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ISOCAM mid-infrared spectroscopy and NIR photometry of the HII complex N4 in the Large Magellanic Cloud
Context: We present the analysis of ISOCAM-CVF and the J, H, andKs photometry data for the HII region complex N4 in the LargeMagellanic Cloud (LMC). Aims: The aim is twofold: 1) to study theconnection between the interstellar medium and the star content of thisregion; 2) to investigate the effects of the lower-than-Galacticmetallicity on dust properties. Methods: A dust-features - gas lines -continuum fitting technique on the whole ISOCAM-CVF data cube allows theproduction of images in each single emission and detailed analysis ofdust (both continuum and bands), and ionized gas. The near infrared(NIR) photometry provides, for the first time, information on thestellar content of N4. Results: The mid-infrared (MIR) spectralcharacteristics of N4 are those expected for an HII region complex, i.e.very similar to those observed in galactic HII regions. The images insingle dust feature bands and gas lines clearly show that the HII regioncore is completely devoid of the carriers responsible for the aromaticfeatures (AFs). On the other hand, the ionized gas arises almostcompletely in this dust cavity, where the two main exciting stars of N4are also located. The HII region models from Stasińska (1982,A&AS, 48, 2999) predict an HII region size that corresponds to theobserved size of the dust cavity. We find evidence that the effect oflower-than-Galactic metallicity (although not as extreme as in the caseof LMC) on the carriers responsible for the AFs is not to prevent theirformation or to modify their chemical properties, but to enhance theirdestruction by the high and hard interstellar radiation field. We arguethat this is the dominant process responsible for the absence of AFs inthe HII region core. We show that this mechanism is more efficient onsmaller dust particles/molecules thus affecting the dust-sizedistribution. We argue that effects on dust-size distribution, ratherthan the different dust properties due to a lower metallicity, should betaken into account when analyzing more distant, relativelylow-metallicity galaxies. Finally, the analysis of the stellar contentof N4 reveals 7 stars: 4 reddened O MS stars and three stars withenvelopes. In particular, one of these seems to be an ultra compact HIIregion containing an embedded young stellar object.Based on observations with ISO, an ESA project with instruments fundedby ESA member states (especially the PI countries: France, Germany, theNetherlands and the United Kingdom) and with the participation of ISASand NASA.
| 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
| Chandra Observation of the Magellanic Cloud Supernova Remnant 0454-67.2 in N9
A Chandra observation has defined the extent of the SNR 0454-67.2 in theLMC H II region N9. The remnant has dimension 2.3 arcmin×3.6 arcminand is elongated in the north-south direction. The brightest emissioncomes from a north-south central ridge that includes three brightpatches. There is good agreement between X-ray and [O III] and [S II]morphology. The remnant is old enough so that optical data give moreinformation about dynamics than do the X-ray data. The supernova (SN)energy release was >=5×1050 ergs, and the age is~3×104 yr. There are several unresolved sources nearby,but none are clearly associated with the remnant. The X-ray spectrum issoft and indicates enhanced Fe abundance in the central region,consistent with a Type Ia SN origin, but a Type II origin cannot beruled out.
| Australia Telescope Compact Array Survey of Candidate Ultracompact and Buried H II Regions in the Magellanic Clouds
We present a systematic survey for ultracompact (UC) H II regions in theMagellanic Clouds. Understanding the physics of massive star formation(MSF) is a critical astrophysical problem. The study of MSF began in ourGalaxy with surveys of UC H II regions, but before now this has not beendone for other galaxies. We selected candidates on the basis of theirInfrared Astronomical Satellite (IRAS) colors and imaged them at 3 and 6cm with the Australia Telescope Compact Array. Nearly all of theobserved regions contain compact radio sources consistent with thermalemission. Many of the sources are related to optically visible H IIregions, and often the radio emission traces the youngest and densestpart of the H II region. The luminosity function and number distributionof Lyman continuum fluxes of the compact radio sources are consistentwith standard stellar and cluster initial mass functions. This type ofsystematic assessment of IRAS diagnostics is important for interpretingSpitzer Space Telescope data, which will probe similar physical scalesin nearby galaxies as IRAS did in the Magellanic Clouds.
| A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| 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 physical structure of Magellanic Cloud H II regions. I. Dataset
We present infrared and optical spectroscopic data for 11 H Ii regionsand one Supernova Remnant in the Large and Small Magellanic Cloud. Theinfrared data have been obtained with the Short Wavelength Spectrometerand Long Wavelength Spectrometer on board the Infrared Space Observatoryas part of a Guaranteed Time Program on H Ii regions in Local GroupGalaxies. Aim of this project is to give a new and improved analysis ofthe physical structure of the sample H Ii regions by combining as muchspectral data as possible. A detailed account is given here of thereduction process, and the quality and reliability of the presentedfluxes are discussed. Based on observations with ISO, an ESA projectwith instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA.
| The PAH emission spectra of Large Magellanic Cloud H II regions
A set of ISOPHOT spectra from a sample of H Ii regions in the LargeMagellanic Cloud (LMC) is presented. In all the spectra, emission bandsarising from Polycyclic Aromatic Hydrocarbons (PAHs) are clearlypresent. These features are observed to vary considerably in relativestrength to each other from source to source and even within 30 Doradus.The LMC spectra have been compared with ISO-SWS spectra from Galactic HIi regions and with the ISOCAM observation towards a quiescent molecularcloud in the SMC (Reach et al. \cite{Reach}). A correlation is foundbetween the I7.7/I11.2 versusI6.2/I11.2 and theI8.6/I11.2 versus I6.2/I11.2ratios. A segregation between the sources in the different types ofenvironment (Milky Way - LMC - SMC) is present. Furthermore, within theLMC observations, a clear distinction between 30 Doradus and non-30Doradus pointings is found. We discuss the variations in the relativestrength of the PAH features in view of the different physicalenvironments and highlight the relation with the PAH/dust ratio and theextinction curve. We conclude that 1) the same conditions responsiblefor the observed trends in the relative PAH-feature strengths alsoaffect the carrier of the 2175 Å bump leading to the differencesin strength of the latter, and 2) the molecular structure is the majorcause of the observed variations in the relative strength of the PAHfeatures. In the SMC and 30 Doradus compact PAH species dominate, whilePAHs with an open, uneven structure are the dominant ones in Galactic HIi regions and the non-30 Dor LMC sources. Based on observations withISO, an ESA project with instruments funded by ESA Member States(especially the PI countries: France, Germany, The Netherlands and theUK) and with the participation of ISAS and NASA.
| Mid-infrared imaging and spectrophotometry of N 66 in the SMC with ISOCAM
We present observations with the mid-infrared camera ISOCAM on board theInfrared Space Observatory of the major star-forming region N 66 in theSmall Magellanic Cloud (SMC) and of its surroundings. These observationswere performed with broad filters and Circular Variable Filters giving aspectral resolution of about 40. In addition, CO(2-1) data arepresented, allowing us to identify and study how hot dust relates withthe different phases of the Interstellar Medium (ISM) present in N66.The spectra are dominated by the strong emission of fine-structure line.Monochromatic maps have been made in the [Ne iiI] 15.6 mu m and [S IV]10.5 mu m line. There are significant differences between theirdistributions, due to the effects of density and of shocks. AromaticInfrared Bands (AIBs) are seen at various places in the field but theyare generally faint. They exhibit a variety of shapes and relativeintensities, suggesting that a diversity of carbonaceous materials arepresent. Silicate emission is also clearly visible in the centralcondensation and in a few others and emission from hot small grains(Very Small Grains, VSGs) longward of 10 mu m is present in the wholeregion. All these dust components are heated by the very strong far-UVradiation of the many young, massive stars contained in the region. Theinterstellar radiation field (ISRF) at 1600 Åis >=105 times the ISRF of the solar neighborhood in the peaks ofmid-infrared emission. The relative contributions of these components(AIB carriers, VSGs and silicate grains) to the mid-infrared spectraseem to depend on the intensity and the hardness of the far-UV field. Ingeneral the 15/6.75 mu m intensity ratio is higher than in relativelyquiescent galactic regions (Cesarsky {et al. \cite{NGC7023}, Abergel etal. \cite{Abergel}) but it is not as high as expected for a linearincrease with ISRF. We interpret this behavior as due to the destructionof both AIBs carriers and VSGs in a very high ISRF. Finally severalstars have been detected at 6.75 mu m. Two are red supergiants; theother stars are blue and the IR emission is due to circumstellar matteror to interstellar matter heated by the star. Based on observations withISO, an ESA project with instruments funded by ESA member states(especially the PI countries: France, Germany, the Netherlands and theUnited Kingdom) and with the participation of ISAS and NASA.
| 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.
| Toward an Understanding of the Mid-Infrared Surface Brightness of Normal Galaxies
We report a mid-infrared color and surface brightness analysis of IC 10,NGC 1313, and NGC 6946, three of the nearby galaxies studied under theInfrared Space Observatory Key Project on Normal Galaxies. Images withless than 9" (170 pc) resolution of these nearly face-on, late-typegalaxies were obtained using the LW2 (6.75 μm) and LW3 (15 μm)ISOCAM filters. Although their global flux ratios are similar andtypical of normal galaxies, they show distinct trends of this colorratio with mid-infrared surface brightness. We find thatIν(6.75 μm)/Iν(15 μm)<~1 onlyoccurs for regions of intense heating activity where the continuum risesat 15 μm and where polycyclic aromatic hydrocarbon destruction canplay an important role. The shape of the color-surface brightness trendalso appears to depend, to the second order, on the hardness of theionizing radiation. We discuss these findings in the context of atwo-component model for the phases of the interstellar medium andsuggest that star formation intensity is largely responsible for themid-infrared surface brightness and colors within normal galaxies,whereas differences in dust column density are the primary drivers ofvariations in the mid-infrared surface brightness between the disks ofnormal galaxies.
| Supernova Remnants in the Magellanic Clouds. III. an X-Ray Atlas of Large Magellanic Cloud Supernova Remnants
We have used archival ROSAT data to present X-ray images of 31 supernovaremnants (SNRs) in the Large Magellanic Cloud (LMC). We have classifiedthese remnants according to their X-ray morphologies, into thecategories of shell-type, diffuse face, centrally brightened,point-source-dominated, and irregular. We suggest possible causes of theX-ray emission for each category and for individual features of some ofthe SNRs.
| 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.
| Spectroscopic binaries in the Large Magellanic Cloud.
Not Available
| Ultraviolet observations of stars in the Magellanic Clouds: a historical bridge from the Sun to R 136 and beyond.
Not Available
| Mid-IR mapping of the region of N 4 in the Large Magellanic Cloud with ISOCAM
We present images of the N 4 region in the Large Magellanic Cloudobtained with ISOCAM on board ISO through broad band filters centered at6.75 and 15 mu m. Far from the three H i I regions contained in the map,the emission at both wavelengths is due to the Unidentified InfraredBands and associated continuum and originates in the external layers ofa molecular cloud complex. The ratio between the intensities at 15 and6.75 mu m is =~ 0.6-0.7 comparable to the 0.55-0.85 ratio found in ourGalaxy. Closer to the H i I regions, this ratio increases when theultraviolet radiation density reaches =~ 10(3) times the radiationdensity near the Sun, due to the contribution of very small grains tothe flux near 15 mu m. The emission at both wavelengths is maximum inthe direction of an interface between the main H i I region N 4A and themolecular cloud, a region very similar to the classical interface of M17 in our Galaxy. We have detected at both mid-IR wavelengths theemission of a M supergiant present in the field. Based on observationswith ISO, an ESA project with instruments funded by ESA member states(especially the PI countries: France, Germany, the Netherlands and theUnited Kingdom) and with the participation of ISAS and NASA.
| 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.
| Carbon monoxyde in the Magellanic Clouds.
Not Available
| Supernova Remnants in OB Associations
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....113.1815C
| 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.
| Extinction characteristics of giant HII regions - Star-forming complexes in the galaxies M33, LMC, and NGC 2403
The discrepancies between the extinction of gas emission and that of thestarlight in giant HII regions, star-forming complexes in the galaxiesM33, LMC, and NGC 2403, were empirically investigated. The extinctionvalues were determined for 30 stars in eight associations in M33. Anempirical relation between the extinction of starlight and that of thegas emission in giant HII regions, star-forming complexes in thegalaxies under study, was obtained.
| Classical H II regions in the Magellanic Clouds. 2: Stellar content
In this second in a series of papers on the nature of classical H IIregions in the Magellanic Clouds I investigate the properties of theunderlying stellar content of the nebulae. Particular emphasis is placedon identifying and classifying the ionizing source(s) for each H IIregion. With the exception of the LMC H II regions DEM 20 and DEM 8c, Ifind that all of the objects in this sample are ionized by more than oneO or B star. Even the faintest H II regions reflect the formation of ahandful of massive, albeit early B type, stars. Typically, one staraccounts for 60% - 70% of the ionizing photons and 2 - 5 less massivestars provide the remaining 30% - 40%. From the statistics of thehottest stars in these H II regions, and from considering all the bluestars contained within each region, the distribution of massive starswith spectral type is consistent with results found in similar galacticH II regions.
| The LMC H II region N4A and its unusual molecular cloud
We present a comprehensive study of the large magellanic cloud (LMC) HII region N4A and its associated molecular cloud using a relativelylarge amount of data obtained with various observational techniques. Weinvestigate the physical characteristics of the H II region, inparticular the morphology, density structure, gas temperature, dustdistribution, chemical abundances, etc. A low-brightness filamentarypattern is brought out in N4A. The possible causes of this feature(stellar wind, supernova explosion, magnetic field, molecular cloud) arediscussed. The overall extinction towards N4A is AVapproximately 0.4 mag. However, it amounts to as high as 3.7 mag in thedirection of a compact knot of ionized gas lying in front of the H IIregion. We investigate the physical parameters of the associatedmolecular cloud extensively observed in transitions (12)CO(1-0),(12)CO(2-1) and (13)CO(1-0). The cloud is unusual mainly because theemission ratio (12)CO(2-1)/(12)CO(1-0) is much greater than 1 in itspart close to N4A. This part is probably a thin molecular sheet in frontof the H II region. Moreover, a prominent velocity structure is detectedin the molecular cloud with two main components at VLSR =272.5 and 278.5 km/s. The ionized gas is redshifted with respect to theCO velocity counterpart by as much as 14.6 km/s. A molecular hydrogenmass of approximately 3 x 104 is derived for the maincomponent of the molecular cloud.
| The initial mass function for massive stars in the Magellanic Clouds. 2: Interstellar reddening toward 14 OB associations
We have used UBV CCD photometry to determine the interstellar reddeningtoward 14 OB associations in the Magellanic Clouds. The tworeddening-free indices available in the UBV system were used to obtainthe reddening estimates. The mean color excesses of the associationsrange from E(B-V) = 0.01-0.26 mag in the Large Magellanic Cloud (LMC)and from E(B-V) = 0.06-0.25 mag in the Small Magellanic Cloud (SMC). Wehave modeled the observed scatter in the color excesses of individualstars within the associations and find that statisically significantdifferential reddening exists in at least some of the associations.
| The initial mass function for massive stars in the Magellanic Clouds. 1: UBV photometry and color-magnitude diagrams for 14 OB associations
UBV charge coupled device (CCD) photometry has been obtained for 14 OBassociations in the Magellanic Clouds using the University of Toronto's0.6 m telescope and the Carnegie Institution of Washington's 1.0 mreflector, both on Las Campanas, Chile. The data are presented and usedto construct color-magnitude diagrams for the purposes of investigatingthe massive-star content of the associations.
| Classical H II regions in the Magellanic Clouds. 1: Physical properties
In the first in a series of papers I present a detailed examination ofthe basic physical properties of a sample of classical H II regions inthe Magellanic Clouds. Narrow band H alpha imaging is used to show thatsuch objects are representative of the most common H II regions in theClouds and that they display a wide range of morphologicalcharacteristics. Spectroscopic data show that, as a result of the lowmetal content, classical H II regions in the Clouds are systematicallyhotter than their Galactic counterparts. The data also suggest theexistence of a correlation between the excitation of an H II region andits emission measure. The spectra of one object, DEM 243, indicates thepresence of a low velocity shock passing through this H II region. Ialso discuss the characteristics of the extinction towards these objectsand speculate that it arises from dust local to the H II regionsthemselves.
| Results of the ESO / SEST Key Programme on Co/ in the Magellanic Clouds - Part One - a Survey of Co/ in the Large Magellanic Cloud and the Small Magellanic Cloud
As the first part of the ESO-Swedish SEST Key Programme on CO in theMagellanic Clouds, we have observed ^12^CO J = 1-0 towards 92 positionsin the LMC and 42 positions in the SMC. In the SMC we searched foremission from H II regions, dark clouds and IRAS infrared sources. Thegenerally negative detection rate of non-IRAS sources in the SMC led toan LMC source selection based on the IRAS results. In both galaxies, COwas detected towards the majority of sources observed. We also observed^13^C0 J = 1-0 towards the brighter ^12^CO sources in the LMC (37) andSMC (9). Compared to the strength of CO lines observed in the Milky WayGalaxy with identical linear resolutions, velocity-integrated COemission is weaker by at least a factor of three in the LMC sources andan order of magnitude in the SMC sources. The mean velocity-integratedisotopic intensity ratio I_12_/I_13_ is 12.5 in the LMC and about 15 inthe SMC. Individual ratios range from 8.5 to 20. These isotopicintensity ratios are two to three times higher than those found inGalactic molecular clouds.
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