions aroundthe central LH9/LH10 complex of N11, finding ~25 new O-type stars fromour spectroscopy. We have observed a relatively large number of Be-typestars that display permitted Fe II emission lines. These are primarilynot in the cluster cores and appear to be associated with classicalBe-type stars, rather than pre main-sequence objects. The presence ofthe Fe II emission, as compared to the equivalent width of Hα, isnot obviously dependent on metallicity. We have also explored therelative fraction of Be- to normal B-type stars in the field-regionsnear to NGC 330 and NGC 2004, finding no strong evidence of a trend withmetallicity when compared to Galactic results. A consequence of serviceobservations is that we have reasonable time-sampling in three of ourFLAMES fields. We find lower limits to the binary fraction of O- andearly B-type stars of 23 to 36%. One of our targets (NGC 346-013) isespecially interesting with a massive, apparently hotter, less luminoussecondary component.
| 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.
| Australia Telescope Compact Array Survey of Candidate Ultracompact and Buried H II Regions in the Magellanic Clouds
We present a systematic survey for ultracompact (UC) H II regions in theMagellanic Clouds. Understanding the physics of massive star formation(MSF) is a critical astrophysical problem. The study of MSF began in ourGalaxy with surveys of UC H II regions, but before now this has not beendone for other galaxies. We selected candidates on the basis of theirInfrared Astronomical Satellite (IRAS) colors and imaged them at 3 and 6cm with the Australia Telescope Compact Array. Nearly all of theobserved regions contain compact radio sources consistent with thermalemission. Many of the sources are related to optically visible H IIregions, and often the radio emission traces the youngest and densestpart of the H II region. The luminosity function and number distributionof Lyman continuum fluxes of the compact radio sources are consistentwith standard stellar and cluster initial mass functions. This type ofsystematic assessment of IRAS diagnostics is important for interpretingSpitzer Space Telescope data, which will probe similar physical scalesin nearby galaxies as IRAS did in the Magellanic Clouds.
| 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.
| Active Star Formation in the N11B Nebula in the Large Magellanic Cloud: A Sequential Star Formation Scenario Confirmed
The second largest H II region in the Large Magellanic Cloud, N11B hasbeen surveyed in the near-IR. We present JHKs images of theN11B nebula. These images are combined with CO (1-->0) emission-linedata and with archival New Technology Telescope and Hubble SpaceTelescope WFPC2 optical images to address the star formation activity ofthe region. IR photometry of all the IR sources detected is given. Weconfirm that a second generation of stars is currently forming in theN11B region. Our IR images show the presence of several bright IRsources that appear to be located toward the molecular cloud as seenfrom the CO emission in the area. Several of these sources show IRcolors with young stellar object characteristics, and they are primecandidates to be intermediate-mass Herbig Ae/Be stars. For the firsttime, an extragalactic methanol maser is directly associated with IRsources embedded in a molecular core. Two IR sources are found at 2"(0.5 pc) of the methanol maser reported position. Additionally, wepresent the association of the N11A compact H II region to the moleculargas, where we find that the young massive O stars have eroded a cavityin the parental molecular cloud, typical of a champagne flow. The N11region turns out to be a very good laboratory for studying theinteraction of winds, UV radiation, and molecular gas. Severalphotodissociation regions are found.Based in part on observations with the NASA/ESA Hubble Space Telescopeobtained from the archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.
| 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.
| 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 and Optical Observations of OB Associations and Field Stars in the Southwest Region of the Large Magellanic Cloud
Using ultraviolet photometry from the Ultraviolet Imaging Telescope(UIT) combined with photometry and spectroscopy from three ground-basedoptical data sets we have analyzed the stellar content of OBassociations and field areas in and around the regions N79, N81, N83,and N94 in the Large Magellanic Cloud. In particular, we compare datafor the OB association Lucke-Hodge 2 (LH 2) to determine how stronglythe initial mass function (IMF) may depend on different photometricreductions and calibrations. Although the data sets exhibit medianphotometric differences of up to 30%, the resulting uncorrected IMFs arereasonably similar, typically Γ~-1.6 in the 5-60 Msolarmass range. However, when we correct for the background contribution offield stars, the calculated IMF flattens to Γ=-1.3+/-0.2 (similarto the Salpeter IMF slope). This change underlines the importance ofcorrecting for field star contamination in determinations of the IMF ofstar formation regions. It is possible that even in the case of anuniversal IMF, the variability of the density of background stars couldbe the dominant factor creating the differences between calculated IMFsfor OB associations. We have also combined the UIT data with the mostextensive of these ground-based optical data sets-the Magellanic CloudPhotometric Survey-to study the distribution of the candidate O-typestars in the field. We find a significant fraction, roughly half, of thecandidate O-type stars are found in field regions, far from any obviousOB associations (in accord with the 1982 suggestions of Garmany, Conti,& Chiosi for O-type stars in the solar neighborhood). These starsare greater than 2' (30 pc) from the boundaries of existing OBassociations in the region, which is a distance greater than most O-typestars with typical dispersion velocities will travel in their lifetimes.The origin of these massive field stars (either as runaways, members oflow-density star-forming regions, or examples of isolated massive starformation) will have to be determined by further observations andanalysis.
| Compact star clusters of the LMC H ii region N11 C
Based on imaging and spectroscopy obtained at the ESO NTT telescope andusing an efficient image analysis algorithm, we study the core of theLMC OB association LH 13, particularly the two compact stellar clustersSk-66deg41, and HNT in the H ii, region N 11C. We resolveSk-66deg41, into 15 components and for the first time the HNTcluster into 70 stars, and derive photometry for the members. Moreover,from medium resolution spectroscopy we determine the spectral types forsixteen stars in N 11C. We compare the color-magnitude diagrams of theclusters with that of the field stars and discuss the cluster ages. Withan age of ~ ,100 Myr, the HNT cluster appears significantly older thanthe very young (<= 5 Myr) Sk-66deg41, starburst. Wesuggest that most of the `field' O-stars in the core of N 11C haveactually been ejected from Sk-66deg41, through dynamicalinteractions in the compact cluster. The properties of theSk-66deg41, and HNT clusters suggest that we are viewingdifferent star formation regions lying at different distances along thesame line of sight. Based on observations obtained at the EuropeanSouthern Observatory, La Silla, Chile
| 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.
| Imaging and spectroscopy of ionized shells and supershells in the Large Magellanic Cloud
Deep H-alpha images of the Large Magellanic Cloud (LMC) have revealedthe presence of numerous supergiant (greater than 300 pc radius) andgiant shells of ionized gas. These structures are generally believed tobe the result of the action of encircled massive stars on thesurrounding interstellar medium. This paper examines the spectral andkinematic signature of this interaction through low and high dispersionspectra obtained for three supergiant and three giant shells in the LMC.One of the giant shells is an x-ray bubble embedded in the 30 Doradusnebula. The emission line ratios, including the lines (O II)lambda-3727, (O III) lambda-5007, (N II) lambda-6584, (S II)lambda-6717,31, in all but the embedded x-ray bubble, are found to beunusual compared to typical H II regions and supernova remnants in thesame galaxy. However, the emission lines and surface brightnesses ofthese structures are generally consistent with models of photoionizedgas having a very low ionization parameter due to the large distancebetween the encircled stars and the gas. Thus, emission from both thesupergiant and giant shell structures appears to be dominated byphotoionization processes. High dispersion spectra reveal that theprofiles of the ionized gas at the edges of supershells are narrow andcontain a single velocity component; spectra of the giant shells revealbroad profiles with multiple velocity components.
| Luminosity functions and color-magnitude diagrams for three OB associations in the Large Magellanic Cloud
Using the point spread function photometry program DAOPHOT, we have usedUBV CCD photometry to construct color-magnitude diagrams and luminosityfunctions for three OB associations in the Large Magellanic Cloud. Theregion LH 76 appears to be completely coeval; the region LH 13 showssome evidence for noncoevality which will need to be checked withspectra of the stars in question. The region LH 105, which lies on thesouthern edge of 30 Doradus, shows significant contamination by anunderlying older population, possibly from previous star forming events.The luminosity functions, which serve as the first step towarddetermining the initial mass function in these regions, are calculated.
| 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.
| A radio continuum study of the Magellanic Clouds. II - The far-infrared/radio correlation in the Large Magellanic Cloud
The correlation between the far-infrared (FIR) and the 6.3 cm radiocontinuum of the Large Magellanic Cloud (LMC) was studied. Radiocontinuum maps and infrared maps are presented. Statistical studiesindicated that the local point-by-point correlation between the two mapsis strong and appears to be due to the correlation between warm FIR andthermal radio emissions both of which are associated with young ionizingstars. The cool FIR/nonthermal radio correlation appears to be stronglylinked to the global FIR/radio correlation. A systematic difference wasobserved between the eastern and western half of the LMC. This asymmetrymay be due to different ages of massive stars or different gas-to dustratios in the two halves.
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