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The Detection of Protostellar Condensations in Infrared Dark Cloud Cores
Infrared dark clouds (IRDCs) are a distinct class of interstellarmolecular cloud identified as dark extinction features against thebright mid-infrared Galactic background. Here we present high angularresolution millimeter continuum images obtained with the IRAM Plateau deBure Interferometer toward four high-mass (200-1800 Msolar)IRDC cores that show evidence for active high-mass star formation(M>8 Msolar). We detect twelve bright (>7 mJy), compact(<~2", <~0.024 pc) condensations toward these cores. Two of thecores (G024.60+00.08 MM1 and G024.60+00.08 MM2) are resolved intomultiple protostellar condensations, while one core (G022.35+00.41 MM1)shows two condensations. The remaining core (G024.33+00.11 MM1) containsa single, compact protostellar condensation with a very rich molecularspectrum, indicating that this is a hot molecular core associated withan early stage in the formation of a high-mass protostar. The derivedgas masses for these condensations suggest that each core is forming atleast one high-mass protostar (Mgas>8 Msolar),and three cores are also forming lower mass protostars(Mgas~2-5 Msolar). A comparison of the ratios ofthe gas masses (MG) to the Jeans masses (MJ) forIRDCs, cores, and condensations, provides broad support for the idea ofhierarchical fragmentation. The close proximity of multiple protostarsof disparate mass indicates that these IRDCs are in the earliestevolutionary states in the formation of stellar clusters.Based on observations carried out with the IRAM Plateau de BureInterferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany),and IGN (Spain).
| Ages and Metallicities of Extragalactic Globular Clusters from Spectral and Photometric Fits of Stellar Population Synthesis Models
Spectra of galaxies contain an enormous amount of information about therelative mixture of ages and metallicities of constituent stars. Wepresent a comprehensive study designed to extract the maximuminformation from spectra of data quality typical in large galaxysurveys. These techniques are not intended for detailed stellarpopulation studies that use high-quality spectra. We test techniques ona sample of globular clusters, which should consist of single stellarpopulations and provide good test cases, using the Bruzual-Charlothigh-resolution stellar population synthesis models to simultaneouslyestimate the ages and metallicities of 101 globular clusters in M31 andthe Magellanic Clouds. The clusters cover a wide range of ages andmetallicities, 4 Myr
| Structure and Mass Segregation in h and χ Persei
We use V magnitudes and spectral types to examine the density structureof h and χ Per. We describe an automatic method for derivingspectral types and compare classifications for observations made at twodifferent facilities. With these data, we measure an extinction to theclusters of E(B-V)=0.52+/-0.07, consistent with other authors. However,there appears to be a correlation between the spectral types of thestars used and the resulting value of the extinction. We compareextinction values measured by different authors using different numbersof stars and reproduce their values by imposing different cuts in the Vmagnitude. This variation in color excess versus spectral type suggeststhat the standard intrinsic colors for the earliest type stars are bluerthan the stars in h and χ Per. We measure centers for h and χPer at α(2000)=2h18m56.4s+/-3.0s,δ(2000)=57deg8'25''+/-23''and α(2000)=2h22m4.3s+/-2.9s,δ(2000)=57deg8'35''+/-25'',respectively. We fit the density structure of the clusters and find coreradii of 1.9 and 2.4 pc, respectively. Integration of the Miller-Scaloinitial mass function suggests overall cluster masses of 5500 and 4300Msolar and central densities of 27 and 50 Msolarpc-3, respectively. We find strong evidence of masssegregation in h Per but not in χ Per. Examination of the dynamicaltimescales, as well as comparisons between the two clusters, suggestthat the mass segregation is partly primordial.
| 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
| The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. I. Results from 19 OB Associations in the Magellanic Clouds
We combine new CCD UBV photometry and spectroscopy with those from theliterature to investigate 19 Magellanic Cloud OB associations thatcontain Wolf-Rayet (W-R) and other types of evolved, massive stars. Ourspectroscopy reveals a wealth of newly identified interesting objects,including early O-type supergiants, a high-mass, double-lined binary inthe SMC, and, in the LMC, a newly confirmed luminous blue variable (LBV;R85), a newly discovered W-R star (Sk -69°194), and a newly foundluminous B[e] star (LH 85-10). We use these data to provide precisereddening determinations and construct physical H-R diagrams for theassociations. We find that about half of the associations may be highlycoeval, with the massive stars having formed over a short period(Δτ<1 Myr). The (initial) masses of the highest massunevolved stars in the coeval clusters may be used to estimate themasses of the progenitors of W-R and other evolved stars found in theseclusters. Similarly, the bolometric luminosities of the highest massunevolved stars can be used to determine the bolometric corrections(BCs) for the evolved stars, providing a valuable observational basisfor evaluating recent models of these complicated atmospheres. What wefind is the following: (1) Although their numbers is small, it appearsthat the W-R stars in the SMC come from only the highest mass (greaterthan 70 Msolar) stars. This is in accord with ourexpectations that at low metallicities only the most massive andluminous stars will have sufficient mass loss to become W-R stars. (2)In the LMC, the early-type WN (WNE) stars occur in clusters whoseturnoff masses range from 30 to 100 Msolar or more. Thissuggests that possibly all stars with mass greater than 30Msolar pass through a WNE stage at LMC metallicities. (3) Theone WC star in the SMC is found in a cluster with a turnoff mass of 70Msolar, the same as that for the SMC WN stars. In the LMC,the WC stars are found in clusters with turnoff masses of 45Msolar or higher, similar to what is found for the LMC WNstars. Thus we conclude that WC stars come from essentially the samemass range as do WN stars and indeed are often found in the sameclusters. This has important implications for interpreting therelationship between metallicity and the WC/WN ratio found in LocalGroup galaxies, which we discuss. (4) The LBVs in our sample come fromvery high mass stars (greater than 85 Msolar), similar towhat is known for the Galactic LBV η Car, suggesting that only themost massive stars go through an LBV phase. Recently, Ofpe/WN9 starshave been implicated as LBVs after one such star underwent an LBV-likeoutburst. However, our study includes two Ofpe/WN9 stars, BE 381 and Br18, which we find in clusters with much lower turnoff masses (25-35Msolar). We suggest that Ofpe/WN9 stars are unrelated to``true'' LBVs: not all ``LBV-like outbursts'' may have the same cause.Similarly, the B[e] stars have sometimes been described as LBV-like.Yet, the two stars in our sample appear to come from a large mass range(30-60 Msolar). This is consistent with other studies,suggesting that B[e] stars cover a large range in bolometricluminosities. (5) The bolometric corrections of early WN and WC starsare found to be extreme, with an average BC(WNE) of -6.0 mag and anaverage BC(WC4) of -5.5 mag. These values are considerably more negativethan those of even the hottest O-type stars. However, similar valueshave been found for WNE stars by applying Hillier's ``standard model''for W-R atmospheres. We find more modest BCs for the Ofpe/WN9 stars(BC=-2 to -4 mag), also consistent with recent analysis done with thestandard model. Extension of these studies to the Galactic clusters willprovide insight into how massive stars evolve at differentmetallicities.
| 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.
| 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
| A catalogue of compact radio sources in and behind the Large Magellanic Cloud
We present the results of a continuum snapshot survey of a 3 deg X 4 degregion of the Large Magellanic Cloud including the area of the giantmolecular cloud and the 30 Doradus nebula. The observations have beencarried out with the Australia Telescope Compact Array (ATCA) at 1.4 and2.4 GHz. Most fields are complete to about 6 mJy peak flux density at1.4 GHz and to about 3 mJy at 2.4 GHz. The positions, peak and integralflux densities of 113 compact (< 54") sources detected at 1.4 GHz andof 70 sources (<34") detected at 2.4 GHz are presented. Positions areaccurate to about 3" and peak flux densities are accurate to about 10%or better, depending on the source position relative to the pointingcenters. 32 of the sources detected at 1.4 GHz are coincident withHα objects in the catalogue of Davies et al.; these are possiblyintrinsic to the LMC. However, we suppose that most are backgroundobjects, since the number vs. flux agrees with predictions ofextragalactic source counts from other surveys. Tables 3 and 4 are alsoavailable electronically at the CDS via ftp cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html TheAustralia Telescope is funded by the Commonwealth of Australia foroperation as a National Facility managed by CSIRO.
| 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.
| Blue-violet spectral evolution of young Magellanic Cloud clusters
We study the integrated spectral evolution in the blue-violet range of97 blue star clusters in the Magellanic Clouds, from those associatedwith gas emission to those as old as a few hundred Myr. Some clustersare dominated by the flux of those massive stars that pass throughevolutionary stages such as Wolf-Rayet, Luminous Blue Variable, Be, andsupergiant stars of different temperatures. The relationships amongspectral features such as absorption and emission lines, Balmerdiscontinuity and Balmer continuum are used to study the spectralevolution of the clusters. Finally, we sort into groups spectra ofsimilar evolutionary stages, creating a template spectral library withpossible applications in stellar populations syntheses of star-forminggalaxies and in the spectral simulation of bursts of star formation withdifferent mean ages and durations.
| 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.
| 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.
| Indications for common origin and gravitational interaction in three binary LMC clusters
Three close pair clusters of the LMC, NGC 2006/SL 538, NGC 2011 a/b, andNGC 2042 a/b, have been studied in order to establish their binarity.The stellar content in the outer region of each cluster has beeninvestigated by means of low resolution objective UK Schmidt prismspectra, the cores of the clusters have been examined using lowresolution integrated IUE spectra, whereas their density profiles andtheir observed dynamical parameters have been derived by means of starcounts. The integrated spectra of their cores and the stellar content oftheir outer cluster regions have shown a common origin and a very youngage (less than 2 x 10 exp 7 yr) for each member of the pairs, whereastheir dynamical study has shown that they are gravitationallyinteracting. Comparing the age of their stellar content with theirdynamical and relaxation times, it has been found that these clustersare physically associated and had no time to relax by stellarencounters, no time to merge, and no time to be destroyed by dynamicalfriction.
| A catalogue of binary star cluster candidates in the Large Magellanic Cloud
A photographic atlas of close pairs of star clusters in the LargeMagellanic Cloud is presented here. The criterion for inclusion ofcluster pairs in the atlas was an upper limit of 18.7 pc for theprojected separation between the centers of the clusters in each pair.Accurate coordinates for the clusters, the projected separations andestimates of the diameters and positional angles are given and some ofthe global properties of the cluster-pair population of the LMC arediscussed. It is found that the individual clusters in pairspreferentially have nearly equal sizes.
| 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.
| The stellar content of binary star clusters in the LMC
The bright stellar content of fifteen binary star clusters in the LMCwas estimated, using film copies of plates taken with the 1.2 U.K.Schmidt telescope to derive the spectral types of the stars in theregion. It was found that all classified stars are brighter than V =17.5 mag and are situated in the large areas around each pair and in aneighboring field. The binary clusters were of different ages, from 8 x10 to the 6th to greater than 6 x 10 to the 8th yr, indicating that theformation of binary clusters occurred at different stages of theGalaxy's dynamical history. Seven of the pairs, the brightest and themost populous, are young clusters. For these, both members of each pairexhibit the same stellar content.
| Long-period variables in the Large Magellanic Cloud. I - Search and discovery
A search for long-period variables has been made in the bar and southernregions of the LMC using a series of I band UKST plates, resulting inthe discovery of 471 Mira variables and 572 SRa variables. By usingmainly automated methods of determining periods and amplitudes ofvariability, corrections for incompleteness have been estimated. TheMiras show a trend toward larger amplitudes and brighter luminositieswith period, both of which should contribute to increase mass-lossrates. The period distribution falls abruptly longward of about 420 daysand shortward of about 140 days, whereas the corresponding limits in thesolar neighborhood are about 450 and about 220 days, suggestingdifferent histories of star formation in the LMC and the Galaxy. Inparticular, there appear to be relatively more old stars in the LMC.
| 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.
| Binary star clusters in the Large Magellanic Cloud
In a survey of the LMC cluster system, double clusters with acenter-to-center separation of less than 1.3 arcmin (18 pc) have beenidentified. It is inferred that a considerable fraction of these doubleclusters must be binaries since the calculated projection effects canaccount for only 31 of them. This inference is strongly supported by thefact that the ages available for some of the culsters of the sample (asdetermined from UBV photometry) are less than the computed times ofmerger or disruption of the binary cluster system. Furthermore, thespace distribution of these pairs indicates that these clusters belongto a very young or young population.
| The radio continuum of the Large Magellanic Cloud. I. The Sources at 6 cm wavelength
Not Available
| 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
Not Available
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