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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
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| Near-infrared surface brightness fluctuations and optical colours of Magellanic star clusters
This work continues our efforts to calibrate model surface brightnessfluctuation luminosities for the study of unresolved stellarpopulations, through a comparison with the data of Magellanic Cloud starclusters. We present here the relation between absoluteKs-band fluctuation magnitude and (V-I) integrated colour,using data from the Two-Micron All-Sky Survey (2MASS) and the DeepNear-Infrared Southern Sky Survey (DENIS), and from the literature. Wecompare the star cluster sample with the sample of early-type galaxiesand spiral bulges studied by Liu et al. We find that intermediate-age toold star clusters lie along a linear correlation with the same slope,within the errors, of that defined by the galaxies in the versus (V-I)diagram. While the calibration by Liu et al. was determined in thecolour range 1.05 < (V-IC)0 < 1.25, oursholds in the interval . This implies, according to Bruzual-Charlot andMouhcine-Lançon models, that the star clusters and the lateststar formation bursts in the galaxies and bulges constitute an agesequence. At the same time, a slight offset between the galaxies and thestar clusters [the latter are ~0.7 mag fainter than the former at agiven value of (V-I)], caused by the difference in metallicity ofroughly a factor of 2, confirms that the versus (V-I) plane maycontribute to break the age-metallicity degeneracy in intermediate-ageand old stellar populations. The confrontation between models and galaxydata also suggests that galaxies with Ks fluctuationmagnitudes that are brighter than predicted, given their (V-I) colour,might be explained in part by longer lifetimes of thermally pulsingasymptotic giant branch stars. A preliminary comparison between the H2MASS data of the Magellanic star clusters and the sample of 47early-type galaxies and spiral bulges observed by Jensen et al. throughthe F160WHubble Space Telescope filter leads to the same basicconclusions: galaxies and star clusters lie along correlations with thesame slope, and there is a slight offset between the star cluster sampleand the galaxies, caused by their different metallicities. Magellanicstar clusters are single populations, while galaxies are compositestellar systems; moreover, the objects analysed live in differentenvironments. Therefore, our findings mean that the relationship betweenfluctuation magnitudes in the near-infrared, and (V-I) might be a fairlyrobust tool for the study of stellar population ages and metallicities,could provide additional constraints on star formation histories, andaid in the calibration of near-infrared surface brightness fluctuationsfor cosmological distance measurements.
| 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.
| Infrared Surface Brightness Fluctuations of Magellanic Star Clusters
We present surface brightness fluctuations (SBFs) in the near-IR for 191Magellanic star clusters available in the Second Incremental and All SkyData releases of the Two Micron All Sky Survey (2MASS) and compare themwith SBFs of Fornax Cluster galaxies and with predictions from stellarpopulation models as well. We also construct color-magnitude diagrams(CMDs) for these clusters using the 2MASS Point Source Catalog (PSC).Our goals are twofold. The first is to provide an empirical calibrationof near-IR SBFs, given that existing stellar population synthesis modelsare particularly discrepant in the near-IR. Second, whereas mostprevious SBF studies have focused on old, metal-rich populations, thisis the first application to a system with such a wide range of ages(~106 to more than 1010 yr, i.e., 4 orders ofmagnitude), at the same time that the clusters have a very narrow rangeof metallicities (Z~0.0006-0.01, i.e., 1 order of magnitude only). Sincestellar population synthesis models predict a more complex sensitivityof SBFs to metallicity and age in the near-IR than in the optical, thisanalysis offers a unique way of disentangling the effects of age andmetallicity. We find a satisfactory agreement between models and data.We also confirm that near-IR fluctuations and fluctuation colors aremostly driven by age in the Magellanic cluster populations and that inthis respect they constitute a sequence in which the Fornax Clustergalaxies fit adequately. Fluctuations are powered by red supergiantswith high-mass precursors in young populations and by intermediate-massstars populating the asymptotic giant branch in intermediate-agepopulations. For old populations, the trend with age of both fluctuationmagnitudes and colors can be explained straightforwardly by evolution inthe structure and morphology of the red giant branch. Moreover,fluctuation colors display a tendency to redden with age that can befitted by a straight line. For the star clusters only,(H-Ks)=(0.21+/-0.03)log(age)-(1.29+/-0.22) once galaxies areincluded, (H-Ks)=(0.20+/-0.02)log(age)-(1.25+/-0.16).Finally, we use for the first time a Poissonian approach to establishthe error bars of fluctuation measurements, instead of the customaryMonte Carlo simulations.This research has made use of the NASA/ IPAC Infrared Science Archive,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under contract with the National Aeronautics and SpaceAdministration.
| 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)).
| Abundances of the planetary nebula Hu 1-2
The ISO and IUE spectra of the ``elliptical'' nebula Hu1-2 are presented. These spectra are combined with new, highresolution spectra in the visual wavelength region to obtain a complete,extinction corrected, spectrum. The chemical composition of the nebulais then calculated and compared to previous determinations. Theabundances determined are the lowest yet found in the analysis of ISOdata. The evolutionary significance is discussed.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.
| 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.
| Pristine CNO abundances from Magellanic Cloud B stars. I. The LMC cluster NGC 2004 with UVES
We present chemical abundances for four main sequence B stars in theyoung cluster NGC 2004 in the Large Magellanic Cloud (LMC). Apart from HII regions, unevolved OB-type stars are currently the only accessiblesource of present-day CNO abundances for the MCs not altered by stellarevolution. Using UVES on the VLT, we obtained spectra of sufficientresolution (R = 20 000) and signal-to-noise (S/N>= 100) to deriveabundances for a variety of elements (He, C, N, O, Mg and Si) with NLTEline formation. This study doubles the number of main sequence B starsin the LMC with detailed chemical abundances. More importantly and incontrast to previous studies, we find no CNO abundance anomalies broughton by e.g. binary interaction or rotational mixing. Thus, this is thefirst time that abundances from H II regions in the LMC can sensibly becross-checked against those from B stars by excluding evolutionaryeffects. We confirm the H II-region CNO abundances to within the errors,in particular the extraordinarily low nitrogen abundance of varepsilon(N) =~ 7.0. Taken at face value, the nebular carbon abundance is 0.16dex below the B-star value which could be interpreted in terms ofinterstellar dust depletion. Oxygen abundances from the two sourcesagree to within 0.03 dex. In comparison with the Galactic thin disk atMC metallicities, the Magellanic Clouds are clearly nitrogen-poorenvironments. Based on observations carried out at the European SouthernObservatory (ESO), Paranal, Chile under programme ID 66.D-0214(A).
| 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.
| MSX, 2MASS, and the LARGE MAGELLANIC CLOUD: A Combined Near- and Mid-Infrared View
The Large Magellanic Cloud (LMC) has been observed by the MidcourseSpace Experiment (MSX) in the mid-infrared and the Two Micron All SkySurvey (2MASS) in the near-infrared. We have performed across-correlation of the 1806 MSX catalog sources and nearly 1.4 million2MASS cataloged point and extended sources and find 1664 matches. Usingthe available color information, we identify a number of stellarpopulations and nebulae, including main-sequence stars, giant stars, redsupergiants, carbon- and oxygen-rich asymptotic giant branch (AGB)stars, planetary nebulae, H II regions, and other dusty objects likelyassociated with early-type stars. A total of 731 of these sources haveno previous identification. We compile a listing of all objects, whichincludes photometry and astrometry. The 8.3 μm MSX sensitivity is thelimiting factor for object detection: only the brighter red objects,specifically the red supergiants, AGB stars, planetary nebulae, and H IIregions, are detected in the LMC. The remaining objects are likely inthe Galactic foreground. The spatial distribution of the infrared LMCsources may contribute to understanding stellar formation and evolutionand the overall galactic evolution. We demonstrate that a combined mid-and near-infrared photometric baseline provides a powerful means ofidentifying new objects in the LMC for future ground-based andspace-based follow-up observations.
| Large-Scale Diffuse X-Ray Emission from the Large Magellanic Cloud
X-ray mosaics of the Large Magellanic Cloud (LMC) taken with the ROSATPosition Sensitive Proportional Counter (PSPC) have revealed extensivediffuse X-ray emission, indicative of hot >=106 K gasassociated with this irregular galaxy on scales from ~10 to >=1000pc. We have selected regions of large-scale (d>=600 pc) diffuse X-rayemission, such as supergiant shells, the LMC Spur, and the LMC Bar, andexamined the physical conditions of the hot gas associated with them. Wefind that for these objects the plasma temperatures range from kT~0.15to 0.60 keV and the derived electron densities range fromne~0.005 to 0.03 cm-3. Furthermore, we haveexamined the fraction of diffuse X-ray emission from the LMC andcompared it to the total X-ray emission. We find that discrete sourcessuch as X-ray binaries and supernova remnants account for ~41% and ~21%of the X-ray emission from the LMC, respectively. In contrast, diffuseX-ray emission from the field and from supergiant shells account for~30% and ~6% of the total X-ray emission, respectively.
| 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.
| 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 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
| 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.
| Searching for Wolf-Rayet Stars in I ZW 18: the Origin of He II Emission
I Zw 18 is the most metal-poor star-forming galaxy known and is an ideallaboratory to probe stellar evolution theory at low metallicities. Usingarchival Hubble Space Telescope WFPC2 imaging and Faint ObjectSpectrograph spectroscopy, we were able to improve previous studies. Weconstructed a continuum-free He II lambda4686 map, which was used toidentify Wolf-Rayet (W-R) stars recently found by ground-basedspectroscopy and to locate diffuse nebular emission. Most of the He IIlambda4686 emission is associated with the northwest stellar clusterclearly displaced from the surrounding shell-like [O III] and Hαemission. We found evidence for He II sources, compatible with five tonine WNL stars and/or compact nebular He II lambda4686 emission, as wellas residual diffuse emission. Only one of them is outside the northwestcluster. We have done an extensive comparison between our results andthe recent ground-based data used by Izotov et al. and Legrand et al. toidentify WN and WC stars in I Zw 18. The differences between the variousdata may be understood in terms of varying slit locations, continuumfits, and contamination by nebular lines. We have calculatedevolutionary tracks for massive stars and synthesis models at theappropriate metallicity (Z ~ 0.02 Z_ȯ). These single-star modelspredict a mass limit M_WR ~ 90 M_ȯ for W-R stars to become WN andWC/WO. For an instantaneous burst model with a Salpeter initial massfunction extending up to M_up ~ 120-150 M_ȯ, our model predictionsare in reasonable agreement with the observed equivalent widths. Ourmodel is also able to fully reproduce the observed equivalent widths ofnebular He II lambda4686 emission due to the presence of WC/WO stars.This quantitative agreement and the spatial correlation of nebular He IIlambda4686 with the stellar cluster and the position of W-R stars shownfrom the ground-based spectra further supports the hypothesis that W-Rstars are responsible for nebular He II emission in extragalactic H IIregions.
| 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.
| 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.
| Cool gas in southern galaxies.
Not Available
| A photometric and spectroscopic study of the association LH47 in the superbubble N44 in the LMC
The OB association LH47 inside the superbubble N44 in the LargeMagellanic Cloud (LMC) is analyzed with CCD photometry. For the brightblue stars spectra in the optical and UV have been taken. The observedU, B, and V magnitudes and colours are converted to Teff andL. From isochrone fits to the colour magnitude diagram (CMD), we find anage of the association of about 5 to 6 Myr. The stellar populationoutside the shell is of similar age, so there is no direct evidence forsequential star formation. The slope of the luminosity function (gamma =0.24) agrees with what has been found for other young associations. Wecompare two methods to derive the slope of the initial mass function(IMF). First we count all stars along the main sequence in the BV CMD.Second we count the stars between stellar evolutionary tracks in theHertzsprung-Russell diagram (HRD). Both methods give mainly the sameresult, i.e. x=1.37 and x=1.22, respectively. The IMF of the starsoutside the shell shows a slightly steeper slope than inside the shell.The slope of the IMF is very similar to values found for otherassociations and open clusters as well as in the solar neighborhood,thus supporting the idea of an universal shape of the IMF. LH47 turnsout to be a well behaved young association embedded in a molecularcloud.
| Observation of DCO^+^ in the Large Magellanic Cloud.
We report on the detection of DCO^+^ and, tentatively, DCN in theirrotational J=2-1 transitions towards the N159 region in the LargeMagellanic Cloud (LMC). These detections of deuterated molecules in anextragalactic source complement those already published by Chin et al.(1996). The resulting estimate of the HCO^+^/DCO^+^ abundance ratio is120_-60_^+60^ (total 2σ uncertainties). Assuming a Galactic D/Hratio for the LMC leads to an estimate of the kinetic temperature of themolecular gas and an upper limit to the fractional electron abundance of10-27K and a few x10^-5^, respectively. The temperature estimate is inaccordance with excitation calculations using a number of othermolecules. The HCO^+^/DCO^+^ abundance ratio in N159 is a factor of afew higher than in cores of Galactic molecular clouds. This is probablydue to a somewhat higher gas temperature and possibly a considerablyhigher electron abundance in N159. The estimated HCN/DCN abundance ratiois 60_-35_^+50^ (total 2σ uncertainties).
| Molecular abundances in the Magellanic Clouds. I. A multiline study of five cloud cores.
Nine Hii regions of the LMC were mapped in ^13^CO(1-0) and three in^12^CO(1-0) to study the physical properties of the interstellar mediumin the Magellanic Clouds. For N113 the molecular core is found to have apeak position which differs from that of the associated Hii region by20". Toward this molecular core the ^12^CO and ^13^CO peak T_MB_ linetemperatures of 7.3K and 1.2K are the highest so far found in theMagellanic Clouds. The molecular concentrations associated with N113,N44BC, N159HW, and N214DE in the LMC and LIRS36 in the SMC wereinvestigated in a variety of molecular species to study the chemicalproperties of the interstellar medium. I(HCO^+^)/I(HCN) andI(HCN)/I(HNC) intensity ratios as well as lower limits to theI(^13^CO)/I(C^18^O) ratio were derived for the rotational 1-0transitions. Generally, HCO^+^ is stronger than HCN, and HCN is strongerthan HNC. The high relative HCO^+^ intensities are consistent with ahigh ionization flux from supernovae remnants and young stars, possiblycoupled with a large extent of the HCO^+^ emission region. The bulk ofthe HCN arises from relatively compact dense cloud cores. Warm orshocked gas enhances HCN relative to HNC. From chemical modelcalculations it is predicted that I(HCN)/I(HNC) close to one should beobtained with higher angular resolution (<~30") toward the cloudcores. Comparing virial masses with those obtained from the integratedCO intensity provides an H_2_ mass-to-CO luminosity conversion factor of1.8x10^20^mol/cm^2^(K.km/s)^-1^ for N113 and2.4x10^20^mol/cm^2^(K.km/s)^-1^ for N44BC. This is consistent withvalues derived for the Galactic disk.
| X-Rays from Superbubbles in the Large Magellanic Cloud. IV. The Blowout Structure of N44
We have used optical echelle spectra along with ROSAT and ASCA X-rayspectra to test the hypothesis that the southern portion of the N44X-ray bright region is the result of a blowout structure. Three piecesof evidence now support this conclusion. First, the filamentary opticalmorphology corresponding with the location of the X-ray bright South Barsuggests the blowout description (Chu et al. 1993). Second, opticalechelle spectra show evidence of high-velocity (90 km s^-1^) gas in theregion of the blowout. Third, X-ray spectral fits show a lowertemperature for the South Bar than the main superbubble region of shell1. Such a blowout can affect the evolution of the superbubble andexplain some of the discrepancy discussed by Oey & Massey (1995)between the observed shell diameter and the diameter predicted on thebasis of the stellar content and the pressure-driven bubble model ofWeaver et al. (1977).
| 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.
| Spectroscopy and Imaging of the High-Ionization LMC H II Region N44C
We present new long-slit spectroscopy and narrow-band CCD imaging of thepeculiar H II region N44C in the Large Magellanic Cloud, which exhibitsstrong He II recombination emission, although the apparent ionizing staris of spectral class O. The spectra cover the range 3300-7400 Angstroms,including the emission lines [Ne V] 3426 Angstroms and [Ar V] 7005Angstroms, which provide important constraints on the stellar radiationfield. We use the emission line spectrum of N44C to constrain thestellar ionizing continuum and to estimate the element abundances forHe, O, N, Ne, S, and Ar in the nebula, to compare with normal LMC H IIregions. The helium and oxygen abundances in N44C appear to be normalcompared to other well-studied H II regions in the LMC. N/O appearssomewhat high (N(+) /O(+) = 0.05 +/- 0.04), but the value is uncertainbecause of the weak [N II] measurement and the very large correction forunseen higher ionization states of nitrogen. The spectral type of the Ostar is uncertain because of severe contamination by nebular emission,but is in the range O3-O7. We also set significant upper limits on theamounts of Ne(+4) and Ar(+4) in N44C, which places severe constraints onthe number of ionizing photons above 60 eV from the star. The observedionization will be compared with results from photoionization modelsusing hot stellar atmospheres.
| 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.
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