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Ýlgili Makaleler
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.
| High spatial resolution radio continuum observations of compact H {II} regions in the Magellanic Clouds
We present high spatial resolution observations of the 6 cm continuumemission of compact H II regions in well-known sites of massive starformation located in the Small and Large Magellanic Clouds. Theobservations include N81 in the SMC, and N4A, N83B, N11A, N160A andN159-5 in the LMC. Some of the compact H II regions are isolated, whileothers are embedded in more diffuse ionised regions. A description ofthe radio morphology of the sources, together with comparisons withother observations, is given in detail. The regions cover a wide rangein size (from ˜ 0.1 to 7 pc), rms electron density (from ˜200 to 6500 cm-3), emission measure (from~3×105 to 2×107 pc cm-6),ionised gas mass (from ˜ 0.2 to 750 Mȯ) and rateof Lyman continuum photons (from ~ 3× 1047 to5×1049 s-1). The spectral types determinedfrom the Lyman continuum fluxes are consistent with opticaldeterminations. We have compared these Magellanic Cloud H II regionswith their Galactic counterparts in terms of size, rms electron densityand Lyman continuum flux. This comparison shows that their propertiesrelate to each other in the same way as those in Galactic H II regions.
| 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.
| 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.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. X. CO emission from star formation regions in LMC and SMC
We present J=1-0 and J=2-1 12CO maps of several star-formingregions in both the Large and the Small Magellanic Cloud, and brieflydiscuss their structure. Many of the detected molecular clouds arerelatively isolated and quite small with dimensions of typically 20 pc.Some larger complexes have been detected, but in all cases the extent ofthe molecular clouds sampled by CO emission is significantly less thanthe extent of the ionized gas of the star-formation region. Very littlediffuse extended CO emission was seen; diffuse CO in between orsurrounding the detected discrete clouds is either very weak or absent.The majority of all LMC lines of sight detected in 13CO hasan isotopic emission ratio I( 12CO)/I( 13CO) ofabout 10, i.e. twice higher than found in Galactic star-formingcomplexes. At the lowest 12CO intensities, the spread ofisotopic emission ratios rapidly increases, low ratios representingrelatively dense and cold molecular gas and high ratios marking COphoto-dissociation at cloud edges.
| The physical structure of Magellanic Cloud H II regions. II. Elemental abundances
Based on a new data set of optical and infrared spectra described inVermeij et al. (\cite{Ronald}), we analyse the gas-phase elementalabundances of a sample of H Ii regions in the Large and Small MagellanicCloud. The combined optical and infrared data set gives us access to allthe ionization stages of astrophysically important elements such assulfur and oxygen. We self-consistently determine the electrontemperatures and densities for the \elem{O}{+}, \elem{S}{++} and\elem{O}{++} ionization zones, and use these parameters in thederivation of the ionic fractions. We discuss the uncertainties on theseionic fractions. The different relations between the electrontemperatures as proposed by Garnett (\cite{Garnett}) and Thuan et al.(\cite{Thuan}) are confronted with our results. We find our electrontemperatures to be consistent with these relations, although therelation between Te,[S Iii] and Te, [O Iii] mightbe slightly steeper than predicted. We investigate the reliability ofthe Ionization Correction Factors (ICFs) used in the derivation of thefull elemental abundances of sulfur and neon. We conclude that theprescription for the ICF used to derive the sulfur abundance as given byStasińska (\cite{Stas1}) for alpha = 3 is accurate for\elem{O}{+}/O > 0.20. No conclusions could be drawn for neon.Avoiding the use of ICFs as much as possible, we then proceed to derivethe full elemental abundances. We calculate a grid of generalphotoionization models to compare our results with the ``bright-line''abundance diagnostics for oxygen (R23) and sulfur (S23(4)). Thereliability of the newly proposed S234 parameter (Oey & Shields\cite{oey}) which includes emission lines from \elem{S}{+}, \elem{S}{++}and \elem{S}{+3} is checked. We find a very good agreement between theS234 models and our analysis results. Finally, we compare the heavyelement-to-oxygen ratios of our sample objects to those of giant H Iiregions in a large sample of low-metallicity blue dwarf galaxies (Izotov& Thuan \cite{Izotov}) and with the results from Kobulnicky &Skillman (\cite{Kobul1}, \cite{Kobul2}) for the irregular galaxies NGC1569 and NGC 4214.
| 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 stellar content, metallicity and ionization structure of H II regions
Observations of infrared fine-structure lines provide direct informationon the metallicity and ionization structure of H Ii regions andindirectly on the hardness of the radiation field ionizing thesenebulae. We have analyzed a sample of Galactic and Magellanic Cloud H Iiregions observed by the Infrared Space Observatory (ISO) to examine theinterplay between stellar content, metallicity and the ionizationstructure of H Ii regions. The observed [S Iv],10.5/[S Iii],18.7 μmand [Ne Ii]I 15.5/[Ne Ii],12.8 μm line ratios are shown to be highlycorrelated over more than two orders of magnitude. We have compared theobserved line ratios to the results of photoionization models usingdifferent stellar energy distributions. The derived characteristics ofthe ionizing star depend critically on the adopted stellar model as wellas the (stellar) metallicity. We have compared the stellar effectivetemperatures derived from these model studies for a few well-studied HIi regions with published direct spectroscopic determinations of thespectral type of the ionizing stars. This comparison supports ourinterpretation that stellar and nebular metallicity influences theobserved infrared ionic line ratios. We can explain the observedincrease in degree of ionization, as traced by the [S Iv]/[S Iii] and[Ne Iii]/[Ne Ii] line ratios, by the hardening of the radiation fielddue to the decrease of metallicity. The implications of our results forthe determination of the ages of starbursts in starburst galaxies areassessed. Based on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, The Netherlands and the UK) and with the participationof ISAS and NASA.
| Principal components analysis for equivalent widths of globular clusters
The method of principal component analysis is applied to a sample of 41Galactic globular clusters and 22 younger star clusters to select someage-sensitive or metallicity-sensitive spectral lines.It is found thatsome spectral lines have great potential to determine the metallicity ofstar clusters, such as CN, CaII K, CaII H and MgI+MgH; and some otherspectral lines are good indicators of age, such as H?, H?, H? and H?.These lines can help us to solve the age-metallicity degeneracy of starclusters.
| The Excitation and Metallicity of Galactic H II Regions from Infrared Space Observatory SWS Observations of Mid-Infrared Fine-Structure Lines
We present mid-infrared Infrared Space Observatory Short-WavelengthSpectrometer (ISO-SWS) observations of the fine-structure emissionslines [Ne II] 12.8 μm, [Ne III] 15.6 μm, [Ne III] 36.0 μm, [ArII] 6.99 μm, [Ar III] 8.99 μm, [S III] 18.7 μm, [S III] 33.5μm, and [S IV] 10.5 μm and the recombination lines Brα andBrβ in a sample of 112 Galactic H II regions and 37 nearbyextra-Galactic H II regions in the LMC, SMC, and M33. We selected oursources from archival ISO-SWS data as those showing prominent [Ne II]12.8 μm or [Ne III] 15.6 μm emissions. The Galactic sources have awide range in galactocentric distance (0kpc<~Rgal<~18kpc), which enables us to study excitation and metallicity variationsover large Galactic scales. We detect a steep rise in the [Ne III] 15.6μm/[Ne II] 12.8 μm, [Ar III] 8.99 μm/[Ar II] 6.99 μm, and [SIV] 10.5 μm/[S III] 33.5 μm excitation ratios from the innerGalaxy outward, and a moderate decrease in metallicity, from ~2Zsolar in the inner Galaxy to ~1 Zsolar in theouter disk. The extra-Galactic sources in our sample show low gasdensity, low metallicity, and high excitation. We find a goodcorrelation between [Ne III] 15.6 μm/[Ne II] 12.8 μm and [Ar III]8.99 μm/[Ar II] 6.99 μm excitation ratios in our sample. Theobserved correlation is well reproduced by theoretical nebular modelsthat incorporate new-generation wind-driven non-LTE model stellaratmospheres for the photoionizing stars. In particular, the non-LTEatmospheres can account for the production of [Ne III] emission in the HII regions. We have computed self-consistent nebular and stellaratmosphere models for a range of metallicities (0.5-2Zsolar). We conclude that the increase in nebular excitationwith galactocentric radius is due to an increase in stellar effectivetemperature (as opposed to a hardening of the stellar spectral energydistributions due to the metallicity gradient). We estimate anintegrated [Ne III] 15.6 μm/[Ne II] 12.8 μm ratio for the Galaxyof 0.8, which puts it well inside the range of values for starburstgalaxies. The good fit between observations and our models support theconclusion of Thornley and coworkers that the low [Ne III] 15.6μm/[Ne II] 12.8 μm ratios observed in extra-Galactic sources aredue to global aging effects. Based on observations with ISO, an ESAproject with instruments funded by ESA member states (especially the PIcountries: France, Germany, the Netherlands, and the United Kingdom)with the participation of ISAS and NASA.
| 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.
| N-Band Observations of Henize 2-10: Unveiling the Dusty Engine of a Starburst Galaxy
As part of an ongoing program to better understand the early stages ofmassive star cluster evolution and the physical conditions for theirformation, we have obtained J, H, K', and N (10.8 μm) images of thenuclear region of the starburst galaxy He 2-10. The N-band images wereobtained with the Gemini North Telescope. In only 10 minutes ofon-source integration time with Gemini, we were able to detect four ofthe five enshrouded clusters, or ``ultradense H II regions'' (UD H IIregions) recently discovered in radio maps. None of these sourcesappears in either the optical Hubble Space Telescope images or thenear-infrared (J, H, and K') images. These sources make up about 60% ofthe total N-band flux from He 2-10 and, we suspect, a similar fractionof the total far-infrared flux measured by IRAS. The inferred spectra ofthe UD H II regions are strikingly similar to those of Galacticultracompact H II regions. We have modeled the radio and IR spectra ofthese UD H II regions under the assumption that they are ``scaled-up''Galactic ultracompact H II regions. From this model, the bolometricluminosity of the brightest cluster alone is estimated to be~2×109 Lsolar. The total mass of the dustand gas in this UD H II region is Mshell~107Msolar. We have also used the observed spectra to placeconstraints on the masses and ages of the stellar clusters enshroudedwithin the UD H II regions. For the brightest UD H II region, we findthat the stellar mass must beMcluster>~2.5×106 Msolar, andthe age must be <~4.8×106 yr, with the most probableage being <~3.6×106 yr. If we assume that the regionis pressure confined and enforce the requirement that the star formationefficiency must be less than ~90%, we find that the age of this stellarcluster must lie within a very narrow range,4×105<τ<5×106 yr. All theclusters within the UD H II regions in He 2-10 are estimated to haveages less than about 5×106 yr and masses greater thanabout 5×105 Msolar. We find that thelogarithmic ratio of the radio to far-infrared flux densities, q, forthe UD H II regions in He 2-10 is ~4 q~2.6 for both He 2-10 as a wholeand NGC 5253, another nearby starburst known to host UD H II regions.These values of q are significantly larger than the average q=2.35 foundfor normal galaxies but are comparable to the values of q found forultraluminous infrared galaxies. We suggest that large q values forstarburst galaxies may indicate that a significant fraction of thefar-infrared flux may arise from thermal dust emission from UD H IIregions. Finally, the possibility that all the far-infrared flux from He2-10 and other starburst galaxies may be produced by regions completelyobscured at wavelengths as long as K' suggests that the well-knowncorrelation between ultraviolet continuum slope andinfrared-to-ultraviolet flux ratio in starbursts cannot be due entirelyto reprocessing of ultraviolet radiation by dust in a foreground screengeometry. In fact, the dust that reddens the ultraviolet continuum slopemust be largely decoupled from the dust that produces the large infraredfluxes in some starbursts. Based on observations obtained at the GeminiObservatory, which is operated by the Association of Universities forResearch in Astronomy, Inc., under a cooperative agreement with theNational Science Foundation on behalf of the Gemini partnership: theNational Science Foundation (US), the Particle Physics and AstronomyResearch Council (UK), the National Research Council (Canada), CONICYT(Chile), the Australian Research Council (Australia), CNPq (Brazil), andCONICET (Argentina).
| 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.
| 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.
| A close look into an intermediate redshift galaxy using STIS
We present a detailed view of a galaxy at z=0.4 which is part of a largedatabase of intermediate redshifts using high resolution images. We usedthe STIS parallel images and spectra to identify the object and obtainthe redshift. The high resolution STIS image (0.05'') enabled us toanalyse the internal structures of this galaxy. A bar along the majoraxis and hot-spots of star formation separated by 0.37'' (1.6 kpc) arefound along the inner region of the galaxy. The analysis of themorphology of faint galaxies like this one is an important step towardsestimating the epoch of formation of the Hubble classification sequence.
| 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.
| 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.
| 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.
| 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
| Gamma-Ray Line Signals from 26Al and 60Fe in the Galaxies of the Local Group
Steady state gamma -line fluxes from the decay of 26Al and 60Fe situatedin the Local Group galaxies LMC, SMC, NGC 6822, IC 1613, M31, and M33are estimated with standard supernova yields and supernova rates basedon blue luminosities and other indicators of massive star formation. Thelargest 26Al flux, 2.0 x 10-6 gamma s-1 cm-2, comes from the LMC, whilethe SMC is estimated to provide an 26Al flux of 3.6 x 10-7 gamma s-1cm-2. Andromeda is surprisingly weak, only about 1.6 x 10-8 gamma s-1cm-2 because of both its distance and its current low rate of starformation. Expected 60Fe fluxes are about 16% of the 26Al flux.Detection of these fluxes is feasible but will probably requiresensitivities greater than those of the International Gamma-RayAstrophysical Laboratory mission. The chief utility of such observationsis the new insight they provide into the mechanisms of supernovaexplosions, the distribution and nature of massive-star--forming regionsin irregular and spiral galaxies, and the nucleosynthetic history ofLocal Group members.
| 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.
| Ultraviolet spectral evolution of star clusters in the IUE library.
The ultraviolet integrated spectra of star clusters and H II regions inthe IUE library have been classified into groups based on their spectralappearance, as well as on age and metallicity information from otherstudies. We have coadded the spectra in these groups according to theirS/N ratio, creating a library of template spectra for futureapplications in population syntheses in galaxies. We define spectralwindows for equivalent width measurements and for continuum tracings.These measurements in the spectra of the templates are studied as afunction of age and metallicity. We indicate the windows with a strongmetallicity dependence, at different age stages.
| A radio continuum study of the Magellanic Clouds. IV. Catalogues of radio sources in the Large Magellanic Cloud at 1.40, 2.45, 4.75, 4.85 and 8.55 GHz.
From observations with the Parkes radio telescope, we present cataloguesof radio sources in the Large Magellanic Cloud at four frequencies:1.40, 2.45, 4.75 and 8.55GHz, and an additional catalogue from a sourceanalysis of the Parkes-MIT-NRAO survey at 4.85GHz. A total of 469sources have been detected at least one of these frequencies, 132 ofwhich are reported here for the first time as radio sources.
| 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.
| A Search for Methanol Masers in the Magellanic Clouds
We report the discovery of a second methanol maser in the LargeMagellanic Cloud and we present the results of synthesis observations ofthis and the methanol maser detected previously. The second discoverywas made using the Australia Telescope National Facility's 64-m Parkesradio telescope during an extensive maser search for 6.6-GHz maseremission from the 5_1_-6_0_ A^+^ transition in both Magellanic Clouds.Spectra were obtained towards 35 HII regions in the Large MagellanicCloud and 13 regions in the Small Magellanic Cloud, and also on a3-arcmin grid over an area 0.3^deg^ square, south of the 30 Doradusnebula. Parkes observations at 12.2 GHz towards the two maser sitesyielded no detectable emission from the 2_0_-3_-1_, E methanoltransition. The results suggest that methanol masers are less abundantin the Magellanic Clouds than in our Galaxy. Observations of the twomasers with the Australia Telescope Compact Array showed one to belocated near the continuum emission peak of the H II region MC18 (N11),while the other appeared to be centred near OH emission on thesouth-eastern boundary of MC23 (N105a).
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