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Star cluster ecology - VII. The evolution of young dense star clusters containing primordial binaries
We study the first ~100 Myr of the evolution of isolated star clustersinitially containing 144179 stars, including 13107 (10 per cent)primordial hard binaries. Our calculations include the effects of bothstellar and binary evolution. Gravitational interactions among the starsare computed by direct N-body integration using high-precision GRAPE-6hardware. The evolution of the core radii and central concentrations ofour simulated clusters are compared with the observed sample of young(<~100Myr) star clusters in the large Magellanic Cloud. Even thoughour simulations start with a rich population of primordial binaries,core collapse during the early phase of the cluster evolution is notprevented. Throughout the simulations, the fraction of binaries remainsroughly constant (~10 per cent). Due to the effects of mass segregationthe mass function of intermediate-mass main-sequence stars becomes asflat as α = -1.8 in the central part of the cluster (where theinitial Salpeter mass function had α = -2.35). About 6-12 per centof the neutron stars were retained in our simulations; the fraction ofretained black holes is 40-70 per cent. In each simulation about threeneutron stars become members of close binaries with a main-sequencecompanion. Such a binary will eventually become an X-ray binary, whenthe main-sequence star starts to fill its Roche lobe. Black holes arefound more frequently in binaries; in each simulated cluster we find ~11potential X-ray binaries containing a black hole. Binaries consisting oftwo white dwarfs are quite common, but few (20-30) are sufficientlyclose that they will merge within a Hubble time due to the emission ofgravitational radiation. Clusters with shorter relaxation times tend toproduce fewer merging white dwarf binaries. The white dwarf binariesthat do merge are all sufficiently massive to produce a Type Iasupernova. The densest cluster produces about twice as many bluestragglers as a field population containing the same number of binaries,and these blue stragglers are more massive, bluer and brighter than inless dense clusters.
| Massive Binaries in High-Mass Star-forming Regions: A Multiepoch Radial Velocity Survey of Embedded O Stars
We present the first multiepoch radial velocity study of embedded youngmassive stars using near-infrared spectra obtained with ISAAC mounted atthe ESO Very Large Telescope, with the aim of detecting massivebinaries. Our 16 targets are located in high-mass star-forming regions,and many of them are associated with known ultracompact H II regions,whose young age ensures that dynamic evolution of the clusters did notinfluence the intrinsic binarity rate. We identify two stars with about90 km s-1 velocity differences between two epochs, provingthe presence of close massive binaries. The fact that two out of the 16observed stars are binary systems suggests that at least 20% of theyoung massive stars are formed in close multiple systems, but may alsobe consistent with most, if not all, young massive stars being binaries.In addition, we show that the radial velocity dispersion of the fullsample is about 35 km s-1, significantly larger than ourestimated uncertainty (25 km s-1). This finding is consistentwith similar measurements of the young massive cluster 30 Dor, whichmight have a high intrinsic binary rate. Furthermore, we argue thatvirial cluster masses derived from the radial velocity dispersion ofyoung massive stars may intrinsically overestimate the cluster mass dueto the presence of binaries.Based on observations collected at the European Southern Observatory atParanal, Chile (ESO programs 64.H-0425, 65.H-0602, and 69.C-0189).
| 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
| Hidden Trigger for the Giant Starburst Arc in M83?
The huge star formation events that occur at some galactic centers donot provide enough clues as to their origin, since the morphologicalsignatures of the triggering mechanism are smeared out in the timescaleof a few orbital revolutions of the galaxy core. Our high spatialresolution three-dimensional near-infrared spectroscopy for the firsttime reveals that a previously known hidden mass concentration islocated exactly at the youngest end of a giant star-forming arc. Thislocation, the inferred average cluster ages, and the dynamical timesclearly indicate that the interloper has left behind a spur of violentstar formation in M83, in a transient event lasting less than oneorbital revolution. The study of the origin (bar funneling orcannibalized satellite) and fate (black hole merging or giant stellarcluster) of this system could provide clues to the question of coregrowing and morphological evolution in grand-design spiral galaxies. Inparticular, our TreeSPH numerical modeling suggests that the two nucleicould coalesce, forming a single massive core in about 60 million yearsor less. This work is based on observations made at the Gemini SouthTelescope.
| On the Similarity between Cluster and Galactic Stellar Initial Mass Functions
The stellar initial mass functions (IMFs) for the Galactic bulge, theMilky Way, other galaxies, clusters of galaxies, and the integratedstars in the universe are composites from countless individual IMFs instar clusters and associations where stars form. These galaxy-scaleIMFs, reviewed in detail here, are not steeper than the cluster IMFsexcept in rare cases. This is true even though low-mass clustersgenerally outnumber high-mass clusters and the average maximum stellarmass in a cluster scales with the cluster mass. The implication is thatthe mass distribution function for clusters and associations is a powerlaw with a slope of -2 or shallower. Steeper slopes, even by a fewtenths, upset the observed equality between large- and small-scale IMFs.Such a cluster function is expected from the hierarchical nature of starformation, which also provides independent evidence for the IMF equalitywhen it is applied on subcluster scales. We explain these results withanalytical expressions and Monte Carlo simulations. Star clusters appearto be the relaxed inner parts of a widespread hierarchy of starformation and cloud structure. They are defined by their own dynamicsrather than by preexisting cloud boundaries.
| Hunting Massive Stars Around the Tarantula
We have studied the N159A region which is located aproximately atα = 5^{h}40^{m}07^{s} and δ = -69°47'47''. This regionbelongs to a major complex called N159 located in the LMC at thesouthern edge of the 30 Dor Nebula. All the complex is ˜50 pcdiameter long and in its interior there are at least six HII regions(N159A, E, F, G, H, K). N159 is an extremely young complex and showscharacteristic features of active stellar fomation. The interest instudying N159 is based in the fact that it is an extragalactic (thoughnear) star forming region, in a low metallicity environment and locatedspatially close to an enormous complex of star formation like 30 Dor.These conditions make N159 an excellent place to study and learn about asubject as sequential star formation, IMF of stars at low metallicities,peculiar objects, etc. In the present work we made a spectrophotometricanalysis of a large number of N159 objects. The images used forspectroscopy were taken with the 2.5-m telescope at Las CampanasObservatory (Chile) during the nights from 26th to 28th November 2003.The images, 25 arc minutes wide, were taken with the Wide FieldReimaging CCD Camera, using masks for multiobjet spectroscopy withmedium spectral resolution.In this study aproximately 150 stars were classificated as a result ofthe analysis of 5 masks. We have found 50 O-type stars, 70 early B typestars and 30 stars of spectral type later than A (which most probablyare field stars) in a region where no spectral classification had beenobtained before.
| Low mass pre-main sequence stars in the Large Magellanic Cloud
As a part of an ongoing effort to characterise the young stellarpopulations in the Large Magellanic Cloud, we present HST-WFPC2 broadand narrow band imaging of two fields with recent star formationactivity in the Tarantula region. A population of objects with Hαand/or Balmer continuum excess was identified. On account of the intenseHα emission (equivalent widths up to several tens of Å), itscorrelation with the Balmer continuum excess and the stars' location onthe HR diagram, we interpret them as low mass ( 1{-}2Mȯ) Pre-Main Sequence stars. In this framework, the datashow that coeval high and low mass stars have significantly differentspatial distributions, implying that star formation processes fordifferent ranges of stellar masses are rather different and/or requiredifferent initial conditions. We find that the overall slope of the massfunction of the young population is somewhat steeper than the classicalSalpeter value and that the star formation density of this youngcomponent is 0.2{-}0.4 Mȯ yr-1kpc-2, i.e. intermediate between the value for an activespiral disk and that of a starburst region. The uncertainties associatedwith the determination of the slope of the mass function and the starformation density are thoroughly discussed.
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| The IMF of the field population of 30 Doradus
The star-formation history and IMF of the field population of the 30Doradus super-association is determined using Wide Field Imagerphotometry. The cluster NGC 2070 and the OB association LH104 are alsostudied and used for comparison. The star-formation history of the 30Doradus super-association appears to be characterized by a largeincrease in star-formation activity 10 Myr to 20 Myr ago. This seems tobe the case across the whole eastern half of the LMC as demonstrated bythe ages of stellar populations as far away as 30 Doradus and Shapley'sConstellation III. Star-formation appears to be occurring at a constantrate in the field and in loose associations, and in bursts in theclusters. The field IMF is found to have almost the exact Salpeter slopein the range 7 ~M_ȯ ≤ M ≤ 40 ~M_ȯ, at odds with previousclaims. We find that, for objects with more complex star-formationhistories, Be stars and selective incompleteness strongly affect thedetermination of the IMF for M > 40~ M_ȯ, naturally explainingthe observed deviation of the high mass IMF slope from the Salpetervalue. The present work supports the idea of a universal IMF.Based on observations obtained with the MPG/ESO 2.2-m telescope at LaSilla Observatory.Tables 1-3 are only available in electronic form athttp://www.edpsciences.org.Full Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/443/851
| L-band (3.5 μm) IR-excess in massive star formation. I. 30 Doradus
L-band data of 30 Doradus at 3.5 μm taken with SPIREX (South PoleInfrared Explorer) is presented. The photometry was combined with 2MASSJHK data at 1.25-2.2 μm. Colour-colour and colour-magnitude diagramsare constructed and used to determine the sources with infrared excess.These are interpreted as circumstellar disks, and enable the fraction ofsources with disks (the cluster disk fraction or CDF) to be determined.We find that ~42% of the sources detected at L-band in 30 Doradus havean IR-excess.
| On the massive stellar population of the super star cluster Westerlund 1
We present new spectroscopic and photometric observations of the youngGalactic open cluster Westerlund 1 (Wd 1) that reveala unique population of massive evolved stars. We identify ~200 clustermembers and present spectroscopic classifications for ~25% of these. Wefind that all stars so classified are unambiguously post-Main Sequenceobjects, consistent with an apparent lack of an identifiable MainSequence in our photometric data to V 20. We are able to identifyrich populations of Wolf Rayet stars, OB supergiants and short livedtransitional objects. Of these, the latter group consists of both hot(Luminous Blue Variable and extreme B supergiant) and cool (YellowHypergiant and Red Supergiant) objects - we find that half the knownGalactic population of YHGs resides within Wd 1. We obtain a meanV-MV ~ 25 mag from the cluster Yellow Hypergiants, implying aMain Sequence turnoff at or below MV =-5 (O7 V or later).Based solely on the masses inferred for the 53 spectroscopicallyclassified stars, we determine an absolute minimum mass of ~1.5 ×10^3~Mȯ for Wd 1. However, considering the completephotometrically and spectroscopically selected cluster population andadopting a Kroupa IMF we infer a likely mass for Wd 1 of~10^5~Mȯ, noting that inevitable source confusion andincompleteness are likely to render this an underestimate. As such, Wd 1is the most massive compact young cluster yet identified in the LocalGroup, with a mass exceeding that of Galactic Centre clusters such asthe Arches and Quintuplet. Indeed, the luminosity, inferred mass andcompact nature of Wd 1 are comparable with those of Super Star Clusters- previously identified only in external galaxies - and is consistentwith expectations for a Globular Cluster progenitor.
| The M/L* Ratio of Young Star Clusters in Galactic Mergers
We point out a strong time evolution of the mass-to-light conversionfactor η commonly used to estimate masses of dense star clustersfrom observed cluster radii and stellar velocity dispersions. We use agasdynamical model coupled with the Cambridge stellar evolution tracksto compute line-of-sight velocity dispersions and half-light radiiweighted by the luminosity. Stars at birth are assumed to follow theSalpeter initial mass function in the range of 0.15-17Msolar. We find that η, and hence the estimated clustermass, increases by factors as large as 3 over timescales of 20 millionyears. Increasing the upper mass limit to 50 Msolar leads toa sharp rise of similar amplitude but in as little as 10 million years.Fitting truncated isothermal (Michie-King) models to the projected lightprofile leads to overestimates of the concentration parameter c ofδc~0.3 compared to the same functional fit applied to theprojected mass density.
| MSX mid-infrared imaging of massive star birth environments - II. Giant HII regions
We conduct a Galactic census of giant HII (GHII) regions, based on theall-sky 6-cm data set of Kuchar & Clark, plus the kinematicdistances obtained by Russeil. From an inspection of mid-infrared (MIR)Mid-course Space Experiment (MSX) and far-IR IRAS Sky Survey Atlasimages, we identify a total of 56 GHII regions in the Milky Way, ofwhich 15 per cent (65 per cent) can be seen at optical (near-IR)wavelengths. The mid to far-IR fluxes from each GHII region aremeasured, and sample the thermal emission from the ubiquitous dustpresent within the exciting clusters of OB stars, arising from theintegrated luminosity of the hot stars heating the cluster dust, forwhich we obtain log L(IR) = 5.5-7.3Lsolar. The MIR 21-μmspatial morphology is presented for each GHII region, and oftenindicates multiple emission sources, suggesting complicated clusterformation. IR colour-colour diagrams are presented, providinginformation concerning the temperature distribution and the opticaldepth of the dust. For the clusters of our study, the dust is notoptically thick to all stellar radiation, thus the measured infraredluminosity is lower than Lbol. As the dust environment of acluster begins to dissipate, the thermal emission and its optical depthought to decrease even before the stars evolve appreciably. We seeevidence of this in our empirical relationship between the integrated IRand Lyman continuum luminosities.
| Infrared mergers and infrared quasi-stellar objects with galactic winds - II. NGC5514: two extranuclear starbursts with LINER properties and a supergiant bubble in the rupture phase
A study of the morphology, kinematics and ionization structure of theinfrared (IR) merger NGC5514 is presented. This study is based mainly onINTEGRAL two-dimensional (2D) spectroscopy (obtained at the 4.2-mWilliam Herschel Telescope, WHT), plus optical and near-IR images. Clearevidence of two extranuclear starbursts with young outflows (OFs) andlow-ionization nuclear emission region (LINER) activity are reported.One of these OFs has generated a supergiant bubble and the other isassociated with an extended complex of HII regions.In the galactic bubble it was found that: (i) the [SII], Hα,[NII], [OI] and [OIII] emission-line maps show a bubble with a distortedellipsoidal shape, with major and minor axes of ~6.5 kpc [13.6 arcsec;at position angle (PA) = 120°+/- 10°] and ~4.5 kpc (9.6 arcsec);(ii) these maps depict four main knots, a very strong one and threeothers more compact and located at the border; (iii) the centre of thebubble is located at ~4.1 kpc (8.5 arcsec) to the west of the mainnucleus; (iv) the WHT spectra show, in this area, two strong components:blue and red emission-line systems, probably associated with emissionfrom the near and far side of the external shell, for which the mean OFvelocities were measured as VOFblue= (-320 +/- 20)kms-1 and VOFred= (+265 +/- 25) kms-1(v) these two components depict LINER properties, probably associatedwith large-scale OF + shocks; (vi) at the east border, the kinematics ofthe ionized gas and the [SII] emission-line maps show an extendedejection of 4 kpc aligned with the PA of the major axis; (vii) threeother ejections were found, two of them perpendicular to the extendedone. Each ejection starts in one of the knots. These results suggestthat the bubble is in the rupture phase.For the complex of giant HII regions it was found that: (i) theHα, [NII] and [SII] emission-line maps show a compact strongemission area (peaking at ~810 pc ~1.7 arcsec, to the east of the secondnucleus) and faint extended emission with an elongated shape, and majorand minor axes of ~5.1 kpc (10.8 arcsec; at PA ~20°) and ~2.9 kpc(6.0 arcsec); (ii) inside this complex, the spectra show HII region andtransition LINER/HII characteristics; (iii) at the border of thisextended HII area the spectra have outflow components and LINERproperties.INTEGRAL 2D [NII], Hα, [SII] and [OIII] velocity fields (VFs) arepresented. These VF maps show results consistent with an expansion ofthe bubble, plus four ejections of ionized gas. The U, B, V, I, J, H andKS images show a pre-merger morphology, from which faintfilaments of emission emerge, centred on the bubble. The ionizationstructure and the physical conditions were analysed using the following2D emission-line ratio and width maps: [SII]/Hα, [NII]/Hα,[OI]/Hα, [OIII]/Hβ and FWHM-[NII]. In the region of thebubble, 100 per cent of the [NII]/Hα and [SII]/Hα ratiosshow very high values (>0.8) consistent with LINER processesassociated with high-velocity shocks. These new results support theprevious proposition that extreme nuclear and `extranuclear' starburstswith galactic winds + shocks play an important role in the evolution ofIR mergers/quasi-stellar objects.
| OH (1720 MHz) Maser Search toward the Large Magellanic Cloud
We have carried out a sensitive search for OH (1720 MHz) masers in theLarge Magellanic Cloud (LMC) toward five regions using the AustraliaTelescope Compact Array. Our source list includes the 30 Doradus regionand four supernova remnants (SNRs): N44, N49, N120, and N132D. Thesedata have a typical resolution of ~8" and rms noise levels of 5-10 mJybeam-1. We have detected OH (1720 MHz) masers in thenortheast part of 30 Doradus and toward the SNR N49. The OH (1720 MHz)maser emission in 30 Doradus is coincident with a cluster of young starsknown as ``knot 1'' and is almost certainly of the star formationvariety. Our spectral resolution (0.68 km s-1) isinsufficient to detect the Zeeman effect from the strongest (~320 mJybeam-1) of the 30 Doradus OH (1720 MHz) masers, leading to anupper limit to the field strength of 6 mG. The weak OH (1720 MHz) maseremission (35 mJy beam-1) detected toward the LMC SNR N49 islocated just west of a previously identified CO cloud and is indicativeof an interaction between the SNR and the molecular cloud. Although thestatistics are low, the detection rate seems consistent with that seenfor Galactic star-forming region and SNR type OH (1720 MHz) masers-bothof which are low.
| Core Formation by a Population of Massive Remnants
Core radii of globular clusters in the Large and Small Magellanic Cloudsshow an increasing trend with age. We propose that this trend is adynamical effect resulting from the accumulation of massive stars andstellar-mass black holes at the cluster centers. The black holes areremnants of stars with initial masses exceeding ~20-25 Msolaras their orbits decay by dynamical friction, they heat the stellarbackground and create a core. Using analytical estimates and N-bodyexperiments, we show that the sizes of the cores so produced and theirgrowth rates are consistent with what is observed. We propose that thismechanism is responsible for the formation of cores in all globularclusters and possibly in other systems as well.
| Characterizing the Stellar Population in NGC 1705-1
We observed the brightest super-star cluster NGC 1705-1 in the nearbydwarf galaxy NGC 1705 with the Space Telescope Imaging Spectrograph inthe echelle mode between 1200 and 3100 Å. The data allow a studyof the young stellar population at hitherto unprecedented spectralresolution and signal-to-noise ratios. A comprehensive list of strongand weak stellar and interstellar absorption lines is given, togetherwith the measured line parameters. Four distinct velocity systems areidentified: stellar lines at the measured H I velocity, blueshiftedinterstellar lines from outflowing gas, Milky Way foreground absorption,and a high-velocity cloud. Comparison with stellar template spectraindicates an equivalent spectral type of B0 to B1, with mostly dwarf andgiant stars contributing. When placed on a theoreticalHertzsprung-Russell diagram, these stars constrain the age of NGC 1705-1to 12+/-31 Myr. Since this age is derived purelyfrom spectroscopy, it is independent of reddening corrections. Acomparison of the observed and theoretical mass-to-light ratio for thederived age was performed. We find no significant evidence for ananomalous initial mass function at the low-mass end, contrary tosuggestions found in the literature. The stellar population of NGC1705-1 is similar to that in other massive clusters, such as 30 Doradusor NGC 1569-A, after taking into account age differences and modeluncertainties. We discuss the difficulty of relating observed andtheoretical mass-to-light ratios because of the unknown gas massfraction lost by the cluster and the uncertain mass-loss rates ofasymptotic giant branch stars in population synthesis models.
| A Study of Cygnus OB2: Pointing the Way toward Finding Our Galaxy's Super-Star Clusters
New optical MK classification spectra have been obtained for 14 OB starcandidates identified by Comerón et al. and presumed to bepossible members of the Cyg OB2 cluster as recently described byKnödlseder. All 14 candidate OB stars observed are indeedearly-type stars, strongly suggesting the remaining 31 candidates byComerón et al. are also early-type stars. A thoroughinvestigation of the properties of these new candidate members comparedwith the properties of the Cyg OB2 cluster star has been completed,using traditional as well as newly revised effective temperature scalesfor O stars. The cooler O star effective temperature scale of Martins etal. gives a very close distance for the cluster (DM=10.4). However, evenusing traditional effective temperature scales, Cyg OB2 appears to beslightly closer (DM=10.8) than previous studies determined (DM=11.2;Massey & Thompson), when the very young age of the stellar cluster(~2×106 yr) is taken into account in fitting the late-Oand early-B dwarfs to model isochrones. Of the 14 new OB stars observedfor this study, as many as half appear to be significantly older thanthe previously studied optical cluster, making their membership in CygOB2 suspect. So, while some of the newly identified OB stars mayrepresent a more extended halo of the Cyg OB2 cluster, the survey ofComerón et al. also picked up a large fraction of nonmembers.Presently, estimates of the very high mass of this cluster(Mcl~104 Msolar and over 100 O stars)first made by Knödlseder remain higher than this study can support.Despite this, the recognition of Cyg OB2 as a more massive and extensivestar cluster than previously realized using 2MASS images, along with therecently recognized candidate super-star cluster Westerlund 1 only a fewkiloparsecs away (Clark & Negueruela), reminds us that we arewoefully underinformed about the massive cluster population in ourGalaxy. Extrapolations of the locally derived cluster luminosityfunction indicate that tens to perhaps a hundred of these very massiveopen clusters (Mcl~104 Msolar,MV~-11) should exist within our galaxy. Radio surveys willnot detect these massive clusters if they are more than a few millionyears old. Our best hope for remedying this shortfall is through deepinfrared searches and follow-up near-infrared spectroscopicobservations, as were used by Comeron et al. to identify candidatemembers of the Cyg OB2 association.
| Supernova Remnant Candidates in the 30 Doradus Nebula
We have searched the 30 Doradus nebula, a giant extragalactic H IIregion, for the presence of embedded supernova remnants usinghigh-resolution radio, optical, and X-ray images. Radio images obtainedat 6 and 3.5 cm with the Australia Telescope Compact Array have anangular resolution of ~2". We compare our radio images with the opticalimages from the Cerro Tololo Inter-American Observatory to distinguishthermal and possible nonthermal components in the nebula. We have alsomapped, for the first time, the extinction across the region at highangular resolution. We identify two sources as supernova remnantcandidates in the nebula. Derived physical parameters for the adiabaticphase imply relatively young remnants (~103 yr) evolving in avery dense environment (>~102 cm-3). We compareour results with high-resolution data from the Chandra X-RayObservatory, but with only a short exposure these preliminary X-ray datalack the sensitivity to resolve our supernova remnant candidates fromintervening hot gas of the diffuse thermal X-ray component of thenebula.
| Giant H II Regions in the Merging System NGC 3256: Are They the Birthplaces of Globular Clusters?
CCD images and spectra of ionized hydrogen in the merging system NGC3256 were acquired as part of a kinematic study to investigate theformation of globular clusters (GCs) during the interactions and mergersof disk galaxies. This paper focuses on the proposition by Kennicutt& Chu that giant H II regions (GHRs), with an Hα luminositygreater than 1.5×1040 ergs s-1, arebirthplaces of young populous clusters (YPCs). Although, compared withsome other interacting systems, NGC 3256 has relatively few (seven)giant H II complexes, these regions are comparable in total flux toabout 85 30 Doradus-like giant H II regions (GHRs). The bluest, massiveYPCs (Zepf et al.) are located in the vicinity of observed 30 Dor GHRs,contributing to the notion that some fraction of 30 Dor GHRs do cradlemassive YPCs, as 30 Dor harbors R136. If interactions induce theformation of 30 Dor GHRs, the observed luminosities indicate that almost900 30 Dor GHRs would form in NGC 3256 throughout its merger epoch. Inorder for 30 Dor GHRs to be considered GC progenitors, this number mustbe consistent with the specific frequencies of globular clustersestimated for elliptical galaxies formed via mergers of spirals (Ashman& Zepf). This only requires that about 10% of NGC 3256's 900 30 DorGHRs harbor YPCs, which survive several gigayears and have masses>=MR136.
| The Chemical Composition of the 30 Doradus Nebula Derived from Very Large Telescope Echelle Spectrophotometry
Echelle spectrophotometry of the 30 Doradus nebula in the LMC ispresented. The data consist of VLT UVES observations in the 3100-10350Å range. The intensities of 366 emission lines have been measured,including 269 identified permitted lines of H0,He0, C0, C+, N+,N++, O0, O+, Ne0,Ne+, S+, S++, Si0,Si+, Si++, Ar+, and Mg+;many of them are produced by recombination only while others mainly byfluorescence. Electron temperatures and densities have been determinedusing different line intensity ratios. The He+,C++, O+, and O++ ionic abundances havebeen derived from recombination lines; these abundances are almostindependent of the temperature structure of the nebula. Alternatively,abundances from collisionally excited lines have been derived for alarge number of ions of different elements; these abundances dependstrongly on the temperature structure. Accurate t2 valueshave been derived from the Balmer continuum and by comparing theC++, O+, and O++ ionic abundancesobtained from collisionally excited and recombination lines. Thechemical composition of 30 Doradus is compared with those of Galacticand extragalactic H II regions. The values of ΔY/ΔO,ΔY/ΔZ, and Yp are also discussed.Based on observations collected at the European Southern Observatory,Chile, proposal ESO 68.C-0149(A).
| Creating Hubble's Technicolor Universe
Not Available
| A Dozen Colliding-Wind X-Ray Binaries in the Star Cluster R136 in the 30 Doradus Region
We analyzed archival Chandra X-ray observations of the central portionof the 30 Doradus region in the Large Magellanic Cloud. The imagecontains 20 X-ray point sources with luminosities between5×1032 and 2×1035 ergs s-1(0.2-3.5 keV). A dozen sources have bright WN Wolf-Rayet or spectraltype O stars as optical counterparts. Nine of these are within ~3.4 pcof R136, the central star cluster of NGC 2070. We derive an empiricalrelation between the X-ray luminosity and the parameters for the stellarwind of the optical counterpart. The relation gives good agreement forknown colliding-wind binaries in the Milky Way Galaxy and for theidentified X-ray sources in NGC 2070. We conclude that probably allidentified X-ray sources in NGC 2070 are colliding-wind binaries andthat they are not associated with compact objects. This conclusioncontradicts earlier studies where it was argued, using ROSAT data, thattwo earlier discovered X-ray sources are accreting black hole binaries.Five of the 18 brightest stars in R136 are not visible in our X-rayobservations. These stars either are single, have low-mass companions,or have very wide orbits. The resulting binary fraction among early-typestars is then unusually high (at least 70%).
| 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 Lives and Deaths of Star Clusters near the Galactic Center
We study the evolution and observability of young, compact star clustersnear the Galactic center, such as the Arches and Quintuplet systems. Theclusters are modeled by integrating the equations of motion of all starswhile accounting for the internal evolution of stars and binaries, aswell as the effect of the Galactic tidal field. We find that clusterswithin 150 pc of the Galactic center dissolve within ~55 Myr, but theirprojected densities drop below the background density in the directionof the Galactic center within only a few megayears, effectively makingthese clusters undetectable after that time. Detailed observations ofthe Arches cluster, taken at face value, suggest that its mass functionis unusually flat and that the cluster contains an overabundance ofstars more massive than 20 Msolar. Our dynamical analysis,however, shows that the observed characteristics of the Arches clusterare consistent with a perfectly normal initial mass function. Theobserved anomalies are then caused by a combination of observationalselection effects and the dynamical evolution of the cluster. Wecalibrate the current parameters of the Arches cluster using a normalinitial mass function and conclude that the cluster is more massive than40,000 Msolar, has a half-mass radius of about 0.35 pc, andis located between 50 and 90 pc from the Galactic center.
| 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.
| Structural Properties of Massive Young Clusters
We have retrieved multicolor WFPC2/Hubble Space Telescope data from theSTScI archive for 27 nearby massive (>~3×104Msolar) young (<20 Myr) star clusters (MYCs). The datarepresents the most complete sample of clearly resolved MYCs to date. Wehave analyzed their structural properties and have found that they canbe classified as either super-star clusters (SSCs) or as scaled OBassociations (SOBAs). SSCs have a compact core possibly surrounded by ahalo while SOBAs have no core. A morphological sequence can beestablished from SSCs with weak halos to SSCs with strong halos toSOBAs, and we propose that this is linked to the original massdistribution of the parent giant molecular clouds. Our results indicatethat a significant fraction of the stars in MYCs dissipate on timescalesof 10 Gyr or less due to the extended character of some of the clusters.Also, SSCs with ages younger than 7 Myr have smaller cores on averagethan those with ages older than 7 Myr, confirming predictions ofnumerical simulations with mass loss.
| High-Mass, OB Star Formation in M51: Hubble Space Telescope Hα and Paα Imaging
We have obtained Hα and Paα emission-line images coveringthe central 3'-4' of M51 using the WFPC2 and NICMOS instruments on theHubble Space Telescope to study the high-mass stellar population. The0.1"-0.2" pixels provide 4.6-9 pc resolution in M51, and theHα/Paα line ratios are used to obtain extinction estimates.A sample of 1373 Hα emission regions is cataloged using anautomated and uniform measurement algorithm. Their sizes are typically10-100 pc. The luminosity function for the Hα emission regions isobtained over the range LHα=1036 to2×1039 ergs s-1. The luminosity function isfitted well by a power law with dN/dlnL~L-1.01. The power lawis significantly truncated, and no regions were found with observedLHα above 2×1039 ergs s-1(uncorrected for extinction). (The maximum seen in ground-based studiesis approximately a factor of 5 higher, very likely because of theblending of multiple regions.) The extinctions derived here increase themaximum intrinsic luminosity to above 1040 ergss-1. The logarithmically binned luminosity function is alsosomewhat steeper (α=-1.01) than that found from ground-basedimaging (α=-0.5 to -0.8)-probably also a result of our resolvingregions that were blended in the ground-based images. The two-pointcorrelation function for the H II regions exhibits strong clustering onscales <=2", or 96 pc. To analyze the variations of H II regionproperties vis-à-vis the galactic structure, the spiral arm areaswere defined independently from millimeter-CO and optical continuumimaging. Although the arms constitute only 25% of the disk surface area,the arms contain 45% of the cataloged H II regions. The luminosityfunction is somewhat flatter in spiral arm regions than in the interarmareas (-0.72 to -0.95) however, this is very likely the result ofincreased blending of individual H II regions in the arms that havehigher surface density. No significant difference is seen in the sizesand electron densities of the H II regions in spiral arm and interarmregions. For 209 regions that had >=5 σ detections in bothPaα and Hα, the observed line ratios indicate visualextinctions in the range AV=0-6 mag. The mean extinction wasAV=3.1 mag (weighting each region equally), 2.4 mag(weighting each by the observed Hα luminosity), and 3.0 mag(weighting by the extinction-corrected luminosity). On average, theobserved Hα luminosities should be increased by a factor of ~10,implying comparable increases in global OB star cluster luminosities andstar formation rates. The full range of extinction-corrected Hαluminosities is between 1037 and 2×1040 ergss-1. The most luminous regions have sizes >=100 pc, so itis very likely that they are blends of multiple regions. This is clearbased on their sizes, which are much larger than the maximum diameter(<=50 pc) to which an H II region might conceivably expand within the~3×106 yr lifetime of the OB stars. It is alsoconsistent with the observed correlation (L~D2) between themeasured luminosities and sizes of the H II regions. We thereforegenerated a subsample of 1101 regions with sizes <=50 pc, which ismade up of those regions that might conceivably be ionized by a singlecluster. Their extinction-corrected luminosities range between2×1037 and 1039 ergs s-1, orbetween two-thirds of M42 (the Orion Nebula) and W49 (the most luminousGalactic radio H II region). The upper limit for individual clusters istherefore conservatively <=1039 ergs s-1,implying QLyc,up~=7×1050 s-1(with no corrections for dust absorption of the Lyman continuum or UVthat escapes to the diffuse medium). This corresponds to cluster masses<=5000 Msolar (between 1 and 120 Msolar). Thetotal star formation rate in M51 is estimated from theextinction-corrected Hα luminosities to be ~4.2 Msolaryr-1 (assuming a Salpeter initial mass function between 1 and120 Msolar), and the cycling time from the neutralinterstellar medium into these stars is 1.2×109 yr. Wedevelop a simple model for the UV output from OB star clusters as afunction of the cluster mass and age in order to interpret constraintsprovided by the observed luminosity functions. The power-law index atthe high-luminosity end of the luminosity function (α=-1.01)impliesN(Mcl)/dMcl~M-2.01cl. Thisimplies that high-mass star formation, cloud disruption due to OB stars,and UV production are contributed to by a large range of cluster masseswith equal effects per logarithmic interval of cluster mass. Thehigh-mass clusters (~1000 Msolar) have a mass such that theinitial mass function is well sampled up to ~120 Msolar, butthis cluster mass is <=1% of that available in a typical giantmolecular cloud. We suggest that OB star formation in a cloud coreregion is terminated at the point that radiation pressure on thesurrounding dust exceeds the self-gravity of the core star cluster andthat this is what limits the maximum mass of standard OB star clusters.This occurs at a stellar luminosity-to-mass ratio of ~500-1000(L/M)solar, which happens for clusters >=750Msolar. We have modeled the core collapse hydrodynamicallyand have found that a second wave of star formation may propagateoutward in a radiatively compressed shell surrounding the core starcluster-this triggered, secondary star formation may be the mechanismfor formation of the super-star clusters seen in starburst galaxies.
| The ionising cluster of 30 Doradus. IV. Stellar kinematics
On the basis of multislit spectroscopy of 180 stars in the ionisingcluster of 30 Doradus we present reliable radial velocities for 55stars. We calculate a radial velocity dispersion of ~35 kms-1 for the cluster and we analyse the possible influence ofspectroscopic binaries in this rather large velocity dispersion. We usenumerical simulations to show that the observations are consistent withthe hypothesis that all the stars in the cluster are binaries, and thetotal mass of the cluster is ~ 5 x 105 Msun. Asimple test shows only marginal evidence for dynamical mass segregationwhich if present is most likely not due to dynamical relaxation. Basedon observations collected at the European Southern Observatory.
| Stellar Dymatics in 30 Doradus
From multislit spectroscopy of 180 stars in the ionising cluster of 30Doradus, we have obtained reliable stellar radial velocities for 58stars. With these we calculated the stellar radial velocity dispersionand found it to be 35 km/s. If the cluster is dinamically relaxed thevirial mass is ~ 1 × 107 Msolar
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