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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.
| 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.
| 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
| Early-type variables in the Magellanic Clouds. I. beta Cephei stars in the LMC bar
A thorough analysis of the OGLE-II time-series photometry of the LargeMagellanic Cloud bar supplemented by similar data from the MACHOdatabase led us to the discovery of three beta Cephei-type stars. Theseare the first known extragalactic beta Cephei-type stars. Two of thethree stars are multiperiodic. Two stars have inferred masses of about10 M_sun while the third is about 2 mag brighter and at least twice asmassive. All three variables are located in or very close to the massiveand young LMC associations (LH 41, 59 and 81). It is therefore veryprobable that the variables have higher than average metallicities. Thiswould reconcile our finding with theoretical predictions of the shapeand location of the beta Cephei instability strip in the H-R diagram.The low number of beta Cephei stars found in the LMC is anotherobservational confirmation of strong dependence of the mechanism drivingpulsations in these variables on metallicity. Follow-up spectroscopicdetermination of the metallicities in the discovered variables willprovide a good test for the theory of pulsational stability in massivemain-sequence stars.
| 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.
| 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.
| 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.
| The detection of X-ray emission from the OB associations of the Large Magellanic Cloud
A systematic study of the X-ray properties of OB associations in theLarge Magellanic Cloud has been carried out using data from the EinsteinObservatory. An excess of young, X-ray-bright supernova remnants isfound in the vicinity of the associations. In addition, diffuse X-rayemission is detected from over two dozen other associations;luminosities in the 0.16-3.5 keV band range from 2 x 10 to the 34th (thedetection threshold) to 10 to the 36th ergs/s. For several of the moreluminous examples, it is shown that emission from interstellar bubblescreated by the OB stellar winds alone is insufficient to explain theemission. It is concluded that transient heating of the bubble cavitiesby recent supernovae may be required to explain the observed X-rays andthat such a scenario is consistent with the number of X-ray-brightassociations and the expected supernova rate from the young stars theycontain.
| CCD photometry of young open clusters in Large Magellanic Cloud - NGC 1712, NGC 1722, and NGC 1727
Color-magnitude diagrams of the young LMC open clusters NGC 1711, 1712,1722, and 1727 shows that all of them are very young objects. NGC 1722and 1727 contain a considerable amount of primordial diffused matter andtheir main sequences are practically unevolved. NGC 1712 is more evolvedand its age is comparable to that of the young populous cluster NGC1711. The approximate luminosity functions of NGC 1722 and 1727 arepractically identical and different from the luminosity functions of NGC1711 and 1712. This difference may be an evolutionary effect.
| Variable stars in NGC 1712, a very young open cluster in Large Magellanic Cloud
Data obtained during nearly eight hours of observation of the LMC opencluster NGC 1712 are analyzed. Nine stars with light variations werediscovered. For star Nos. 1, 3, 4, 5, 7, and 8, the absolute magnitudesand colors match the values expected for the main sequence stars whosespectral type is between B2 and B9. Star Nos. 2, 6, and 9 are suspectedto be either double stars or premain-sequence objects.
| Young stars and bubbles in the Large Megellanic Cloud
The generating mechanisms of bubbles are investigated on a galaxy-widescale for the Large Magellanic Cloud. Several formation processes forring-shaped and filamentary emission regions are considered, andformulas are given for the time dependence of the shell radius takingthe interaction of supernovas and stellar winds into account. Theparameters of associations and H II regions are compiled, reduced to ahomogeneous system, and presented. Correlations between associationparameters and emission region parameters are investigated. It is foundthat stellar content versus emission region diameter, H-alpha fluxversus FUV flux, star surface density versus H-alpha brightness, and FUVflux versus stellar content of blue stars all show correlations withcoefficients greater than 0.4. A diameter-age diagram for bubbleevolution is depicted in which the H II region evolution effect and thestellar wind effect are separated.
| A catalogue of stellar associations in the Large Magellanic Cloud.
Abstract image available at:http://adsabs.harvard.edu/abs/1970AJ.....75..171L
| A Catalogue of Clusters in The LMC
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