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OB stellar associations in the Large Magellanic Cloud: Survey of young stellar systems
The method developed by Gouliermis et al. (\cite{Gouliermis00}, PaperI), for the detection and classification of stellar systems in the LMC,was used for the identification of stellar associations and openclusters in the central area of the LMC. This method was applied on thestellar catalog produced from a scanned 1.2 m UK Schmidt Telescope Platein U with a field of view almost 6\fdg5 x 6\fdg5, centered on the Bar ofthis galaxy. The survey of the identified systems is presented herefollowed by the results of the investigation on their spatialdistribution and their structural parameters, as were estimatedaccording to our proposed methodology in Paper I. The detected openclusters and stellar associations show to form large filamentarystructures, which are often connected with the loci of HI shells. Thederived mean size of the stellar associations in this survey was foundto agree with the average size found previously by other authors, forstellar associations in different galaxies. This common size of about 80pc might represent a universal scale for the star formation process,whereas the parameter correlations of the detected loose systems supportthe distinction between open clusters and stellar associations.
| A statistical study of binary and multiple clusters in the LMC
Based on the Bica et al. (\cite{bica}) catalogue, we studied the starcluster system of the LMC and provide a new catalogue of all binary andmultiple cluster candidates found. As a selection criterion we used amaximum separation of 1farcm4 corresponding to 20 pc (assuming adistance modulus of 18.5 mag). We performed Monte Carlo simulations andproduced artificial cluster distributions that we compared with the realone in order to check how many of the found cluster pairs and groups canbe expected statistically due to chance superposition on the plane ofthe sky. We found that, depending on the cluster density, between 56%(bar region) and 12% (outer LMC) of the detected pairs can be explainedstatistically. We studied in detail the properties of the multiplecluster candidates. The binary cluster candidates seem to show atendency to form with components of similar size. When possible, westudied the age structure of the cluster groups and found that themultiple clusters are predominantly young with only a few cluster groupsolder than 300 Myr. The spatial distribution of the cluster pairs andgroups coincides with the distribution of clusters in general; however,old groups or groups with large internal age differences are mainlylocated in the densely populated bar region. Thus, they can easily beexplained as chance superpositions. Our findings show that a formationscenario through tidal capture is not only unlikely due to the lowprobability of close encounters of star clusters, and thus the evenlower probability of tidal capture, but the few groups with largeinternal age differences can easily be explained with projectioneffects. We favour a formation scenario as suggested by Fujimoto &Kumai (\cite{fk}) in which the components of a binary cluster formedtogether and thus should be coeval or have small age differencescompatible with cluster formation time scales. Table 6 is only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/547
| Arc-Shaped and Spheroidal Stellar Complexes
Complexes of young clusters and high-luminosity stars in the shape ofregular, circular arcs have been found in a number of galaxies, firstand foremost the LMC, NGC 6946, and M83. These shapes are found even instrongly inclined galaxies, suggesting that the observed arcs areprojections of partial spherical shells. Obviously, these stellar shellsmust have formed from gaseous shells swept up by some source of centralpressure and become gravitationally unstable. The power of this sourcecorresponds to several dozen supernova explosions; however, its natureremains unclear. A central cluster providing a source of O stars andsupernovae is usually absent. The presence of multiple arcs locatedclose to each other can be explained by the fall of a swarm of fragmentsor by the progenitor stars originating in a single peculiar starcluster, implying the existence of stellar objects capable of givingrise to explosions with energies an order of magnitude higher than thoseof individual supernovae. The same objects may be responsible forgamma-ray bursts. It may be that only the most massive clusters withfrequent or especially powerful supernova explosions are capable ofproducing HI supershells. Otherwise, it is impossible to explain why nosupershells have been found around numerous clusters that should becapable of producing them according to current theories. The presence ofstar clusters in shell-like structures provides extremely importantinformation about the physical conditions in and the ages of the initialgaseous shells, making stellar arcs the best available laboratory forstudies of triggered star formation.
| X-Rays from Superbubbles in the Large Magellanic Cloud. VI. A Sample of Thirteen Superbubbles
We present ROSAT observations and analysis of thirteen superbubbles inthe Large Magellanic Cloud. Eleven of these observations have not beenpreviously reported. We have studied the X-ray morphology of thesuperbubbles and have extracted and analyzed their X-ray spectra.Diffuse X-ray emission is detected from each of these superbubbles, andX-ray emission is brighter than that theoretically expected for awind-blown bubble, suggesting that the X-ray emission from thesuperbubbles has been enhanced by interactions between the superbubbleshell and interior supernova remnants. We have also found significantpositive correlations between the X-ray luminosity of a superbubble andits Hα luminosity, expansion velocity, and OB star count. Further,we have found that a large fraction of the superbubbles in the sampleshow evidence of breakout regions, where hot X-ray-emitting gas extendsbeyond the Hα shell.
| Stellar luminosity functions of rich star clusters in the Large Magellanic Cloud
We show the results of deep V and I HST photometry of 6 rich starclusters in the Large Magellanic Cloud with different ages andmetallicities. The number of stars with measured magnitudes in eachcluster varies from about 3000 to 10 000 stars. We build stellar densityand surface brightness profiles for the clusters and extract half-lightradii and other structural parameters for each. We also obtainluminosity functions, Phi (M_V), down to M_V =~ 6 (M/Msun>~ 0.9), and investigate their dependence with distance from thecluster centre well beyond their half-light radius. In all clusters wefind a systematic increase in the luminosity functions slope, Delta logPhi (M_V) / Delta (M_V), with radial distance from the centre. Among theclusters displaying significant mass segregation are the 2 youngest inthe sample: NGC 1805, NGC 1818. For these two clusters we obtain presentday mass functions. The NGC 1818 mass function is in excellent agreementwith that derived by other authors, also using HST data. The youngcluster mass function slopes differ, that of NGC 1805 beingsystematically steeper than NGC 1818. Since these are very young stellarsystems (tau <~ 40 Myrs), these variations may reflect the initialconditions rather than evolution due to internal dynamics.
| OB Stellar Associations in the Large Magellanic Cloud: Identification Method
We describe an objective method for the identification of stellar OBassociations in the Large Magellanic Cloud under the assumption thatthey are loose, unbound stellar systems with a young OB stellarcomponent. The method is based on star counts and spectralclassification. First we detect the areas where an enhancement of starnumber density occurs above 3 σ of the average field density inlarge regions. The boundaries at 3 σ provide the size andmorphology of the detected stellar concentrations. Further examinationat different magnitude ranges allows us to select the systems with abright stellar component within the detected areas. In the second step,star counts around the peak density of each detected stellarconcentration provide a typical value of the projected half-mass radius,in order to calculate the central density using the appropriate massfunction slope. The central density, being a crucial parameter for thebound and unbound systems, has been used as a tentative criterion forthe distinction between open clusters and associations. Finally,spectral classification from objective-prism plates provides furtherevidence for the existence of OB-type stars in these concentrations. Thefaintest magnitude at which the various systems were detected is foundto be independent of the presence or absence of gas and varies by up to4 mag. An explanation for this effect is the possible existence ofpre-main-sequence stars that are not visible in the optical region.
| 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.
| Gamma ray bursts versus OB associations: do they trigger star formation?
We discuss differences in shapes, expansion velocities and fragmentationtimes of structures created by an energy deposition from a singleGamma-Ray Burst (GRB) and an OB association to the ISM. After theinitial inflation, supershells produced by GRBs are almost static orslowly expanding, contrary to more rapidly expanding supershells createdby OB associations. We discuss the position of the energy sourcerelative to the symmetry plane of the galaxy: observed arc-likestructures can be the most dense part of structures formed by anexpansion from a source above or below the galactic plane. Arcs may alsoform if the expansion takes place inside a giant HI cloud. We try toreproduce the size, the age, and the average distance between OBassociations in the Sextant region at the edge of LMC 4.
| 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.
| Triggered star formation in the LMC4/Constellation III region of the Large Magellanic Cloud
The origin of a regular, 600-pc-long arc of young stars and clusters inthe Constellation III region of the Large Magellanic Cloud isconsidered. The circular form of this arc suggests that the pre-stellargas was uniformly swept up by a central source of pressure. In thecentre of the arc are six ~30-Myr-old A-type supergiant stars and aCepheid variable of similar age, which may be related to the source ofthis pressure. We calculate the expansion of a bubble around a clusterof this age, and show that it could have triggered the formation of thearc at the right time and place. Surrounding the central old stars andextending well outside the young arc is the LMC4 superbubble and giantHI shell. We show how this superbubble and shell could have formed bythe continued expansion of the 15-Myr-old cavity, following starformation in the arc and the associated new pressures. The age sequenceproposed here was not evident in the recent observations by Olsen et al.and Braun et al. because the first generation stars in the centre of theLMC superbubble are relatively faint and scarce compared to the moresubstantial population of stars less than 15 Myr old that formedthroughout the region in a second generation. These considerations leadto an examination of the origin of the LMC4/Constellation III region andother large rings in the LMC and other galaxies. Their size andcircularity could be the result of low galactic shear and a thick disc,with several generations of star formation in their interiors now toofaint to be seen.
| 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.
| No stellar age gradient inside supergiant shell LMC4
The youngest stellar populations of a 'J'-shaped region inside thesupergiant shell (SGS) LMC4 have been analysed with CCD photometry in B,V passbands. This region consists of 2 coherent strips, one from theeast to the west reaching about 400;pc across the OB superassociationLH77 and another extending about 850;pc from south to north. Thestandard photometric methods yield 25 colour-magnitude diagrams (CMDs)which were used for age determination of the youngest star population byisochrone fitting. The resultant ages lie in the range from 9;Myr to16;Myr without correlation with the distance to the LMC4 centre. Wetherefore conclude that there must have been one triggering event forstar formation inside this great LMC SGS with a diameter of 1.4;kpc. Weconstruct the luminosity function and the mass function of five regionsconsisting of 5 fields to ensure that projection effects don't mask theresults. The slopes lie in the expected range (gamma in [0.22;0.41] andGamma in [-1.3;-2.4] respectively). The greatest values of the slopeoccur in the north, which is caused by the absence of a young,number-dominating star population. We have calculated the rate withwhich supernovae (SNe) have exploded in LMC4, based on the finding thatall stars are essentially coeval. A total of 5--7*10(3) supernovae hasdumped the energy of 10(54.5) ;erg over the past 10;Myr into LMC4, infact enough to tear the original star-forming cloud apart in the timespan between 5 and 8;Myr after the starformation burst. We conclude thatLMC4 can have been formed without a contribution from stochasticself-propagating star formation (SSPSF), although the ring of youngassociations and HiI\ regions around the edge have been triggered by theevents inside LMC4. Based on observations collected at the EuropeanSouthern Observatory (ESO), La Silla, Chile.
| Ultraviolet ages of young clusters in the Magellanic Clouds.
Following a previous investigation on the integrated UV colours ofstellar clusters (Barbero et al. 1990), we study the calibration of theultraviolet colour index C(15-31) in terms of cluster age, usingobservations by the International Ultraviolet Explorer of 29 young andpopulous clusters of the Large Magellanic Cloud (LMC), and of the SmallMagellanic Cloud (SMC). The study is limited to the range of ages5x10^6^ to 8x10^8^yr, which is free from contamination by HorizontalBranch stars. It is shown that in this range of ages the theoreticalsequence C(15-31) vs. age agrees well with the one derived by combiningthe observed colour index C(15-31) with the ages determined viaisochrone fitting to the colour-magnitude diagrams while systematicdifferences, which are discussed on here, exist with respect to the agecalibration by Meurer, Cacciari and Freeman (1990). The present agecalibration C(15-31) vs. log(t), provided in an analytical form, isfinally used to determine the ages of the 29 clusters in our sample,including 13 objects for which no determination was available via theisochrone fitting method.
| Ultraviolet spectral evolution of star clusters in the IUE library.
The ultraviolet integrated spectra of star clusters and H II regions inthe IUE library have been classified into groups based on their spectralappearance, as well as on age and metallicity information from otherstudies. We have coadded the spectra in these groups according to theirS/N ratio, creating a library of template spectra for futureapplications in population syntheses in galaxies. We define spectralwindows for equivalent width measurements and for continuum tracings.These measurements in the spectra of the templates are studied as afunction of age and metallicity. We indicate the windows with a strongmetallicity dependence, at different age stages.
| Ultraviolet interstellar absorption lines in the LMC: Searching for hidden SNRs
Strong x-ray emission detected in Large Magellanic Cloud (LMC)superbubbles has been explained as the result of interior supernovaremnants (SNRs) hitting the dense superbubble shell. Such SNRs cannot befound using conventional criteria. We thus investigate the possibilityof using the interstellar absorption properties in the ultraviolet (UV)as a diagnostic of hidden SNR shocks. The International UltravioletExplorer (IUE) archives provide the database for this pilot study. Theycontain high-dispersion spectra of several stars in x-ray brightsuperbubbles. To distinguish the effects of SNR shocks from those oflocal stellar winds and a global hot halo around the LMC, we includedcontrol objects in different environments. We find that almost allinterstellar absorption properties can be explained by the interstellarenvironment associated with the objects. Summarizing the two mostimportant results of this study: (1) a large velocity shift between thehigh-ionization species (C IV and Si IV) and the low-ionization species(S II, Si II, and C II*) is a diagnostic of hidden SNR shocks; however,the absence of a velocity shift does not preclude the existence of SNRshocks; (2) there is no evidence that the LMC is uniformly surrounded byhot gas; hot gas is preferentially found associated with largeinterstellar structures like superbubbles and supergiant shells, whichmay extend to large distances from the plane.
| 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.
| 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.
| Analysis of the UV spectra of young clusters of the Large Magellanic Cloud
UV and visual spectral energy distributions of young generations havebeen synthesized from evolutionary tracks in the HR diagram and modelatmospheres. The influence of several parameters has also been analyzed.Their UV portions have been checked with the UV spectra of 24 clustersof the LMC obtained by Cassatella et al. (1987). The models generallyagree well with the observations and make it possible to derive ages forvarious assumed abundances. Ages thus derived have been compared, in anage-age diagram, with those obtained from the integrated UBV photometry.While there are no systematic differences between these two agedeterminations, a fraction of clusters displays a large scatter, largerthan what is expected from the observational errors alone. Possiblecauses for this scatter are briefly analyzed.
| X-rays from superbubbles in the Large Magellanic Cloud
Diffuse X-ray emission not associated with known supernova remnants(SNRs) are found in seven Large Magellanic Cloud H II complexesencompassing 10 OB associations: N44, N51D, N57A, N70, N154, N157 (30Dor), and N158. Their X-ray luminosities range from 7 x 10 to the 34thergs/s in N57A to 7 x 10 to the 36th ergs/s in 30 Dor. All, except 30Dor, have simple ring morphologies, indicating shell structures.Modeling these as superbubbles, it is found that the X-ray luminositiesexpected from their hot interiors fall an order of magnitude below theobserved values. SNRs close to the center of a superbubble add verylittle emission, but it is calculated that off-center SNRs hitting theionized shell could explain the observed emission.
| The cluster system of the Large Magellanic Cloud
A new catalog of clusters in the Large Magellanic Cloud has beenconstructed from searches of the IIIa-J component of the ESO/SERCSouthern Sky Atlas. The catalog contains coordinate and diametermeasurements of 1762 clusters in a 25 deg x 25 deg area of sky centeredon the LMC, but excluding the very crowded 3.5 sq deg region around theBar. The distribution of these clusters appears as two superimposedelliptical systems. The higher density inner system extends over about 8deg; the lower density outer system can be represented by a 13 deg x 10deg disk inclined at 42 deg to the line of sight. There are suggestionsof two weak 'arms' in the latter.
| Ultraviolet colors as age indicators for LMC clusters
Empirical correlations are found between log ages and the intrinsicultraviolet colors for 27 LMC clusters. The problems and limitations ofusing these correlations as age indicators for LMC clusters and otherstellar populations are discussed. The correlations are used to estimatethe ages of two LMC clusters of unknown age (NGC 1968 and NGC 1974) andthe nuclei of two nearby blue compact dwarf galaxies (NGC 1705 and NGC5253). For the latter two objects optical- and ultraviolet-based ageestimates are in good agreement.
| The age calibration of integrated ultraviolet colors and young stellar clusters in the Large Magellanic Cloud
Integrated colors in selected far-UV bands are presented for a largesample of Large Magellanic Cloud (LMC) clusters. Theoreticalcalculations of these integrated colors are derived and discussed. Thelocation in the two-color diagram C(18-28), C(15-31) is expected to be asensitive but smooth function of cluster age for ages in the range 5 to800 million yr. Theoretical results appear in very good agreement withthe observed colors of LMC clusters. From this comparison, the gap inthe observed colors is suggested to be caused by the lack of LMCclusters in the range of ages between 200 million to one billion yr. Thetwo-color location of old globulars is discussed, also in connectionwith available data for the M31 clusters.
| The ultraviolet spectra of M31 globular clusters
Ultraviolet spectra of 11 of the brightest globular clusters in M31 showthat some exhibit residual flux below 3000 A, greater than that expectedfrom the bright, evolved stars in the cluster. There seems to be noapparent correlation of the strength of this ultraviolet flux withparameters such as metallicity, U-B color, visual magnitude, X-rayemission, or location within the parent galaxy. However, comparison ofthe ultraviolet colors of the M31 globular clusters with those in theGalaxy and in the Large Magellanic Cloud suggests that the M31 clustersmay contain a high percentage of blue horizontal-branch stars or thatsome clusters could be as young as about 2 x 10 to the 9th yr.
| Integrated UV magnitudes of the Large Magellanic Cloud associations
UV photographs (2600 A, 350 A passband) of the LMC have been obtained bythe S183 experiment during a Skylab mission. The background is estimatedand a method for deriving the integrated fluxes is presented. Theintegrated magnitudes of about 50 associations and isocontours of theirintensities are given, along with the B and V integrated magnitudes of13 associations.
| 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.
| Ultraviolet observations by the IUE of 31 clusters of the Large Magellanic Cloud
Ultraviolet observations of 31 star clusters of the Large MagellanicCloud obtained by the International Ultraviolet Explorer are presented.The clusters, mostly globular, span the range of ages from about 10 tothe 7th to 10 to the 10th yr. The sample includes several young globularclusters, which have no counterpart in the Galaxy. The paper is devotedto the study of the basic properties of the clusters in the ultraviolet,such as the interstellar reddening, the spatial extension in theultraviolet, and the ultraviolet color indices. These data are essentialfor a later study of the evolutionary stage of the clusters. Oneimportant result of this study is that a discontinuity exists in the UVtwo-color diagram, a fact which deserves further investigation, since itmight support the suggestion made by other authors that an active starformation phase took place in the Large Magellanic Cloud starting about100 million yr ago.
| Age determination of extragalactic H II regions
The H II region evolution models of Copetti et al. (1984) were comparedwith observational data of H II regions in the Magellanic Clouds, M 33,M 101 and of 'isolated extragalactic H II regions'. IMF with chi = 3 or2.5 are inconsistent with a large number of H II regions. The moreuniform age distribution of isolated extragalactic H II regions obtainedthrough an IMF with chi = 2 suggests that this value is more realisticthan chi = 1 or 1.5. The H II region age estimates indicate a burst ofstar formation about 5.5 + or - 1.0 10 to the -6th yr ago in the LMC andabout 2.3 + or - 0.9 x 10 to the 6th yr ago in the SMC. The observedforbidden O III/H-beta gradient in M 33 and M 101 must be caused bycolor temperature variation of the radiation ionizing the H II regions.
| Shapley constellation III - A region of self-propagating star formation
The H I dynamics and stellar content of the Shapley III/supergiant loopLMC 4 region are studied. There are three components, one associatedwith the disk H I with a mean velocity of 285 km/s with respect to thelocal standard of rest, and the other two with shells of gas ejectedabove and below the plane of the LMC with an expansion velocity of 36km/s. The disk H I shows a ringlike structure, 1.8 kpc in diameter witha very pronounced central hole, and the H II regions form a ring 1.4 kpcin diameter. Star formation appears to have started approximately 15 Myrago near the center and has propagated outwards at a constant speed of36 km/s. Newly formed stars generate a local overpressure in the ISMwhich drives a shock in the intercloud medium, sweeping over and arounddense H I clouds and so initiating new episodes of star formation.
| 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
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