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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
| Low-excitation blobs in the Magellanic Clouds
Aims.We study an unknown, or very poorly known, interstellar H IIcomponent in the Magellanic Clouds. This is the first study ever devotedto this class of objects, which we call low-excitation blobs (LEBs). Methods: We used low-dispersion spectroscopy carried out at ESO toobtain emission line intensities of Hα, Hβ, and [O III](λλ 4959 + 5007) for 15 objects in the Large MagellanicCloud and 14 objects in the Small Magellanic Cloud. Results aredisplayed in excitation ([O III]/Hβ ratio) versus Hβluminosity diagrams. Results: We show the presence of an LEB componentin the Magellanic Clouds and study its relationship with the alreadyknown class of high-excitation blobs (HEBs). The newly found LEBs arelower excitation counterparts of HEBs and are powered by less massiveexciting stars. Further study of LEBs is expected to provide new piecesof information for a better understanding the low mass end of the upperinitial mass function in the Magellanic Clouds.
| The VLT-FLAMES survey of massive stars: observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region
We present new observations of 470 stars using the Fibre Large ArrayMulti-Element Spectrograph (FLAMES) instrument in fields centered on theclusters NGC 330 and NGC 346 in the Small Magellanic Cloud (SMC), andNGC 2004 and the N11 region in the Large Magellanic Cloud (LMC). Afurther 14 stars were observed in the N11 and NGC 330 fields using theUltraviolet and Visual Echelle Spectrograph (UVES) for a separateprogramme. Spectral classifications and stellar radial velocities aregiven for each target, with careful attention to checks for binarity. Inparticular, we have investigated previously unexplored regions aroundthe central LH9/LH10 complex of N11, finding ~25 new O-type stars fromour spectroscopy. We have observed a relatively large number of Be-typestars that display permitted Fe II emission lines. These are primarilynot in the cluster cores and appear to be associated with classicalBe-type stars, rather than pre main-sequence objects. The presence ofthe Fe II emission, as compared to the equivalent width of Hα, isnot obviously dependent on metallicity. We have also explored therelative fraction of Be- to normal B-type stars in the field-regionsnear to NGC 330 and NGC 2004, finding no strong evidence of a trend withmetallicity when compared to Galactic results. A consequence of serviceobservations is that we have reasonable time-sampling in three of ourFLAMES fields. We find lower limits to the binary fraction of O- andearly B-type stars of 23 to 36%. One of our targets (NGC 346-013) isespecially interesting with a massive, apparently hotter, less luminoussecondary component.
| IR Study of N11 in the LMC
N11 is a large complex in the LMC with many regions of star formation atdifferent evolution stages. It is characterized by a huge cavity (80×60pc) with a 5 Myr central cluster. It is surrounded by severalionized clouds where young where the youngest O star population isdeveloping (Walborn & Parker 1992, ApJ, 399, 87; Barbá et al.2003, AJ, 125, 1940). Current star formation is taking place in N11A andN11B. New millimeter data (CO and continuum) show that dust isassociated ``with the young stellar population, while the central cavityis relatively clean''. In this work we present a morphological study ofthe gas in this region obtained with observation of the IR emission gaslines Br γ and Paschen β. These images are compared to Hα and molecular gas images of N11. The main goal of this studywill consist in a study of the extinction in this region usingmultiwavelength imaging of the gas and dust.
| 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.
| The LMC H II region N 214C and its peculiar nebular blob
We study the Large Magellanic Cloud H ii region N 214C using imaging andspectroscopy obtained at the ESO New Technology Telescope. On the basisof the highest resolution images so far obtained of the OB associationLH 110, we show that the main exciting source of the H ii region, Sk-71°51, is in fact a tight cluster of massive stars consisting of atleast 6 components in an area ~4'' wide. Spectroscopic observationsallow us to revise the spectral type of the main component (# 17) to O2V ((f*)) + OB, a very rare, hot type. We also classify several otherstars associated with N 214C and study the extinction and excitationcharacteristics of the H ii region. Moreover, we obtain BVR photometryand astrometry of 2365 stars and from the corresponding color-magnitudediagram study the stellar content of N 214C and the surrounding LH 110.Furthermore, we discover a striking compact blob of ionized gas in theouter northern part of N 214C. A spherical structure of ~5'' in radius(~1.3 pc), it is split into two lobes by absorbing dust runningdiametrically through its center. We discuss the possible nature of thisobject.
| 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.
| Stellar populations associated with the LMC Papillon Nebula
We study the Large Magellanic Cloud Papillon Nebula (N 159-5), aconspicuous High Excitation Blob (HEB) lying in the star forming complexN 159. Using JHK near-infrared photometry obtained at the ESO VLT withthe ISAAC camera, we examine the stellar populations associated with thePapillon, tracing their history using stellar evolution models. Twopopulations are revealed: one composed of young, massive stars with anage ˜ 3 Myr, and a second consisting of older lower mass stars ofage spreading between 1 and 10 Gyr. We analyze the properties of thosepopulations and discuss their significance in the context of N 159. Wealso estimate that if the star at the center of the Papillon is singleits initial mass is ˜ 50 Mȯ and it is affected byan extinction AV ˜ 7 mag.Based on observations obtained at the European Southern Observatory,Paranal, Chile; Program 66.C-0172(A).Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/422/129
| 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).
| The stellar environment of SMC N81
We present near infrared JHK imaging of the Small Magellanic Cloudcompact H II region N81 using the ISAAC camera at the ESO Very LargeTelescope (Antu). Our analysis of the stellar environment of this youngmassive star region reveals the presence of three new stellarpopulations in the surrounding field which are mainly composed of lowmass stars. The main population is best fitted by evolutionary modelsfor ~ 2 Msun, stars with an age of 1 Gyr. We argue thatthese populations are not physically associated with the H II regionN81. Instead they are the result of a number of low mass star formingevents through the depth of the SMC south of its Shapley's wing. Thepopulations can rather easily be probed due to the low interstellarextinction in that direction.Based on observations obtained at the European Southern Observatory,Paranal, Chile; Program 69.A-0123(A).Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. These observations are associated withprogram # 6535.
| 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)).
| 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.
| Active Star Formation in the N11B Nebula in the Large Magellanic Cloud: A Sequential Star Formation Scenario Confirmed
The second largest H II region in the Large Magellanic Cloud, N11B hasbeen surveyed in the near-IR. We present JHKs images of theN11B nebula. These images are combined with CO (1-->0) emission-linedata and with archival New Technology Telescope and Hubble SpaceTelescope WFPC2 optical images to address the star formation activity ofthe region. IR photometry of all the IR sources detected is given. Weconfirm that a second generation of stars is currently forming in theN11B region. Our IR images show the presence of several bright IRsources that appear to be located toward the molecular cloud as seenfrom the CO emission in the area. Several of these sources show IRcolors with young stellar object characteristics, and they are primecandidates to be intermediate-mass Herbig Ae/Be stars. For the firsttime, an extragalactic methanol maser is directly associated with IRsources embedded in a molecular core. Two IR sources are found at 2"(0.5 pc) of the methanol maser reported position. Additionally, wepresent the association of the N11A compact H II region to the moleculargas, where we find that the young massive O stars have eroded a cavityin the parental molecular cloud, typical of a champagne flow. The N11region turns out to be a very good laboratory for studying theinteraction of winds, UV radiation, and molecular gas. Severalphotodissociation regions are found.Based in part on observations with the NASA/ESA Hubble Space Telescopeobtained from the archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.
| Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. IX. The giant LMC HII region complex N 11
The second-brightest star formation complex in the Large MagellanicCloud, N 11, was surveyed extensively in the J = 1-0 transition of12CO. In this paper we present maps and a cataloguecontaining the parameters of 29 individual molecular clouds in thecomplex, although more may be present. The distribution of molecular gasin the N 11 complex is highly structured. In the southwestern part of N11, molecular clouds occur in a ring or shell surrounding the major OBstar association LH 9. In the northeastern part, a chain of molecularclouds delineates the rim of one of the so-called supergiant shells inthe LMC. There appears to be very little diffuse molecular gasin-between the individual well-defined clouds, especially in thesouthwestern ring. Most of the clouds have dimensions only slightlylarger than those of the survey beam, i.e. diameters of 25 pc or less. Asubset of the clouds mapped in J= 1-0 12CO transition wasalso observed in the J= 2-1 12CO transition, and in thecorresponding transitions of 13CO. Clouds mapped in J= 2-112CO with a two times higher angular resolution show further,clear substructure. The elements of this substructure, however, havedimensions once again comparable to those of the mapping beam. For a fewclouds, sufficient information was available to warrant an attempt atmodelling their physical parameters. They contain fairly warm(Tkin = 60-150 K) and moderately dense (nH_2 =3000 cm-3) gas. The northeastern chain of CO clouds, althoughlacking in diffuse intercloud emission, is characteristic of the morequiescent regions of the LMC, and appears to have been subject torelatively little photo-processing. The clouds forming part of thesouthwestern shell or ring, however, are almost devoid of diffuseintercloud emission, and also exhibit other characteristics of anextreme photon-dominated region (PDR).
| Spectrophotometry of six high-excitation compact HII regions in the Magellanic Clouds
Series of CCD long-slit spectra have been obtained in the 3600 - 10000Å range, with the ESO 1.5m telescope, for the six brightestcompact HII regions in the LMC and SMC: N 11A, N 160 A1-A2 and N 88A, N81 and N 26A-B, respectively. For each region the spectral type of itscomplex exciting source is given. From the emission-line intensities wehave derived the gas electron density and temperature, and computed thechemical abundances of He, O, N, Ne, S, and Ar, which we compare withthe ones found for other HII regions in the Magellanic Clouds.
| 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.
| A statistical study of binary and multiple clusters in the LMC
Based on the Bica et al. (\cite{bica}) catalogue, we studied the starcluster system of the LMC and provide a new catalogue of all binary andmultiple cluster candidates found. As a selection criterion we used amaximum separation of 1farcm4 corresponding to 20 pc (assuming adistance modulus of 18.5 mag). We performed Monte Carlo simulations andproduced artificial cluster distributions that we compared with the realone in order to check how many of the found cluster pairs and groups canbe expected statistically due to chance superposition on the plane ofthe sky. We found that, depending on the cluster density, between 56%(bar region) and 12% (outer LMC) of the detected pairs can be explainedstatistically. We studied in detail the properties of the multiplecluster candidates. The binary cluster candidates seem to show atendency to form with components of similar size. When possible, westudied the age structure of the cluster groups and found that themultiple clusters are predominantly young with only a few cluster groupsolder than 300 Myr. The spatial distribution of the cluster pairs andgroups coincides with the distribution of clusters in general; however,old groups or groups with large internal age differences are mainlylocated in the densely populated bar region. Thus, they can easily beexplained as chance superpositions. Our findings show that a formationscenario through tidal capture is not only unlikely due to the lowprobability of close encounters of star clusters, and thus the evenlower probability of tidal capture, but the few groups with largeinternal age differences can easily be explained with projectioneffects. We favour a formation scenario as suggested by Fujimoto &Kumai (\cite{fk}) in which the components of a binary cluster formedtogether and thus should be coeval or have small age differencescompatible with cluster formation time scales. Table 6 is only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/547
| The physical structure of Magellanic Cloud H II regions. I. Dataset
We present infrared and optical spectroscopic data for 11 H Ii regionsand one Supernova Remnant in the Large and Small Magellanic Cloud. Theinfrared data have been obtained with the Short Wavelength Spectrometerand Long Wavelength Spectrometer on board the Infrared Space Observatoryas part of a Guaranteed Time Program on H Ii regions in Local GroupGalaxies. Aim of this project is to give a new and improved analysis ofthe physical structure of the sample H Ii regions by combining as muchspectral data as possible. A detailed account is given here of thereduction process, and the quality and reliability of the presentedfluxes are discussed. Based on observations with ISO, an ESA projectwith instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA.
| The 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.
| 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 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.
| STIS spectroscopy of newborn massive stars in SMC N81
Using Hubble Space Telescope observations with STIS, we study the mainexciting stars of N81, a high excitation compact H II, region in theSmall Magellanic Cloud (SMC). These far UV observations are the firstspectroscopic measurements of stars in such a region and reveal featurescharacteristic of an O6-O8 stellar type. The astonishing weakness oftheir wind profiles and their sub-luminosity (up to ~ 2 mag fainter inMV than the corresponding dwarfs) make these stars a uniquestellar population in the Magellanic Clouds. Our analysis suggests thatthey are probably in the Hertzsprung-Russell diagram locus of aparticularly young class of massive stars, the so-called Vz luminosityclass, as they are arriving at the zero age main sequence. Based 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 contract NAS5-26555.
| Detection and study of the compact HII region N26A-B in the Small Magellanic Cloud
This paper presents new imagery and spectrophotometric results for theN26 HII region in the Small Magellanic Cloud. The observations usingmonochromatic images and low-resolution spectra (3700-10 000 Å)reveal a compact and complex nebula composed of two cores A and B whereA in the region of Hβ is brighter than B by a factor ~ 5 anddistance of 2arcsec . The core A of FWHM ~ 2farcs 1 or 0.6 pc presents ahigh excitation [O III] lambda lambda5007 +4959/Hβ up to ~ 8 and ahigh reddening E(B-V) <= 0.6, while the core B is less excited buthas a higher reddening >=0.8. Each core contains one exciting source;the brighter one should be responsible for the high excitation of A. Theapparent spectral type of the two cores ranges from O7 to O9 V and thegas electron density and temperature were derived from the absorptionand emission-line intensities. The total mass of the ionized gas isevaluated at 13 Msun. The chemical abundances of He, O, N,Ne, S, and Ar were computed. These abundances seem consistent withaverage abundances for SMC HII regions, except N that appears slightlyoverabundant. N26A-B is comparable to the objects previously observed inthe LMC and SMC that we have called ``blobs''.
| HST observations of the LMC compact \ion{H}{ II} region N 11A
We present a study of the LMC compact H ii region N 11A using HubbleSpace Telescope imaging observations which resolve N 11A and reveal itsunknown nebular and stellar features. The presence of a sharp ionizationfront extending over more than 4'' (1 pc) and fine structure filamentsas well as larger loops indicate an environment typical of massive starformation regions, in agreement with high [O iii]/Hβ line ratios. N11A is a young region, as deduced from its morphology, reddening, andespecially high local concentration of dust, as indicated by the Balmerdecrement map. Our observations also reveal a cluster of stars lyingtowards the central part of N 11A. Five of the stars are packed in anarea less than 2'' (0.5 pc), with the most luminous one being a mid Otype star. N 11A appears to be the most evolved compact H ii region inthe Magellanic Clouds so far studied. Based on observations with theNASA/ESA Hubble Space Telescope obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| Circumstellar masers in the Magellanic Clouds
Results are presented of a search for 22 GHz H_2O616->523, 43 GHz SiOv=1(J=1->0),86 GHz SiOv=1(J=2->1) and 129 GHzSiOv=1(J=3->2) maser emission from bright IRAS pointsources in the Small and Large Magellanic Clouds - mostly circumstellarenvelopes around obscured red supergiants and Asymptotic Giant Branchstars (OH/IR stars). The aim of this effort was to test whether thekinematics of the mass loss from these stars depends on metallicity.H_2O maser emission was detected in the red supergiants IRAS 04553-6825and IRAS 05280-6910, and tentatively in the luminous IR object IRAS05216-6753 and the AGB star IRAS 05329-6708, all in the LMC.SiOv=1(J=2->1) maser emission was detected in IRAS04553-6825. The double-peaked H_2O maser line profiles of IRAS04553-6825 and IRAS 05280-6910, in combination with the OH (and SiO)maser line profiles, yield the acceleration of the outflows from thesestars. The outflow velocity increases between the H_2O masing zone nearthe dust-formation region and the more distant OH masing zone from v ~18 to 26 km s-1 for IRAS 04553-6825 and from v ~ 6 to 17 kms-1 for IRAS 05280-6910. The total sample of LMC targets isanalysed in comparison with circumstellar masers in the Galactic Centre.The photon fluxes of circumstellar masers in the LMC are found to bevery similar to those in the Galactic Centre. The expansion velocitiesin the LMC appear to be ~ 20% lower than for similarly bright OH masersin the Galactic Centre, but the data are still consistent with nodifference in expansion velocity. OH/IR stars in the LMC appear to haveslower accelerating envelopes than OH/IR stars in the Galactic Centre.The masers in the LMC have blue-asymmetric emission profiles. This maybe due to the amplification of stellar and/or free-free radiation,rather than the amplification of dust emission, and may be morepronounced in low metallicity envelopes. The SiO maser strengthincreases with the photometric amplitude at 2.2 mu m but is independentof the photometric amplitude at 10 mu m. This suggests a strongconnection between shocks in the dust-free SiO masing zone and the dustformation process. The LMC masers obey the same trend as the GalacticCentre masers. Appendices describe H_2O maser emission from themoderately mass-losing AGB star R Dor in the Milky Way, optical echellespectroscopy of IRAS 04553-6825, and the properties of circumstellarmasers in the Galactic Centre.
| Very young massive stars in the Small Magellanic Cloud, revealed by HST
High spatial resolution imaging with the Hubble Space Telescope allowedus to resolve the compact H ii region N 81 lying in the Small MagellanicCloud (SMC). We show the presence of a tight cluster of newborn massivestars embedded in this nebular ``blob'' of ~ 10'' across. This is thefirst time the stellar content and internal morphology of such an objectis uncovered. These are among the youngest massive stars in this galaxyaccessible to direct observations at ultraviolet and opticalwavelengths. Six of them are grouped in the core region of ~ 2''diameter, with a pair of the main exciting stars in the very centerseparated by only 0''27 or 0.08 pc. The images display violent phenomenasuch as stellar winds, shocks, ionization fronts, typical of turbulentstarburst regions. Since the SMC is the most metal-poor galaxyobservable with very high angular resolution, these observations provideimportant templates for studying star formation in the very distantmetal-poor galaxies which populate the early Universe. Based 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 contractNAS\,5-26555.}
| 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.
| Obscured AGB stars in the Magellanic Clouds. I. IRAS candidates
We have selected 198 IRAS sources in the Large Magellanic Cloud, and 11in the Small Magellanic Cloud, which are the best candidates to bemass--loosing AGB stars (or possibly post--AGB stars). We used thecatalogues of \cite[Schwering \& Israel (1990)]{ref42} and\cite[Reid et al. (1990)]{ref36}. They are based on the IRAS pointedobservations and have lower detection limits than the Point SourceCatalogue. We also made cross-identifications between IRAS sources andoptical catalogues. Our resulting catalogue is divided in 7 tables.Table \ref{tab1} lists optically known red supergiants and AGB stars forwhich we found an IRAS counterpart (7 and 52 stars in the SMC and LMC,respectively). Table \ref{tab2} lists ``obscured'' (or ``cocoon'') AGBstars or late-type supergiants which have been identified as such inprevious works through their IRAS counterpart and JHKLM photometry (2SMC and 34 LMC sources; no optical counterparts). Table \ref{tab3} listsknown planetary nebulae with an IRAS counterpart (4 SMC and 19 LMC PNe).Table \ref{tab4} lists unidentified IRAS sources that we believe to begood AGB or post--AGB or PNe candidates (11 SMC and 198 LMC sources).Table~\ref{tab5} lists unidentified IRAS sources which could be any typeof object (23 SMC and 121 LMC sources). Table \ref{tab6} lists IRASsources associated with foreground stars (29 SMC and 135 LMC stars).Table \ref{tab7} lists ruled out IRAS sources associated with HIIregions, hot stars, etc... We show that the sample of IRAS AGB stars inthe Magellanic Clouds is very incomplete. Only AGB stars more luminousthan typically 10^4 L_\odot and with a mass-loss rate larger thantypically 5 10^{-6} M_\odot/yr could be detected by the IRAS satellite.As a consequence, one expects to find very few carbon stars in the IRASsample. We also expect that most AGB stars with intermediate mass--lossrates have not been discovered yet, neither in optical surveys, nor inthe IRAS survey. Tables 1 to 8 are also available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Formation of the nebular complex N11 in the Large Magellanic Cloud.
N 11, the second largest nebula of the LMC, is formed of a large bubblesurrounded by 9 bright nebulae and filaments. We have observed thisnebular complex with a scanning Fabry-Perot interferometer at Hαand [OIII] 5007 wavelengths. The kinematics of this field agrees withthe results of the stellar content and of the molecular studies, andshows that such a structure can be the consequence of a sequential starformation. The elements of this result are deduced from the energeticinput inside the ionized gas by the stellar winds of the associationsLH9 and LH10. The evaluated dynamical lifespan of small nebular entitiesassociated with the more massive stars of LH10, are found significantlyshorter than for the gas ionized by the association LH9. Thereforesequential star formation can be triggered at the shock boundaries ofthe nebula excited by an OB association in a time scale shorter thanpreviously found, and of at most a few 10^6^yrs. It thus leads to theevidence for a new class of giant bubble, designated as "ring of HIIregions" bubble with several distinctive characteristics. It isdiscussed that the bubbles of this type are formed by sequential starformation over time scales shorter than previously found for somesuperbubbles.
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