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Infrared Surface Brightness Fluctuations of Magellanic Star Clusters
We present surface brightness fluctuations (SBFs) in the near-IR for 191Magellanic star clusters available in the Second Incremental and All SkyData releases of the Two Micron All Sky Survey (2MASS) and compare themwith SBFs of Fornax Cluster galaxies and with predictions from stellarpopulation models as well. We also construct color-magnitude diagrams(CMDs) for these clusters using the 2MASS Point Source Catalog (PSC).Our goals are twofold. The first is to provide an empirical calibrationof near-IR SBFs, given that existing stellar population synthesis modelsare particularly discrepant in the near-IR. Second, whereas mostprevious SBF studies have focused on old, metal-rich populations, thisis the first application to a system with such a wide range of ages(~106 to more than 1010 yr, i.e., 4 orders ofmagnitude), at the same time that the clusters have a very narrow rangeof metallicities (Z~0.0006-0.01, i.e., 1 order of magnitude only). Sincestellar population synthesis models predict a more complex sensitivityof SBFs to metallicity and age in the near-IR than in the optical, thisanalysis offers a unique way of disentangling the effects of age andmetallicity. We find a satisfactory agreement between models and data.We also confirm that near-IR fluctuations and fluctuation colors aremostly driven by age in the Magellanic cluster populations and that inthis respect they constitute a sequence in which the Fornax Clustergalaxies fit adequately. Fluctuations are powered by red supergiantswith high-mass precursors in young populations and by intermediate-massstars populating the asymptotic giant branch in intermediate-agepopulations. For old populations, the trend with age of both fluctuationmagnitudes and colors can be explained straightforwardly by evolution inthe structure and morphology of the red giant branch. Moreover,fluctuation colors display a tendency to redden with age that can befitted by a straight line. For the star clusters only,(H-Ks)=(0.21+/-0.03)log(age)-(1.29+/-0.22) once galaxies areincluded, (H-Ks)=(0.20+/-0.02)log(age)-(1.25+/-0.16).Finally, we use for the first time a Poissonian approach to establishthe error bars of fluctuation measurements, instead of the customaryMonte Carlo simulations.This research has made use of the NASA/ IPAC Infrared Science Archive,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under contract with the National Aeronautics and SpaceAdministration.
| 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).
| 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.
| The distribution of stars in three regions in the northeastern part of the Large Magellanic Cloud
B,V-CCD photometry has been obtained for stars in two fields in the NEpart of the Large Magellanic Cloud (LMC) and photographic photometry,and a calibrating CCD sequence, for a third field. The colour-magnitudediagrams (CMDs) and luminosity functions (LFs) for main sequence (MS)and red giant (RG) stars in the fields are established. The red giantclumps (RGCs) are found to be bimodal and to contain stars from one oldpopulation, >= 10 Gyr, and one younger, >= 0.3-4 Gyr old. Analysesof published data for a number of LMC fields reveal similar compositionsof their RGCs. These epochs of star formation are common to most of thestudied LMC fields; only those in or close to now active star-formingregions contain a considerably younger population. Comparisons withrecent HST results for main-sequence stars permit the identification ofstar formation events ~ 1 and 2 Gyr ago. The data are used to estimatethe masses of Population II and the intermediate-age Population I to ~ 2and 7.5*E(8) {M}_ȯ, respectively, meaning that the burst(s) of starformation which occurred between 4 and 0.3 Gyr ago were about 3 to 4times as productive as the original star production in the LMC. Based onObservations carried out at the European Southern Observatory, La Silla,Chile
| Obscured AGB stars in the Magellanic Clouds. I. IRAS candidates
We have selected 198 IRAS sources in the Large Magellanic Cloud, and 11in the Small Magellanic Cloud, which are the best candidates to bemass--loosing AGB stars (or possibly post--AGB stars). We used thecatalogues of \cite[Schwering \& Israel (1990)]{ref42} and\cite[Reid et al. (1990)]{ref36}. They are based on the IRAS pointedobservations and have lower detection limits than the Point SourceCatalogue. We also made cross-identifications between IRAS sources andoptical catalogues. Our resulting catalogue is divided in 7 tables.Table \ref{tab1} lists optically known red supergiants and AGB stars forwhich we found an IRAS counterpart (7 and 52 stars in the SMC and LMC,respectively). Table \ref{tab2} lists ``obscured'' (or ``cocoon'') AGBstars or late-type supergiants which have been identified as such inprevious works through their IRAS counterpart and JHKLM photometry (2SMC and 34 LMC sources; no optical counterparts). Table \ref{tab3} listsknown planetary nebulae with an IRAS counterpart (4 SMC and 19 LMC PNe).Table \ref{tab4} lists unidentified IRAS sources that we believe to begood AGB or post--AGB or PNe candidates (11 SMC and 198 LMC sources).Table~\ref{tab5} lists unidentified IRAS sources which could be any typeof object (23 SMC and 121 LMC sources). Table \ref{tab6} lists IRASsources associated with foreground stars (29 SMC and 135 LMC stars).Table \ref{tab7} lists ruled out IRAS sources associated with HIIregions, hot stars, etc... We show that the sample of IRAS AGB stars inthe Magellanic Clouds is very incomplete. Only AGB stars more luminousthan typically 10^4 L_\odot and with a mass-loss rate larger thantypically 5 10^{-6} M_\odot/yr could be detected by the IRAS satellite.As a consequence, one expects to find very few carbon stars in the IRASsample. We also expect that most AGB stars with intermediate mass--lossrates have not been discovered yet, neither in optical surveys, nor inthe IRAS survey. Tables 1 to 8 are also available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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 study of clusters and field stars in two regions in the Large Magellanic Cloud.II. Colour-magnitude diagrams and luminosity functions.
The stellar populations in two regions of the Large Magellanic Cloud,one in the NW and one in the SW, are analysed to a limiting absolutevisual magnitude of about M_V_=3.5mag Colour-magnitude diagrams arepresented for three faint clusters in each region as well as for thepopulations in their surrounding fields. The clusters in the NW regionhave ages between 0.1 and 0.5Gyr and metallicities close to solarwhereas those in the SW are about 1Gyr old and of lower metallicity.Also the field populations in the two regions have different characters.Both contain strong components of ages around 1 to 3Gyr. In the NWregion a young component, 0.2-0.6Gyr, exists which is completely missingin the SW region. The latter contains a weak but well identifiedgeneration about 7-10Gyr old which may be traced also in the NW region.Comparisons with other regions in the Large Cloud are carried out. TheSW part of the LMC differs from the others by an almost total lack ofyoung generations. The luminosity functions in many parts of the LargeCloud are otherwise rather similar. The generations identified hereconfirm that the initial star formation in the LMC, more than 7Gyr ago,was weak and that a much stronger star production occurred during theperiod 0.5 to 4.0Gyr ago.
| A study of clusters and field stars in two regions in the Large Magellanic Cloud. I. CCD photometry in B and V.
We present CCD photometry in the Johnson BV system to about V=23mag forthe stars in six clusters and the surrounding fields in two regions ofdifferent nature in the Large Magellanic Cloud. The analysis includesestimations of loss of measured stars due to crowding effects.
| Large scale structure of the ionized gas in the magellanic clouds
Calibrated H alpha CCD images taken with the 'Parking Lot Camera' areused to investigate the large scale properties of the ionized gas in theMagellanic Clouds. The integrated H alpha luminosity, star formation,and the large-scale diffuse emission are studied and compared with othergalaxies. The diffuse gas component accounts for 30%-40% of the total Halpha emission in both galaxies. The integrated star formation rates andthe fraction of diffuse emission are typical for Magellanic irregulargalaxies. The H alpha maps are used to derive a set of physicalparameters for known supergiant shells in the Large Magellanic Cloud(LMC) and Small Magellanic Cloud (SMC). The H alpha emissiondistribution have also been compared to the distribution of H I columndensity. Evidence for a star formation threshold is discussed.
| 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.
| Stellar associations in the Large Magellanic Cloud. I. LH 15, LH 47, 48, 49, LH 52, 53, LH 83, LH 91, 95.
Five LMC stellar groups of stellar associations around LH 15, 47, 52,83, 91 have been examined in order to define their boundaries, theirstellar content and their dynamical behaviour. Spectral classificationof individual stars from objective prism spectra and star count indirect plates taken with the 1.2m U.K. Schmidt Telescope in Australiahave provided the observational material for this investigation.Isodensity contour maps and the distribution of spectral types reveals15 associations (some are newly detected) that contain sometimes apartfrom the OB type stars, faint A type stars and late type supergiantsgiving evidence for a continuous star formation. Their central massdensity indicates that all of them are unbound systems with disruptiontimes not longer than their age derived from their stellar population'sevolutionary stage.
| 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 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.
| 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.
| Age calibration and age distribution for rich star clusters in the Large Magellanic Cloud
An empirical relation is presented for estimating the ages of rich starclusters in the Large Magellanic Cloud (LMC), to within a factor ofabout 2, from their integrated UBV colors. The calibration is based onpublished ages for 58 LMC clusters derived from main-sequencephotometry, integrated spectra, or the extent of the asymptotic giantbranches. Using stellar population models, a sample of LMC clusters moremassive than about 10,000 solar masses is isolated, which is correctedfor incompleteness as a function of magnitude. An unbiased agedistribution for three clusters is then determined. The number ofclusters decreases with increasing age in a manner that is qualitativelysimilar to the age distribution for the open clusters in our Galaxy. TheLMC age distribution is, however, flatter, and the median age of theclusters is greater. If the formation rate has been approximatelyconstant over the history of the two galaxies, then the age distributionobtained here implies that clusters are disrupted more slowly in theLMC. The results contain no evidence for bursts in the formation ofclusters, although fluctuations on small time scales and slow variationsover the lifetime of the LMC cannot be ruled out.
| 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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