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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
| An empirical calibration of sulphur abundance in ionised gaseous nebulae
We have derived an empirical calibration of the abundance of S/H as afunction of the S{23} parameter, defined using the bright sulphur linesof [SII] and [SIII]. Contrary to the case for the widely used O{23}parameter, the calibration remains single valued up to the abundancevalues observed in the disk HII regions. The calibration is based on alarge sample of nebulae for which direct determinations of electrontemperatures exist and the sulphur chemical abundances can be directlyderived. ICFs, as derived from the [SIV] 10.52 μ emission line (ISOobservations), are shown to be well reproduced by Barker's formula for avalue of α = 2.5. Only about 30% of the objects in the samplerequire ICFs larger than 1.2. The use of the proposed calibration opensthe possibility of performing abundance analysis with red to IRspectroscopic data using S/H as a metallicity tracer.
| A 12CO J = 4-->3 High-Velocity Cloud in the Large Magellanic Cloud
We present Antarctic Submillimeter Telescope and Remote Observatoryobservations of 12CO J=4-->3 and 12[C I]emission in the 30 Doradus complex in the Large Magellanic Cloud. Wedetected strong 12CO J=4-->3 emission toward R140, amultiple system of Wolf-Rayet stars located on the rim of the expandingH II shell surrounding the R136 cluster. We also detected ahigh-velocity gas component as a separate feature in the 12COJ=4-->3 spectrum. This component probably originates from molecularmaterial accelerated as a result of the combined motion induced by thestellar winds and explosions of supernovae, including severalfast-expanding H II shells in the complex. The lower limit on the totalkinetic energy of the atomic and molecular gas component is~2×1051 ergs, suggesting that this comprises only 20%of the total kinetic energy contained in the H II complex structure.
| Antarctic Submillimeter Telescope and Remote Observatory Observations of 12CO J=4-->3 Emission from the N44 Complex in the Large Magellanic Cloud
We present Antarctic Submillimeter Telescope and Remote Observatory(AST/RO) observations of 12CO J=4-->3 and [C I] emissionin the N44 H II complex in the Large Magellanic Cloud. We detectedstrong 12CO J=4-->3 emission toward the H II region knownas N44BC, which is located on the rim of an expanding giant shell in theN44 region. Analysis with a photodissociation region model showed thatthe 12CO J=4-->3 emitting cloud is very dense, withn0~105 cm-3. We also note that there isa high-velocity component associated with the 12CO J=4-->3emission. This probably originates from molecular material acceleratedas a result of the motion induced by the expanding giant shellsurrounding LH 47 in the N44 complex. We found that the kinetic energyof this high-velocity gas observed in the 12CO J=4-->3emission toward the rim of the expanding H II shell is at least a factorof 4 higher than that derived for the H I and H II gas in this region.
| 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.
| WR bubbles and He II emission
We present the very first high quality images of the He Ii lambda 4686emission in three high excitation nebulae of the Magellanic Clouds. Afourth high excitation nebula, situated around the WR star BAT99-2, wasanalysed in a previous letter. Using VLT FORS data, we investigate themorphology of the ring nebulae around the early-type WN stars BAT99-49& AB7. We derive the total He Ii fluxes for each object and comparethem with the most recent theoretical WR models. Whilst the ionizationof the nebula around BAT99-49 can be explained by a WN star oftemperature 90-100 kK, we find that the He Ii emission measure of thenebula associated with AB7 requires an He+ ionizing fluxlarger than predicted for the hottest WN model available. Using Hα, [O I]ii and He I lambda 5876 images along with long-slit spectroscopy,we investigate the physical properties of these ring nebulae and findonly moderate chemical enrichment.We also surveyed seven other LMC WR stars but we failed to detect any HeIi emission. This holds also true for BAT99-9 which had been proposed toexcite an He Ii nebula. Four of these surveyed stars are surrounded by aring nebula, and we use the FORS data to derive their chemicalcomposition: the nebula around BAT99-11 shows a N/O ratio and an oxygenabundance slightly lower than the LMC values, while the nebula aroundBAT99-134 presents moderate chemical enrichment similar to the one seennear BAT99-2, 49 and AB7. Comparing the WR stars of the LMC, BAT99-2 and49 appear unique since similar stars do not reveal high excitationfeatures.The third high excitation nebula presented in this paper, N44C, does notharbor stars hotter than mid-O main sequence stars. It was suggested tobe a fossil X-ray nebula ionized by the transient LMC X-5. Ourobservations of N44C reveal no substantial changes in the excitationcompared to previous results reported in the literature. Therefore, weconclude that either the recombination timescale of the X-ray nebula hasbeen underestimated or that the excitation of the nebula is produced byanother, yet unknown, mechanism.Based on observations collected at the European Southern Observatory,Cerro Paranal, Chile (ESO No. 68.C-0238(A,B)).
| 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.
| Structure and Dynamics of Candidate O Star Bubbles in N44
Dynamical studies of superbubbles and Wolf-Rayet ring nebulae showdiscrepancies from the standard adiabatic model for windblown bubbles.We therefore study the physical properties and kinematics of threecandidate bubbles blown by single O stars to evaluate whether thesediscrepancies are also found in these simpler objects. Our samplecandidates are N44 F, N44 J, and N44 M, in the outskirts of the H IIcomplex N44 in the Large Magellanic Cloud. We have obtained ground-basedand Hubble Space Telescope emission-line images and high-dispersionechelle spectra for these objects. From the Hα luminosities andthe [O III]/Hα ratios of these nebulae, we estimate the spectraltypes of the ionizing stars to be O7 V, O9.5 V, and O9.5 V for N44 F,N44 J, and N44 M, respectively. We find that the observed expansionvelocity of 12 km s-1 for N44 F is consistent with thestellar wind luminosity expected from the central ionizing star, aspredicted by the standard bubble model. The observed upper limits forthe expansion velocities of N44 J and N44 M are also compatible with theexpected values, within the uncertainties. We also report the discoveryin N44 F of strongly defined dust columns, similar to those seen in theEagle Nebula. The photoevaporation of these dense dust features may bekinematically important and may actually govern the evolution of theshell. The inclusion of photoevaporation processes may thus underminethe apparent agreement between the observed bubble dynamics and thesimple adiabatic models.
| The Effects of Dust in Simple Environments: Large Magellanic Cloud H II Regions
We investigate the effects of dust on Large Magellanic Cloud (LMC)H II region spectral energy distributions usingarcminute-resolution far-ultraviolet (FUV), Hα, far-infrared(FIR), and radio images. Widely used indicators of the amount of lightlost to dust (attenuation) at Hα and in the FUV correlate witheach other, although often with substantial scatter. There are twointeresting systematic discrepancies: First, Hα attenuationsestimated from the Balmer decrement are lower than those estimated fromthe Hα-to-thermal radio luminosity ratio. Our data, at this stage,cannot unambiguously identify the source of this discrepancy. Second,the attenuation at 1500 Å and the UV spectral slope, β,correlate, although the slope and scatter are substantially differentfrom the correlation first derived for starbursting galaxies by Calzettiet al. Combining our result with those of Meurer et al. forultraluminous infrared galaxies and Calzetti et al. for starburstinggalaxies, we conclude that no single relation between β and 1500Å attenuation is applicable to all star-forming systems.
| A CO Survey of the LMC with NANTEN: III. Formation of Stellar Clusters and Evolution of Molecular Clouds
In order to elucidate star formation in the LMC, we made a completestudy of CO clouds with NANTEN. In the present paper, we compare 55giant molecular clouds (GMCs), whose physical quantities were welldetermined, with young objects, such as young stellar clusters and HIIregions. We find that the GMCs are actively forming stars and clusters;23 and 40 are found to be associated with the clusters and the HIIregions, respectively. The clusters associated with the GMCs aresignificantly young; ~ 85% of them are younger than ~ 10 Myr. Inaddition, compact groups of the young clusters are often found at thepeak position of the GMCs, e.g., N 159 and N 44, while much loosergroups are away from the GMCs. This suggests that the clusters areformed in groups and disperse as they become old. The distributions ofthe CO, [CII], and UV indicate that the GMCs are likely to be rapidlydissipated within several Myr due to UV photons from the clusters. Wealso estimate the evolutionary time scale of the GMCs; they form starsin a few Myr after their birth, and form clusters during the next fewMyr, and are dissipated in the subsequent few Myr.
| 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.
| The HE II Emitting Nebula N44C in the Large Magellanic Cloud: Optical/Ultraviolet Spectroscopy of the Nebula and Its Ionizing Star
We present HST spectroscopy and imaging, along with new ground-basedspectroscopy and ROSAT HRI imaging, of the He II emitting nebula N44Cand its ionizing star. A GHRS spectrogram of the ionizing star yields aspectral type of about O7 for the star. The lack of P Cygni profiles forSi IV and C IV indicates that the star is not a supergiant. The nebularabundances in the ionized gas are consistent with average abundances forLMC H II regions, with the possible exception that nitrogen may beenhanced. Enrichment by a former evolved companion star is not evident.A long-slit echelle spectrogram in Hα+[N II] shows no evidence forhigh-velocity gas in N44C. This rules out high-velocity shocks as thesource of the nebular He II emission. A 108 ks ROSAT HRI image of N44Cshows no X-ray point source to a 3 σ upper limitLX<1034 ergs s-1 in the 0.1-2.0 keVband. Based on new measurements of the electron density in the He IIemitting region, we derive recombination timescales of ~20 yr forHe+2 and ~4 yr for Ne+4. If N44C is a fossil X-rayionized nebula, this places severe constraints on when the putativeX-ray source could have turned off. The presence of strong [Ne IV]emission in the nebula is puzzling if the ionizing source has turnedoff. It is possible the system is related to the Be X-ray binaries,although the O star in N44C does not show Be characteristics at thepresent time. Monitoring of X-rays and He II emission from the nebula,as well as a radial velocity study of the ionizing star, are needed tofully understand the emission line spectrum of N44C. Based in part onobservations with the NASA/ESA Hubble Space Telescope obtained at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy, Inc., under NASA contractNAS5-26555.
| 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.
| The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. I. Results from 19 OB Associations in the Magellanic Clouds
We combine new CCD UBV photometry and spectroscopy with those from theliterature to investigate 19 Magellanic Cloud OB associations thatcontain Wolf-Rayet (W-R) and other types of evolved, massive stars. Ourspectroscopy reveals a wealth of newly identified interesting objects,including early O-type supergiants, a high-mass, double-lined binary inthe SMC, and, in the LMC, a newly confirmed luminous blue variable (LBV;R85), a newly discovered W-R star (Sk -69°194), and a newly foundluminous B[e] star (LH 85-10). We use these data to provide precisereddening determinations and construct physical H-R diagrams for theassociations. We find that about half of the associations may be highlycoeval, with the massive stars having formed over a short period(Δτ<1 Myr). The (initial) masses of the highest massunevolved stars in the coeval clusters may be used to estimate themasses of the progenitors of W-R and other evolved stars found in theseclusters. Similarly, the bolometric luminosities of the highest massunevolved stars can be used to determine the bolometric corrections(BCs) for the evolved stars, providing a valuable observational basisfor evaluating recent models of these complicated atmospheres. What wefind is the following: (1) Although their numbers is small, it appearsthat the W-R stars in the SMC come from only the highest mass (greaterthan 70 Msolar) stars. This is in accord with ourexpectations that at low metallicities only the most massive andluminous stars will have sufficient mass loss to become W-R stars. (2)In the LMC, the early-type WN (WNE) stars occur in clusters whoseturnoff masses range from 30 to 100 Msolar or more. Thissuggests that possibly all stars with mass greater than 30Msolar pass through a WNE stage at LMC metallicities. (3) Theone WC star in the SMC is found in a cluster with a turnoff mass of 70Msolar, the same as that for the SMC WN stars. In the LMC,the WC stars are found in clusters with turnoff masses of 45Msolar or higher, similar to what is found for the LMC WNstars. Thus we conclude that WC stars come from essentially the samemass range as do WN stars and indeed are often found in the sameclusters. This has important implications for interpreting therelationship between metallicity and the WC/WN ratio found in LocalGroup galaxies, which we discuss. (4) The LBVs in our sample come fromvery high mass stars (greater than 85 Msolar), similar towhat is known for the Galactic LBV η Car, suggesting that only themost massive stars go through an LBV phase. Recently, Ofpe/WN9 starshave been implicated as LBVs after one such star underwent an LBV-likeoutburst. However, our study includes two Ofpe/WN9 stars, BE 381 and Br18, which we find in clusters with much lower turnoff masses (25-35Msolar). We suggest that Ofpe/WN9 stars are unrelated to``true'' LBVs: not all ``LBV-like outbursts'' may have the same cause.Similarly, the B[e] stars have sometimes been described as LBV-like.Yet, the two stars in our sample appear to come from a large mass range(30-60 Msolar). This is consistent with other studies,suggesting that B[e] stars cover a large range in bolometricluminosities. (5) The bolometric corrections of early WN and WC starsare found to be extreme, with an average BC(WNE) of -6.0 mag and anaverage BC(WC4) of -5.5 mag. These values are considerably more negativethan those of even the hottest O-type stars. However, similar valueshave been found for WNE stars by applying Hillier's ``standard model''for W-R atmospheres. We find more modest BCs for the Ofpe/WN9 stars(BC=-2 to -4 mag), also consistent with recent analysis done with thestandard model. Extension of these studies to the Galactic clusters willprovide insight into how massive stars evolve at differentmetallicities.
| The Low End of the Initial Mass Function in Young Large Magellanic Cloud Clusters. I. The Case of R136
We report the result of a study in which we have used very deepbroadband V and I Wide Field Planetary Camera 2 images of the R136cluster in the Large Magellanic Cloud from the Hubble Space Telescopearchive to sample the luminosity function below the detection limit of2.8 Msolar previously reached. In these new deeper images, wedetect stars down to a limiting magnitude of mF555W=24.7 (~=1mag deeper than previous works) and identify a population of red starsevenly distributed in the surrounding of the R136 cluster. A comparisonof our color-magnitude diagram with recently computed evolutionarytracks indicates that these red objects are pre-main-sequence stars inthe mass range 0.6-3 Msolar. We construct the initial massfunction (IMF) in the 1.35-6.5 Msolar range and find that,after correcting for incompleteness, the IMF shows a definite flatteningbelow ~=2 Msolar. We discuss the implications of this resultfor the R136 cluster and for our understanding of starburst galaxyformation and evolution in general. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by AURA for NASA under contract NAS5-26555, and observations obtained at the European Southern Observatory,La Silla.
| 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.
| Distribution of stellar mass in young star clusters of our Galaxy and nearby galaxies
Stellar mass distribution in young star clusters of our Galaxy, theMagellanic Clouds and the nearby local groups of galaxies has been usedto investigate the universality of initial mass function and presence ofmass segregation in these systems. There is no obvious dependence of theMF slope on either galactocentric distance or age of the galactic openstar clusters. A comparison of initial mass function slopes that havebeen measured in star clusters and associations of our and nearbygalaxies indicates that the slope is independent of the spatialconcentration of the star formed, galactic characteristics includingmetallicity, and at least down to 0.85 M?, the stellar mass range.Effects of mass segregation have been observed in good number of youngstellar groups of our Galaxy and Magellanic Clouds. As their ages aremuch smaller than their dynamical evolution times, star formationprocesses seems to be responsible for the observed mass segregation inthem.
| Hubble Space Telescope Wide Field Planetary Camera 2 Imaging of Shocks in Superbubbles
Bright X-ray emission has been detected in superbubbles in the LargeMagellanic Cloud (LMC), and it is suggested that supernova remnants(SNRs) near the inner-shell walls are responsible for this X-rayemission. To identify SNR shocks in superbubble interiors, we haveobtained Hubble Space Telescope Wide Field Planetary Camera 2emission-line images of the X-ray-bright superbubbles DEM L152 and DEML192 and the X-ray-dim superbubble DEM L106. We use these images toexamine the shell morphology and [S II]/Hα ratio variations indetail. Of these three superbubbles, DEM L152 has the highest X-raysurface brightness, the most filamentary nebular morphology, the largestexpansion velocity (~40 km s-1), and the highest [SII]/Hα ratio (0.4-0.6). Its [S II]/Hα ratio increasesoutward and peaks in sharp filaments along the periphery. DEM L192 has amoderate X-ray surface brightness, a complex but not filamentarymorphology, a moderate expansion velocity (35 km s-1), and alow [S II]/Hα ratio (~0.15). DEM L106 is not detected in X-rays.Its shell structure is amorphous and has embedded dusty features; itsexpansion velocity is less than 10 km s-1. None of the threesuperbubbles show morphological features in the shell interior that canbe identified as directly associated with SNR shocks, indicating thatthe SNR shocks have not encountered very dense material. We find thatthe [S II]/Hα ratios of X-ray-bright superbubbles are stronglydependent on the UV radiation field of the encompassed OB associations.Therefore, a tight correlation between [S II]/Hα ratio and X-raysurface brightness in superbubbles should not exist. We also find thatthe filamentary morphologies of superbubbles are associated with largeexpansion velocities and bright X-ray emission.
| An empirical calibration of nebular abundances based on the sulphur emission lines
We present an empirical calibration of nebular abundances based on thestrong emission lines of [Sii] and [Siii] in the red part of thespectrum through the definition of a sulphur abundance parameterS23. This calibration presents two important advantagesagainst the commonly used one based on the optical oxygen lines: itremains single-valued up to abundances close to solar and is almostindependent of the degree of ionization of the nebula.
| 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.
| Molecular abundance variations in the Magellanic Clouds
We have observationally studied the effect of metallicity andfar-ultraviolet (FUV) radiation on the physical conditions and themolecular abundances in interstellar clouds in the Small and the LargeMagellanic Clouds (SMC and LMC, respectively). Spectral line emissionfrom a number of molecules was observed in a sequence of clouds withpositions in and between the 30 Doradus (30Dor) and the southern part ofthe N159 region in the LMC, and in one cloud (N27, also denoted LIRS 49)located in the SMC bar. Physical conditions and molecular abundanceswere estimated from the observational data by excitation and radiativetransfer calculations. A comparison of the molecular abundances inclouds in the SMC, the LMC, and the Galaxy is presented. We also reportthe first detection of hydrogen sulphide (ortho-H2S) in anextragalactic source, detections of methanol (CH3OH) inthermal emission and methyl acetylene (CH3CCH), and atentative detection of thio-formaldehyde (H2CS) in N159W. Theabundances (relative to H_2) of molecular species (except CO) in the LMCsources and in N27 are estimated to be typically 5x10(-10) , and1x10(-10) , respectively. These values apply to the gas volume definedby the CO line emission. Relative to Galactic clouds, the abundances inN159W (our reference cloud) are five to twenty times lower. In two ofthe clouds: N27 and the centremost cloud in 30Dor (30Dor-10), thederived abundances deviate significantly from those in the other cloudsin our sample, by being on the average six and eight times lower,respectively. In N27, the most likely explanation is the lowermetallicity in the SMC, whereas the underabundance in 30Dor-10 isprobably mainly caused by a more rapid photodissociation due to the moreintense FUV radiation in this area. An alternative explanation for theunderabundances in both N27 and 30Dor-10 would be a higher H/H_2 ratioinside these molecular clouds. The ethynyl radical (C_2H), with anestimated average abundance of 5x10(-9) in seven clouds in the LMC and3x10(-9) in two clouds in the SMC, is the most abundant observed tracemolecule after the CO isotopomers. Qualitatively, the high C_2Habundance can be explained as reflecting the C(+) -rich and FUVphoton-rich environment, i.e., a chemistry characteristic forphoton-dominated regions. For N27 we have, using HCO(+) and H(13) CO(+)data, estimated the gas-phase (12) C/(13) C ratio to be 40-90, a rangethat encompasses the values found in N159W and in Galactic disc clouds.In all clouds in our sample, the number density estimates from anexcitation analysis of CS, SO, HCO(+) , HCN and H_2CO are in the range(1-100)*E(4) cm(-3) . CO data gives a lower limit of a few*E(3) cm(-3) .However, the average densities (estimated from the virial mass) aresignificantly lower, typically a fewx10(2) cm(-3) , suggesting that theclouds (as probed by trace molecules) are very clumpy withvolume-filling factors <<1. In N159W, where our data-base is byfar most extensive, the number density and the kinetic temperature ofmolecular hydrogen in the dense part of the gas are estimated to be(1-10)*E(5) cm(-3) and 25+/-10 K, respectively. The correspondingnumbers in N27 are, although based on less data than in N159W,(5-50)x10(4) cm(-3) and 15+/-5 K. Thus, the metallicity differencebetween the LMC and the SMC does not seem to affect the density and thetemperature of the gas dramatically. In the SMC, the CO(J=1-0)/HCO(+)(J=1-0) line intensity ratio follows the same trend with respect to thestar-formation activity as in the LMC: a lower ratio is found in cloudswith a more vigorous star-formation activity. A similar trend is alsoexhibited by the CO(J=1-0)/C_2H(N=1-0) line intensity ratio in the LMC.Based on observations using the Swedish-ESO Submillimetre Telescope(SEST) at the European Southern Observatory (ESO), La Silla, Chile.Figs. 3-14 are only available in the electronic version of this paper.
| 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 Multiphase Medium in the Interstellar Complex N44
We have obtained high-resolution H I observations of N44, one of thelargest H II complexes in the Large Magellanic Cloud. The distributionand internal motions of the H I gas show dynamic effects of fast stellarwinds and supernova blasts. Numerous H I holes are detected, with themost prominent two corresponding to the optically identifiedsuperbubbles Shell 1 and Shell 2. The H I gas associated with Shell 1shows an expansion pattern similar to that of the ionized gas shell, butthe mass and kinetic energy of the H I shell are 3-7 times those of theionized gas shell. The total kinetic energy of the neutral and ionizedgas of Shell 1 is still more than a factor of 5 lower than expected in apressure-driven superbubble. It is possible that the central OBassociation was formed in a molecular cloud, and a visible superbubblewas not fully developed until the ambient molecular gas had beendissociated and cleared away. This hypothesis is supported by theexistence of a molecular cloud toward N44 and the fact that the apparentdynamic age of the superbubble Shell 1 is much shorter than the age ofits OB association LH 47. Accelerated H I gas is detected at SNR0523-679. The mass and kinetic energy in the associated H I gas are alsomuch higher than those in the ionized gas of SNR 0523-679. Studies ofinterstellar gas dynamics using ionized gas alone are clearlyinadequate; neutral gas components must be included.
| 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.
| The C^18O/C^17O ratio in the Large Magellanic Cloud
We report detections of J=3D2-1 line emission from the carbon monoxideisotopomers (13) CO, C(18) O and C(17) O in the molecular clouds N159W,N113, N44BC, and N214DE in the Large Magellanic Cloud (LMC). (13) CO andC(18) O lines were observed in two additional clouds: N159S in the LMCand N27 in the Small Magellanic Cloud (SMC). While (13) CO was detectedin both of them, only upper limits to the C(18) O line emission wereobtained. Statistical-equilibrium excitation and radiative transfercalculations were made to infer molecular column densities from theobserved line intensities. We estimate an average gas-phase C(18)O/C(17) O abundance ratio of 1.6+/-0.3 in the LMC. This is significantlylower than typical values found in Galactic clouds (by a factor of two)and in centres of starburst galaxies (by a factor of five). We use theC(18) O/C(17) O abundance ratio as a measure of the elemental (18)O/(17) O abundance ratio. Provided that current theories of thenucleosynthesis involving (17,18) O apply, then the low (18) O/(17) Oratio suggests that massive stars have contributed little to the metalenrichment of the interstellar medium in the LMC in the past. This maybe caused by a steep initial mass function (which appears to be the casefor field stars in the Magellanic Clouds and in the Galaxy) togetherwith a low average star-formation rate. This explanation contrasts withthe present situation in prominent star-formation regions in the LMC,such as 30 Doradus, which form stars at a considerable rate and appearto have initial mass functions similar to star clusters in the Galaxy.The apparent spatial constancy of the (18) O/(17) O abundance ratio, thenominal values for the individual clouds vary between 1.6 and 1.8,indicates a well mixed interstellar medium and/or that thestar-formation activity took place globally in the LMC in the past. Inthe SMC we obtained a lower limit of 17 for the (13) CO/C(18) O ratio(the LMC average is 30), possibly indicating a low (18) O abundance hereas well. Our data suggests a correlation between the (18) O/(17) Oabundance ratio and the metallicity. The high (18) O/(17) O abundanceratio in centres of starburst galaxies could reflect a high metallicity,mainly caused by a high star-formation rate, possibly but notnecessarily together with an initial mass function biased towardsmassive stars.
| Extinction of H II regions in the Large Magellanic Cloud
The extinction properties of H II regions in the Large Magellanic Cloudare investigated using radio continuum data obtained from the MolongloObservatory Synthesis Telescope, digitized and calibrated H-alpha data,and published Balmer decrement measurements. The resultingextinction-color excess diagram suggests that (1) most H II regions inthe Magellanic Clouds have similar extinction properties to the Galacticones, (2) all imaginable gas/dust configurations are possible, and (3)the extinction of some highly reddened H II region cores originatesexternally in cocoon shells. The puzzle of different extinction-colorexcess ratios of Galactic and extragalactic H II regions is explained asbeing due to the different populations of observed samples rather thanany intrinsic differences. The extinction of the observed Galactic H IIregions produced by foreground dust overwhelms the internal extinction,while the situation in the observed extragalactic H II regions is justthe opposite.
| The Chemical Composition of H II Regions in the Magellanic Clouds: New Calculations Using Modern Atomic Data
Not Available
| Cool gas in southern galaxies.
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| 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.
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