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Clustered Star Formation in the Small Magellanic Cloud. A Spitzer/IRAC View of the Star-Forming Region NGC 602/N 90
We present Spitzer/IRAC photometry on the star-forming H II region N 90,related to the young stellar association NGC 602 in the Small MagellanicCloud. Our photometry revealed bright mid-infrared sources, which weclassify with the use of a scheme based on templates and models of redsources in the Milky Way, and criteria recently developed from theSpitzer Survey of the SMC for the selection of candidate Young StellarObjects (YSOs). We detected 57 sources in all four IRAC channels in a6.2'×4.8' field of view centered on N 90; 22of these sources are classified as candidate YSOs. We compare thelocations of these objects with the position of optical sources recentlyfound in the same region with high-resolution HST/ACS imaging of NGC 602by Schmalzl and coworkers, and we find that 17 candidate YSOs have oneor more optical counterparts. All of these optical sources areidentified as pre-main-sequence stars, thus indicating ongoing clusteredstar formation events in the region. The positions of the detected YSOsand their related PMS clusters give a clear picture of the current starformation in N 90, according to which the young stellar associationphotoionizes the surrounding interstellar medium, revealing the H IInebula, and triggering sequential star formation events mainly along theeastern and southern rims of the formed cavity of the parental molecularcloud.Research supported by the Deutsche Forschungsgemeinschaft (GermanResearch Foundation).
| The Role of Evolutionary Age and Metallicity in the Formation of Classical Be Circumstellar Disks. I. New Candidate Be Stars in the LMC, SMC, and Milky Way
We present B, V, R, and Hα photometry of eight clusters in theSmall Magellanic Cloud, five in the Large Magellanic Cloud, and threeGalactic clusters and use two-color diagrams (2-CDs) to identifycandidate Be star populations in these clusters. We find evidence thatthe Be phenomenon is enhanced in low-metallicity environments, based onthe observed fractional early-type candidate Be star content of clustersof age 10-25 Myr. Numerous candidate Be stars of spectral types B0-B5were identified in clusters of age 5-8 Myr, challenging the suggestionof Fabregat & Torrejon that classical Be stars should only be foundin clusters at least 10 Myr old. These results suggest that asignificant number of B-type stars must emerge onto the zero-age mainsequence as rapid rotators. We also detect an enhancement in thefractional content of early-type candidate Be stars in clusters of age10-25 Myr, suggesting that the Be phenomenon does become more prevalentwith evolutionary age. We briefly discuss the mechanisms that mightcontribute to such an evolutionary effect. A discussion of thelimitations of using the 2-CD technique to investigate the roleevolutionary age and/or metallicity play in the development of the Bephenomenon is offered, and we provide evidence that other B-type objectsof very different nature, such as candidate Herbig Ae/Be stars, maycontaminate the claimed detections of Be stars via 2-CDs.
| Protostars, Dust Globules, and a Herbig-Haro Object in the LMC Superbubble N51D
Using Spitzer Space Telescope and Hubble Space Telescope observations ofthe superbubble N51D, we have identified three young stellar objects(YSOs) in dust globules and made the first detection of a Herbig-Haroobject outside the Galaxy. The spectral energy distributions of theseYSOs suggest young massive stars with disk, envelope, and outflowcavities. The interstellar conditions are used to assess whether thestar formation was spontaneous or induced by external pressure.
| The Initial Mass Function toward the Low-Mass End in the Large Magellanic Cloud with Hubble Space Telescope WFPC2 Observations
We present V- and I-equivalent Hubble Space Telescope (HST) WFPC2photometry of two areas in the Large Magellanic Cloud: the southern partof the stellar association LH 52, located on the western edge of thesupershell LMC 4, and a field between two associations, which is locatedon the southwestern edge of the shell and accounts for the generalbackground field of the galaxy. The HST WFPC2 observations reachmagnitudes as faint as V=25 mag, much deeper than have been observedearlier in stellar associations in the LMC. We determine the massfunction (MF) for main-sequence stars in the areas. Its slope in bothareas is steeper for stars with masses M<~2 Msolar(-4<~Γ<~-6) than for stars of M>~2 Msolar(-1<~Γ<~-2). Thus, as far as the field of the LMC isconcerned, the MF does not have a uniform slope throughout its observedmass range. The MF of the general field of the LMC was found previouslyto be steeper than the MF of a stellar association for massive starswith M>~5 Msolar. We conclude that this seems to also bethe case toward lower masses down to M~1 Msolar. Our dataallow us to construct the field-subtracted, incompleteness-corrected,main-sequence MF of the southwestern part of the young stellarassociation LH 52, which accounts for the initial mass function (IMF) ofthe system. Its mean slope is found to be comparable to, but moreshallow than, a typical Salpeter IMF (Γ~=-1.12+/-0.24) for massesdown to ~1 Msolar. We found indications that the IMF of theassociation probably is ``top heavy,'' owing to the large number ofintermediate-mass stars in the field of the system, while the generalLMC field is found to be responsible for the low-mass population, withM<~2 Msolar, observed in both fields. This findingsuggests that the local conditions seem to favor the formation of highermass stars in associations, and not in the background field. No evidencefor flattening of the IMF toward the low-mass regime or for a lower masscutoff in the IMF was detected in our data.
| Energy Crisis in the Superbubble DEM L192 (N51D)
Superbubbles surrounding OB associations provide ideal laboratories inwhich to study the stellar energy feedback problem, because the stellarenergy input can be estimated from the observed stellar content of theOB associations, and the interstellar thermal and kinetic energies ofsuperbubbles are well defined and easy to observe. We have used DEML192, also known as N51D, to carry out a detailed case study of theenergy budget in a superbubble, and we find that the expected amount ofstellar mechanical energy injected into the interstellar medium,(18+/-5)×1051 ergs, exceeds the amount of thermal andkinetic energies stored in the superbubble,(6+/-2)×1051 ergs. Clearly, a significant fraction ofthe stellar mechanical energy must have been converted into other formsof energy. The X-ray spectrum of the diffuse emission from DEM L192requires a power-law component to explain the featureless emission at1.0-3.0 keV. The origin of this power-law component is unclear, but itmay be responsible for the discrepancy between the stellar energy inputand the observed interstellar energy in DEM L192.
| The XMM-NEWTON View of the LMC Superbubble N51D
Not Available
| 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.
| Feedback of massive stars on the ISM: a XMM-Newton view of the LMC superbubble N 51D
N 51D (= DEM L 192) appears at first glance as a nearly circular, 120 pcdiameter bubble of ionized gas around the LMC OB association LH 54. Adeeper look reveals a complex web of filaments and deviations fromradial expansion. Using a deep XMM pointing centered on N 51D we findthat diffuse soft X-ray emitting gas fills the whole superbubble asdelineated by the Hα filaments. Contrary to recent findings forgalactic winds, the correlation between Hα and X-ray surfacebrightness is not good. The X-ray spectrum of this diffuse gas cannotbe fitted with the LMC abundance pattern, but implies an overabundanceof at least oxygen and neon, consistent with recent enrichment fromsupernovae Type II. Some indications for enhanced mixing at thebrightest region of the Hα shell and for a beginning outflow ofthe hot gas were also detected.
| A very massive spectroscopic binary in the LH 54 OB association in the Large Magellanic Cloud
We announce the discovery of a new early-type, double-linedspectroscopic binary in the LH 54 OB association in the Large MagellanicCloud. We present a V light curve and radial velocities. We investigatethe possible configurations of the system, concluding that it probablycontains the most massive star measured at the present, with a mass ofthe order of 100 Msolar, while its companion hasapproximately 50 Msolar.
| 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 XMM-NEWTON view of the LMC superbubble N51D.
Not Available
| 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.
| An Empirical Test and Calibration of H II Region Diagnostics
We present spectrophotometry in the 3600-9700 Å region for asample of 39 H II regions in the Galaxy and Magellanic Clouds, for whichindependent information is available on the spectral types and effectivetemperatures of the ionizing stars. The spectra have been used toevaluate nebular diagnostics of stellar temperature, metal abundance,and ionization parameter, and to compare the observed behavior of theline indices with predictions of nebular photoionization models. Weobserve a strong degeneracy between forbidden-line sequences produced bychanges in stellar Teff and metal abundance, which severelycomplicates the application of many forbidden-line diagnostics toextragalactic H II regions. Our data confirm however that the Edmunds& Pagel [O II]+[O III] abundance index and the Vílchez &Pagel η' index provide more robust diagnostics of metalabundance and stellar effective temperature, respectively. A comparisonof the fractional helium ionization of the H II regions with stellartemperature confirms the reliability of the spectral type versusTeff calibration for the relevant temperature rangeTeff<=38,000 K. We use empirical relations between thenebular hardness indices and Teff to reinvestigate the casefor systematic variations in the stellar effective temperatures and theupper initial mass functions of massive stars in extragalactic H IIregions. The data are consistent with a significant softening of theionizing spectra (consistent with cooler stellar temperatures) withincreasing metal abundance, especially for Z<=Zsolar.However, unresolved degeneracies between Z and Teff stillcomplicate the interpretation of this result.
| 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.
| Dust and Stellar Populations in the Large Magellanic Cloud
We present an analysis of line-of-sight extinction measurements obtainedusing data from the Magellanic Clouds Photometric Survey (Zaritsky,Harris, & Thompson), which provides four-filter photometry for millionsof stars in the Large Magellanic Cloud. We find that visual extinctionsare typically larger by several tenths of a magnitude for stars witheffective temperatures greater than 12,000 K than for stars witheffective temperatures between 5500 and 6500 K. Several repercussions ofthis population-dependent extinction are discussed. In particular, LMCdistance measurements that utilize old stellar populations, but useextinctions derived from OB stars, may be biased low. As a specificexample, we show that the LMC distance modulus derived from field redclump stars is revised upward relative to published measurements by ~0.2mag if one uses the extinction measured for a matched stellarpopulation. Conversely, measurements that utilize the youngest stars aresubject to greater, and more variable, extinction leading preferentiallyto results that may be biased high. Population-dependent extinctionaffects the interpretation of color-magnitude diagrams and results in aneffective absorption law that is steeper than that intrinsic to the dustfor unresolved stellar systems. We further explore the relation betweenthe stellar populations and dust by comparing our extinction map to the100 μm image of the region and identifying potential heating sourcesof the dust. We find that although regions of high 100 μm flux areassociated with young stars, young stars are not necessarily associatedwith regions of high 100 μm flux and that ~50% of the 100 μm fluxis emitted beyond the immediate regions of high OB stellar density. Weconclude that 100 μm flux should be used with caution as a starformation tracer, particularly for studies of star formation withingalaxies. Finally, we reproduce the observed extinction variationbetween the hot and cold stellar populations with a simple model of thedistribution of the stars and dust in which the scale height of thecooler stars is much greater than that of the dust (which is twice thatof the OB stars; Harris, Zaritsky, & Thompson).
| 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.
| The fourth catalogue of Population I Wolf-Rayet stars in the Large Magellanic Cloud
The catalogue provides for each of the 134 W-R stars of Population Ipresently known in the Large Magellanic Cloud, accurate equatorialcoordinates, photometric data, spectral classification, binary status,correlation with OB associations and HII regions. The miscellaneousdesignations of the stars are also listed. Although completeness is notpretended, results published during the last decade are highlighted inthe notes given for each individual star. A uniform set of findingcharts is presented. Figures 2 to 12 only in the electronic version athttp://edpsciences.com
| 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.
| Recent Star Formation in Shapley Constellation III in the Large Magellanic Cloud
We present UBV photometry of four fields within Shapley ConstellationIII and one field on the edge of the shell. Our fields cover roughly 20%of the region, mostly in the southern half. Determinations are made ofages of the fields, the star formation densities, and the initial massfunction (IMF) slopes. The field-age determinations inside theconstellation show ages between 12 and 16 Myr uncorrelated with distancefrom the center, while the age of the field on the edge of theconstellation shows an age of around 6-7 Myr. The southern part of theconstellation shows star formation densities and IMF slopes typical ofOB associations and giant H ii regions, while the northern part showssignificantly fewer intermediate-mass stars and a steeper IMF slope. Wecompare these properties of Constellation III with those of 30 Doradus,another LMC star-forming region of comparable size to Constellation III.Although the regions formed from roughly the same amount of gas, weestimate that 30 Doradus formed a few times more stars thanConstellation III.
| Shell Formation and Star Formation in Superbubble DEM 192
Was star formation in the OB associations, LH 51 and LH 54, triggered bythe growth of the superbubble DEM 192? To examine this possibility, weinvestigate the stellar contents and star formation history and modelthe evolution of the shell. H-R diagrams constructed from UBV photometryand spectral classifications indicate highly coeval star formation, withthe entire massive star population having an age of <~2-3 Myr.However, LH 54 is constrained to an age of ~3 Myr by the presence of aW-R star, and the initial mass function (IMF) for LH 51 suggests a lowermass limit implying an age of 1-2 Myr. There is no evidence of anearlier stellar population to create the superbubble, but the modeledshell kinematics are consistent with an origin due to the strongeststellar winds of LH 54. It might therefore be possible that LH 54created the superbubble, which in turn may have triggered the creationof LH 51. Within the errors, the spatial distribution of stellar massesand IMF appear uniform within the associations. We reinvestigate theestimates for stellar wind power L_w(t) during the H-burning phase andnote that revised mass-loss rates yield a significantly different formfor L_w(t) and may affect stellar evolution timescales. We also modelsuperbubble expansion into an ambient medium with a sudden,discontinuous drop in density and find that this can easily reproducethe anomalously high-shell expansion velocities seen in manysuperbubbles.
| 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.
| LMC HII region luminosities versus observed ionizing stars
We use the stellar census of OB associations in the Large MagellanicCloud (LMC) to predict the H-alpha luminosities of the host HII regions,based on results from stellar atmosphere models. These values arecompared to the observed HII region luminosities, yielding an estimatefor the mean fraction of H-ionizing photons that escape the localnebulae in this sample. We formally estimate that, overall, 0% to 51% ofthe ionizing radiation escapes the local HII regions and is available toionize the warm, ionized medium in the LMC. We find both nebulae thatappear to be density-bounded, and ones that appear to beradiation-bounded.
| 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.
| Comparison of H II region luminosities with observed stellar ionizing sources in the Large Magellanic Cloud
We estimate the total predicted Lyman continuum emission rates of OBassociations for which the complete census of O star spectral typesexists. The results are compared to the observed H-alpha luminosities ofthe host H II regions. We find evidence for substantial leakage ofionizing photons from some H II regions, while others appear to beradiation-bounded. We estimate that overall for the LMC, 0-51 percent ofthe ionizing radiation escapes the local nebulae, and would be availableto ionize the diffuse, warm, ionized medium (WIM) in that galaxy. Thisrange of values is consistent with the observed 35 percent fraction ofH-alpha luminosity emitted by the WIM in the LMC, as well as thecorresponding fractions observed in other nearby galaxies. It istherefore possible that photoionization by O stars is indeed thedominant ionization mechanism for the WIM.
| Supernova Remnants in OB Associations
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....113.1815C
| UBV Photometry of OB Associations within Superbubbles of the Large Magellanic Cloud
This work presents UBV photometry of the stellar populations associatedwith seven superbubble nebulae and five classical H II regions in theLarge Magellanic Cloud. Although the nebular morphology of thesuperbubbles appears to be substantially evolved compared to theclassical nebulae, the color-magnitude diagrams do not reveal anynoticeable correlation between the resident stellar population andnebular morphology. The photometry presented here will be used in aforthcoming paper to examine further the stellar content and dynamics ofthese superbubbles.
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