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On the relationship between auroral and nebular oxygen line intensities in spectra of H II regions
We investigate relationships between observed auroral and nebular oxygenline fluxes in spectra of H ii regions. We find a relation that ismetallicity-dependent at low metallicities, but becomes independent ofmetallicity (within the uncertainties of the available data) above12+logO/H ~ 8.25, i.e. there is one-to-one correspondence (theff-relation) between auroral and nebular oxygen line fluxes in spectraof high-metallicity H ii regions. The ff-relation allows one to estimatethe flux in the auroral line from strong oxygen line measurements only.This solves the problem of the electron temperature (and, consequently,abundance) determination in high-metallicity H ii regions. Theff-relation confirms the basic idea of the “empirical”method, proposed by Pagel et al. (1979, MNRAS, 189, 95) a quarter of acentury ago, that the oxygen abundance in H ii region can be esimatedfrom the strong oxygen lines measurements only.

Statistical Confirmation of a Stellar Upper Mass Limit
We derive the expectation value for the maximum stellar mass(mmax) in an ensemble of N stars, as a function of theinitial mass function (IMF) upper mass cutoff (mup) and N. Westatistically demonstrate that the upper IMF of the local massive starcensus observed thus far in the Milky Way and Magellanic Clouds clearlyexhibits a universal upper mass cutoff around 120-200 Msolarfor a Salpeter IMF, although the result is more ambiguous for a steeperIMF.

Massive spectroscopic binaries in the Magellanic Clouds
We present results of our ongoing observing program on search andstudies of massive stars (O type) in binary systems in our neighbourgalaxies, the Magellanic Clouds. Radial velocity orbits are presentedfor two new binaries, one in the Small Magellanic Cloud and another inthe Large Magellanic Cloud.Fellow of CONICET, Argentina. Visiting Astronomer, CASLEO, Argentina.

OB stellar associations in the Large Magellanic Cloud: Survey of young stellar systems
The method developed by Gouliermis et al. (\cite{Gouliermis00}, PaperI), for the detection and classification of stellar systems in the LMC,was used for the identification of stellar associations and openclusters in the central area of the LMC. This method was applied on thestellar catalog produced from a scanned 1.2 m UK Schmidt Telescope Platein U with a field of view almost 6\fdg5 x 6\fdg5, centered on the Bar ofthis galaxy. The survey of the identified systems is presented herefollowed by the results of the investigation on their spatialdistribution and their structural parameters, as were estimatedaccording to our proposed methodology in Paper I. The detected openclusters and stellar associations show to form large filamentarystructures, which are often connected with the loci of HI shells. Thederived mean size of the stellar associations in this survey was foundto agree with the average size found previously by other authors, forstellar associations in different galaxies. This common size of about 80pc might represent a universal scale for the star formation process,whereas the parameter correlations of the detected loose systems supportthe distinction between open clusters and stellar associations.

The relation between radio flux density and ionising ultra-violet flux for HII regions and supernova remnants in the Large Magellanic Cloud
We present a comparison between the Parkes radio surveys (Filipovic etal. 1995) and Vacuum Ultra-Violet (VUV) surveys (Smith et al. 1987) ofthe Large Magellanic Clouds (LMC). We have found 72 sources in common inthe LMC which are known HII regions (52) and supernova remnants (SNRs)(19). Some of these radio sources are associated with two or more UVstellar associations. A comparison of the radio flux densities andionising UV flux for HII regions shows a very good correlation, asexpected from theory. Many of the Magellanic Clouds (MCs) SNRs areembedded in HII regions, so there is also a relation between radio andUV which we attribute to the surrounding HII regions.

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( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/547

Early-type variables in the Magellanic Clouds. I. beta Cephei stars in the LMC bar
A thorough analysis of the OGLE-II time-series photometry of the LargeMagellanic Cloud bar supplemented by similar data from the MACHOdatabase led us to the discovery of three beta Cephei-type stars. Theseare the first known extragalactic beta Cephei-type stars. Two of thethree stars are multiperiodic. Two stars have inferred masses of about10 M_sun while the third is about 2 mag brighter and at least twice asmassive. All three variables are located in or very close to the massiveand young LMC associations (LH 41, 59 and 81). It is therefore veryprobable that the variables have higher than average metallicities. Thiswould reconcile our finding with theoretical predictions of the shapeand location of the beta Cephei instability strip in the H-R diagram.The low number of beta Cephei stars found in the LMC is anotherobservational confirmation of strong dependence of the mechanism drivingpulsations in these variables on metallicity. Follow-up spectroscopicdetermination of the metallicities in the discovered variables willprovide a good test for the theory of pulsational stability in massivemain-sequence stars.

A photoionization model of the compact H II region G29.96-0.02
We present a detailed photoionization model of G29.96-0.02 (hereafterG29.96), one of the brightest Galactic Ultra Compact H Ii (UCHII)regions in the Galaxy. This source has been observed extensively atradio and infrared wavelengths. The most recent data include a completeISO (SWS and LWS) spectrum, which displays a remarkable richness inatomic fine-structure lines. The number of observables is twice as greatas the number available in previous studies. In addition, most atomicspecies are now observed in two ionization stages. The radio andinfrared data on G29.96 are best reproduced using a nebular model withtwo density components: a diffuse (ne ~ 680 cm-3)extended (~ 1 pc) component surrounding a compact ( ~ 0.1 pc) dense(ne ~ 57 000 cm-3) core. The properties of theionizing star were derived using state-of-the-art stellar atmospheremodels. CoStar models yield an effective temperature of ~30+2-1 kK whereas more recent non-LTE lineblanketed atmospheres with stellar winds indicate somewhat highervalues, Teff ~ 32-38 kK. This range in Teff iscompatible with all observational constraints, including near-infraredphotometry and bolometric luminosity. The range 33-36 kK is alsocompatible with the spectral type O5-O8 determined by \citet{WH97} whenrecent downward revisions of the effective temperature scale of O starsare taken into account. The age of the ionizing star of G29.96 is foundto be a few 106 yr, much older than the expected lifetime ofUCHII regions. Accurate gas phase abundances are derived with the mostrobust results being Ne/S = 7.5 and N/O = 0.43 (1.3 and 3.5 times thesolar values, respectively). 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) andwith the participation of ISAS and NASA.

Calibrating Nebular Diagnostics of T[ scriptstyle star ]and Abundance
We obtained nebular spectroscopy of LMC H II regions having classifiedstellar populations, thereby strongly constraining the ionization inputparameters. Using photoionization models, we then evaluate theperformance of nebular diagnostics of T[ scriptstyle star ]andabundance. We introduce [Ne III]/H beta as a nebular diagnostic of theionizing stellar T[ scriptstyle star ]. In contrast to the widely-usedensuremath {eta ^prime } parameter, [Ne III]/H beta has greatersensitivity to mid and early O-stars, and is robust to nebularmorphology and the presence of shocks. We present a preliminarycalibration of both T[ scriptstyle star ]diagnostics for LMCmetallicity. We also introduce S[234] equiv ([S II] + [S III] + [SIV])/H beta as a diagnostic of S abundance. S[234] is much lesssensitive to the nebular ionization parameter than is S[23] or R[23].The intensity of [S IV]10.5 mu m is easily estimated from the opticaland near-IR line ratios. We present calibrations of S[23] and S[234]that are reliable at metallicities Z ≲ 0.5Z[ scriptstyle sun ].

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.

Two New Massive Binary Stars in the Magellanic Clouds
The discovery and preliminary spectroscopic orbits of two early O typebinaries in very young open clusters in the Magellanic Clouds isreported. The binaries are NGC 346--1 in the Small Magellanic Cloud, andHDE 270145 in NGC 2122 in the Large Magellanic Cloud.

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.

Lyman Continuum Extinction by Dust in H II Regions of Galaxies
We examine Lyman continuum extinction (LCE) in H II regions by comparinginfrared fluxes of 49 H II regions in the Galaxy, M31, M33, and theLarge Megellanic Cloud with estimated production rates of Lymancontinuum photons. A typical fraction of Lyman continuum photons thatcontribute to hydrogen ionization in the H II regions of three spiralgalaxies is <~50%. The fraction may become smaller as the metallicity(or dust-to-gas ratio) increases. We examine the LCE effect on estimatedstar formation rates of galaxies. The correction factor for the Galacticdust-to-gas ratio is 2-5.

Interstellar Bubbles in Two Young H II Regions
Massive stars are expected to produce wind-blown bubbles in theinterstellar medium; however, ring nebulae, suggesting the existence ofbubbles, are rarely seen around main-sequence O stars. To search forwind-blown bubbles around main-sequence O stars, we have obtainedhigh-resolution Hubble Space Telescope WFPC2 images and high-dispersionechelle spectra of two pristine H II regions, N11B and N180B, in theLarge Magellanic Cloud. These H II regions are ionized by OBassociations that still contain O3 stars, suggesting that the H IIregions are young and have not hosted any supernova explosions. Ourobservations show that wind-blown bubbles in these H II regions can bedetected kinematically, but not morphologically, because their expansionvelocities are comparable to or only slightly higher than the isothermalsound velocity in the H II regions. Bubbles are detected aroundconcentrations of massive stars, individual O stars, and even an evolvedred supergiant (a fossil bubble). Comparisons between the observedbubble dynamics and model predictions show a large discrepancy (1-2orders of magnitude) between the stellar wind luminosity derived frombubble observations and models and that derived from observations ofstellar winds. The number and distribution of bubbles in N11B differfrom those in N180B, which can be explained by the difference in therichness of stellar content between these two H II regions. Most of thebubbles observed in N11B and N180B show a blister structure, indicatingthat the stars were formed on the surfaces of dense clouds. Numeroussmall dust clouds, similar to Bok globules or elephant trunks, aredetected in these H II regions, and at least one of them hosts on-goingstar formation.

On the oxygen abundance determination in HII regions. High-metallicity regions
This is our second paper devoted to the problem of line intensity -oxygen abundance calibration starting from the idea of McGaugh(\cite{mcg91}) that the strong oxygen lines ([OII] lambda lambda 3727,3729 and [OIII] lambda lambda 4959, 5007) contain the necessaryinformation to determine accurate abundances in HII regions. In theprevious study (Pilyugin 2000) the corresponding relations were obtainedfor the low-metallicity HII regions (12+log O/H <= 7.95, the lowerbranch of the O/H - R23 diagram). The high-metallicity HIIregions (12+log O/H >= 8.2, the upper branch of the O/H -R23 diagram) are considered in the present study. A relationof the type O/H=f(P, R23) between oxygen abundance and thevalue of abundance index R23, introduced by Pagel et al.(\cite{pag79}), and the excitation parameter P (which is defined here asthe contribution of the radiation in [OIII] lambda lambda 4959, 5007lines to the ``total" oxygen radiation) has been derived empiricallyusing the available oxygen abundances determined via measurement of atemperature-sensitive line ratio [OIII]4959,5007/[OIII]4363(Te-method). By comparing oxygen abundances inhigh-metallicity HII regions derived with the Te-method andthose derived with the suggested relations (P-method), it was found thatthe precision of oxygen abundance determination with the P-method isaround 0.1 dex (the mean difference for the 38 HII regions considered is~ 0.08 dex) and is comparable to that of the Te-method. Arelation of the type Te=f(P, R23) between electrontemperature and the values of abundance index R23 and theexcitation parameter P was derived empirically using the availableelectron temperatures determined via measurement oftemperature-sensitive line ratios. The maximum value of differencesbetween electron temperatures determined via measurement oftemperature-sensitive line ratios and those derived with the suggestedrelation is around 1000 K for HII regions considered here, the meanvalue of differences for 38 HII regions is ~ 500 K, which is the sameorder of magnitude as the uncertainties of electron temperaturedeterminations in high-metallicity HII regions via measuredtemperature-sensitive line ratios.

Calibration of Nebular Emission-Line Diagnostics. II. Abundances
We examine standard methods of measuring nebular chemical abundances,including estimates based on direct Te measurements and alsoemission-line diagnostics. We use observations of the LMC H II regionsDEM L199, DEM L243, DEM L301, and DEM L323, the ionizing stars of whichhave classifications ranging from O7 to WN3. Following common practice,we assume a two-zone Te structure given by T(O++)and T(O+) to compute ionic abundances. We compare withphotoionization models tailored to the observed properties of theindividual objects, and we emphasize the importance of correctlyrelating Te in the two zones, which can otherwise causeerrors of ~0.2 dex in abundance estimates. The data show no spatialvariations or local metallicity enhancements to within 0.1-0.15 dex inany of the objects, notably including DEM L199, which hosts threeWolf-Rayet stars. Our data agree well with both the modeled R23 and S23abundance diagnostics for O and S. We present the first theoreticaltracks for S23, which are in excellent agreement with a larger availabledata set. However, contrary to earlier suggestions, S23 is much moresensitive to the ionization parameter (U) than is R23. This occursbecause S23 does not sample S IV, which is often a significantpopulation. We therefore introduce S234≡([S II]+[S III]+[SIV])/Hβ and demonstrate that it is virtually independent of U.Predicted and observed spatial variations in S234 are thus dramaticallydecreased in contrast to S23. The intensity of [S IV] 10.5 μm can beeasily estimated from the simple correspondence between [S IV]/[S III]and [O III]/[O II]. Using this method to estimate S234 for data in theliterature yields excellent agreement with our model tracks, hence wegive a theoretical calibration for S234. Our models show that thedouble-valued structure of S23 and S234 remains an important problem asfor R23, and, at present, we consider calibrations of these Sdiagnostics reliable only at Z<~0.5Zsolar. However, theslightly larger dynamic range and excellent compatibility withtheoretical predictions suggest the S parameters to be more effectiveabundance diagnostics than R23.

An Empirical Test and Calibration of H II Region Diagnostics
We present spectrophotometry in the 3600-9700 Å region for asample of 39 H II regions in the Galaxy and Magellanic Clouds, for whichindependent information is available on the spectral types and effectivetemperatures of the ionizing stars. The spectra have been used toevaluate nebular diagnostics of stellar temperature, metal abundance,and ionization parameter, and to compare the observed behavior of theline indices with predictions of nebular photoionization models. Weobserve a strong degeneracy between forbidden-line sequences produced bychanges in stellar Teff and metal abundance, which severelycomplicates the application of many forbidden-line diagnostics toextragalactic H II regions. Our data confirm however that the Edmunds& Pagel [O II]+[O III] abundance index and the Vílchez &Pagel η' index provide more robust diagnostics of metalabundance and stellar effective temperature, respectively. A comparisonof the fractional helium ionization of the H II regions with stellartemperature confirms the reliability of the spectral type versusTeff calibration for the relevant temperature rangeTeff<=38,000 K. We use empirical relations between thenebular hardness indices and Teff to reinvestigate the casefor systematic variations in the stellar effective temperatures and theupper initial mass functions of massive stars in extragalactic H IIregions. The data are consistent with a significant softening of theionizing spectra (consistent with cooler stellar temperatures) withincreasing metal abundance, especially for Z<=Zsolar.However, unresolved degeneracies between Z and Teff stillcomplicate the interpretation of this result.

Calibration of Nebular Emission-Line Diagnostics. I. Stellar Effective Temperatures
We present a detailed comparison of optical H II region spectra tophotoionization models based on modern stellar atmosphere models. Weexamine both spatially resolved and integrated emission-line spectra ofthe H II regions DEM L323, DEM L243, DEM L199, and DEM L301 in the LargeMagellanic Cloud. The published spectral classifications of the dominantstars range from O7 to WN3, and morphologies range from Strömgrensphere to shell structure. Two of the objects include SNR contamination.The overall agreement with the predictions is generally within 0.2 dexfor major diagnostic line ratios. An apparent pattern in the remainingdiscrepancies is that the predicted electron temperature is ~1000 Khotter than observed. [Ne III] intensities are also slightlyoverpredicted, which may or may not be related. We model the shockemission for the SNR-contaminated objects and find excellent agreementwith the observations for composite shock and photoionized spectra. DEML301's emission apparently results from both shocks and density-boundedphotoionization. The existence of contaminating shocks can be difficultto ascertain in the spatially integrated spectra. Our analysis of thecomplex DEM L199 allows a nebular emission-line test of unprecedenteddetail for WR atmospheres. Surprisingly, we find no nebular He IIλ4686 emission, despite the fact that both of the dominant WN3stars should be hot enough to fully ionize He I in their atmospheres.The nebular diagnostics are again in excellent agreement with the data,for stellar models not producing He+-ionizing photons. Theoptical diagnostics are furthermore quite insensitive to the ionizingenergy distribution for these early WR stars. We confirm that the η'emission-line parameter is not as useful as hoped for determining theionizing stellar effective temperature, T*. Both empiricallyand theoretically, we find that it is insensitive forT*>~40 kK and that it also varies spatially. Theshock-contaminated objects show that η' will also yield a spuriouslyhigh T* in the presence of shocks. It is furthermoresensitive to shell morphology. We suggest [Ne III]/Hβ as anadditional probe of T*. Although it is abundance dependent,[Ne III]/Hβ has higher sensitivity to T*, is independentof morphology, and is insensitive to shocks in our objects. Theseobservations should be useful data points for a first empiricalcalibration of nebular diagnostics of T*, which we attemptfor LMC metallicity.

Distribution of stellar mass in young star clusters of our Galaxy and nearby galaxies
Stellar mass distribution in young star clusters of our Galaxy, theMagellanic Clouds and the nearby local groups of galaxies has been usedto investigate the universality of initial mass function and presence ofmass segregation in these systems. There is no obvious dependence of theMF slope on either galactocentric distance or age of the galactic openstar clusters. A comparison of initial mass function slopes that havebeen measured in star clusters and associations of our and nearbygalaxies indicates that the slope is independent of the spatialconcentration of the star formed, galactic characteristics includingmetallicity, and at least down to 0.85 M?, the stellar mass range.Effects of mass segregation have been observed in good number of youngstellar groups of our Galaxy and Magellanic Clouds. As their ages aremuch smaller than their dynamical evolution times, star formationprocesses seems to be responsible for the observed mass segregation inthem.

Ultraviolet Imaging Polarimetry of the Large Magellanic Cloud. II. Models
Motivated by new sounding-rocket wide-field polarimetric images of theLarge Magellanic Cloud (reported simultaneously by Cole et al.), we haveused a three-dimensional Monte Carlo radiation transfer code toinvestigate the escape of near-ultraviolet photons from young stellarassociations embedded within a disk of dusty material (i.e., a galaxy).As photons propagate through the disk, they may be scattered or absorbedby dust. Scattered photons are polarized and tracked until they escapethe dust layer, allowing them to be observed; absorbed photons heat thedust, which radiates isotropically in the far-infrared where the galaxyis optically thin. The code produces four output images: near-UV andfar-IR flux, and near-UV images in the linear Stokes parameters Q and U.From these images we construct simulated UV polarization maps of theLMC. We use these maps to place constraints on the star+dust geometry ofthe LMC and the optical properties of its dust grains. By tuning themodel input parameters to produce maps that match the observedpolarization maps, we derive information about the inclination of theLMC disk to the plane of the sky and about the scattering phase functiong. We compute a grid of models with i=28 deg, 36 deg, and 45 deg, andg=0.64, 0.70, 0.77, 0.83, and 0.90. The model that best reproduces theobserved polarization maps has i=36 deg+2-5 andg~0.7. Because of the low signal-to-noise in the data, we cannot placefirm constraints on the value of g. The highly inclined models do notmatch the observed centrosymmetric polarization patterns around brightOB associations or the distribution of polarization values. Our modelsapproximately reproduce the observed ultraviolet photopolarimetry of thewestern side of the LMC; however, the output images depend on many inputparameters and are nonunique. We discuss some of the limitations of themodels and outline future steps to be taken; our models make somepredictions regarding the polarization properties of diffuse lightacross the rest of the LMC.

A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. II. The Large Magellanic Cloud
A survey of extended objects in the Large Magellanic Cloud was carriedout on the ESO/SERC R and J Sky Survey Atlases, checking entries inprevious catalogs and searching for new objects. The census provided6659 objects including star clusters, emission-free associations, andobjects related to emission nebulae. Each of these classes containsthree subclasses with intermediate properties, which are used to infertotal populations. The survey includes cross identifications amongcatalogs, and we present 3246 new objects. We provide accuratepositions, classification, and homogeneous measurements of sizes andposition angles, as well as information on cluster pairs andhierarchical relation for superimposed objects. This unification andenlargement of catalogs is important for future searches of fainter andsmaller new objects. We discuss the angular and size distributions ofthe objects of the different classes. The angular distributions show twooff-centered systems with different inclinations, suggesting that theLMC disk is warped. The present catalog together with its previouscounterpart for the SMC and the inter-Cloud region provide a totalpopulation of 7847 extended objects in the Magellanic System. Theangular distribution of the ensemble reveals important clues on theinteraction between the LMC and SMC.

The WR and O-type star population predicted by massive star evolutionary synthesis
Evolutionary calculations of massive single stars and of massive closebinaries that we use in the population number synthesis (PNS) code arepresented. Special attention is given to the assumptions/uncertaintiesinfluencing these stellar evolutionary computations (and thus the PNSresults). A description is given of the PNS model together with theinitial statistical distributions of stellar parameters needed toperform number synthesis.We focus on the population of O-type stars andWR stars in regions where star formation was continuous in time and instarburst regions. We discuss the observations that have to be explainedby the model. These observations are then compared to the PNSpredictions.We conclude that: . probably the majority of the massivestars are formed as binary components with orbital period between 1 dayand 10 yr; most of them interact. . at most 8% of the O-type stars arerunaways due to a previous supernova explosion in a binary; recentstudies of pulsar space velocities and linking the latter to the effectof asymmetrical supernova explosions, reveal that only a smallpercentage of these runaways will have a neutron star companion. . withpresent day stellar evolutionary computations, it is difficult toexplain the observed WR/O number ratio in the solar neighbourhood and inthe inner Milky Way by assuming a constant star formation rate, with orwithout binaries. The observed ratio for the Magellanic Clouds is betterreproduced. . the majority of the single WR stars may have had a binarypast. . probably merely 2-3% (and certainly less than 8%) of all WRstars have a neutron star companion. . a comparison between theoreticalprediction and observations of young starbursts is meaningful only ifbinaries and the effect of binary evolution are correctly included. Themost stringent feature is the rejuvenation caused by mass transfer.

LMC HII region luminosities versus observed ionizing stars
We use the stellar census of OB associations in the Large MagellanicCloud (LMC) to predict the H-alpha luminosities of the host HII regions,based on results from stellar atmosphere models. These values arecompared to the observed HII region luminosities, yielding an estimatefor the mean fraction of H-ionizing photons that escape the localnebulae in this sample. We formally estimate that, overall, 0% to 51% ofthe ionizing radiation escapes the local HII regions and is available toionize the warm, ionized medium in the LMC. We find both nebulae thatappear to be density-bounded, and ones that appear to beradiation-bounded.

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.

Comparison of H II region luminosities with observed stellar ionizing sources in the Large Magellanic Cloud
We estimate the total predicted Lyman continuum emission rates of OBassociations for which the complete census of O star spectral typesexists. The results are compared to the observed H-alpha luminosities ofthe host H II regions. We find evidence for substantial leakage ofionizing photons from some H II regions, while others appear to beradiation-bounded. We estimate that overall for the LMC, 0-51 percent ofthe ionizing radiation escapes the local nebulae, and would be availableto ionize the diffuse, warm, ionized medium (WIM) in that galaxy. Thisrange of values is consistent with the observed 35 percent fraction ofH-alpha luminosity emitted by the WIM in the LMC, as well as thecorresponding fractions observed in other nearby galaxies. It istherefore possible that photoionization by O stars is indeed thedominant ionization mechanism for the WIM.

Adaptive Optics Near-Infrared Imaging of R136 in 30 Doradus: The Stellar Population of a Nearby Starburst
Abstract image available at:http://adsabs.harvard.edu/abs/1996ApJ...466..254B

The Lyman-Continuum Fluxes and Stellar Parameters of O and Early B-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...460..914V&db_key=AST

Morphology and stellar content of complexes in the LMC.
Three LMC stellar aggregates and two LMC stellar complexes locatedinside the constellations Shapley I, IV, IX, and X have been examined inorder to study their morphology and properties. Star counts wereperformed on excellent quality film copies of direct plates taken withthe 1.2m U.K. Schmidt telescope. They have been used for derivingisodensity contour mapping of the four studied regions. Low dispersionobjective prism plates taken with the same telescope were also used toclassify the spectra of the stars down to M_V_~0.0mag. Combination ofthe two sets of data was used to define the boundaries of these regions,their age, the density and the spatial distribution of their OB stars.It is therefore found that the bright and massive OB type stars are thepredominant stellar component of the four studied regions. They areembedded in a fainter and less massive stellar component within theboundaries of a region, revealed by the isopleths, where the limitingdetection magnitude is down to M_V_~1.5mag. Thus it appears that thestellar content of the complexes and aggregates is made not only by thestars as massive as ~40Msun_, but they also contain lowermass stars, at least ~3Msun_. The spatial distribution ofearly type stars (down to M_V_~0.0mag) shows a gradient which reveals aregion coinciding with the one, defined by the isopleths, where fainterstars are also located. For the near future, we plan to study whetherthe gradient of the radial distribution of the early type starsrepresents the mass distribution of the molecular cloud from which thesestellar structures are formed, or it is due to sequential star formationprocess and/or expansion because of the high stellar winds of the verymassive stars.

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.

The Initial Mass Function and Massive Star Evolution in the OB Associations of the Northern Milky Way
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...454..151M&db_key=AST

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Observation and Astrometry data

Right ascension:05h48m55.00s
Apparent magnitude:99.9

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NGC 2000.0NGC 2122

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