Contents
Images
Upload your image
DSS Images Other Images
Related articles
Surface Brightness Profiles for a Sample of LMC, SMC, and Fornax Galaxy Globular Clusters
We use Hubble Space Telescope archival images to measure central surfacebrightness profiles of globular clusters around satellite galaxies ofthe Milky Way. We report results for 21 clusters around the LMC, fivearound the SMC, and four around the Fornax dwarf galaxy. The profileswere obtained using a recently developed technique based on measuringintegrated light, which is tested on an extensive simulated data set.Our results show that for 70% of the sample, the central photometricpoints of our profiles are brighter than previous measurements usingstar counts with deviations as large as 2 mag arcsec-2. About40% of the objects have central profiles deviating from a flat centralcore, with central logarithmic slopes continuously distributed between-0.2 and -1.2. These results are compared with those found for a sampleof Galactic clusters using the same method. We confirm the knowncorrelation in which younger clusters tend to have smaller core radii,and we find that they also have brighter central surface brightnessvalues. This seems to indicate that globular clusters might be bornrelatively concentrated, and that a profile with an extended flat coremight not be the ideal choice for initial profiles in theoreticalmodels.
| Characterization of open cluster remnants
Context: Despite progress in the theoretical knowledge of open clusterremnants and the growing search for observational identifications inrecent years, open questions still remain. The methods used to analyzeopen cluster remnants and criteria to define them as physical systemsare not homogeneous. In this work we present a systematic method forstudying these objects that provides a view of their properties andallows their characterization. Aims: Eighteen remnant candidates areanalyzed by means of photometric and proper motion data. These dataprovide information on objects and their fields. We establish criteriafor characterizing open cluster remnants, taking observationaluncertainties into account. Methods: 2MASS J and H photometry isemployed (i) to study structural properties of the objects by means ofradial stellar density profiles, (ii) to test for any similarity betweenobjects and fields with a statistical comparison method applied to thedistributions of stars in the CMDs, and (iii) to obtain ages, reddeningvalues, and distances from the CMD, taking an index of isochrone fitinto account. The UCAC2 proper motions allowed an objective comparisonbetween objects and large solid angle offset fields. Results: Theobjective analysis based on the present methods indicates 13open-cluster remnants in the sample. Evidence of the presence of binarystars is found, as expected for dynamically evolved systems. Finally, weinfer possible evolutionary stages among remnants from the structure,proper motion, and CMD distributions. The low stellar statistics forindividual objects is overcome by means of the construction of compositeproper motion and CMD diagrams. The distributions of remnants in thecomposite diagrams resemble the single-star and unresolved binary stardistributions of open clusters.
| The VLT-FLAMES survey of massive stars: wind properties and evolution of hot massive stars in the Large Magellanic Cloud
We have studied the optical spectra of a sample of 28 O- and earlyB-type stars in the Large Magellanic Cloud, 22 of which are associatedwith the young star forming region N11. Our observations sample thecentral associations of LH9 and LH10, and the surrounding regions.Stellar parameters are determined using an automated fitting method(Mokiem et al. 2005), which combines the stellar atmosphere codefastwind (Puls et al. 2005) with the genetic algorithm basedoptimisation routine pikaia (Charbonneau 1995). We derive an age of 7.0± 1.0 and 3.0 ± 1.0 Myr for LH9 and LH10, respectively.The age difference and relative distance of the associations areconsistent with a sequential star formation scenario in which stellaractivity in LH9 triggered the formation of LH10. Our sample containsfour stars of spectral type O2. From helium and hydrogen line fitting wefind the hottest three of these stars to be 49{-}54 kK (compared to45{-}46 kK for O3 stars). Detailed determination of the helium massfraction reveals that the masses of helium enriched dwarfs and giantsderived in our spectroscopic analysis are systematically lower thanthose implied by non-rotating evolutionary tracks. We interpret this asevidence for efficient rotationally enhanced mixing leading to thesurfacing of primary helium and to an increase of the stellarluminosity. This result is consistent with findings for SMC stars byMokiem et al. (2006). For bright giants and supergiants no such massdiscrepancy is found; these stars therefore appear to follow tracks ofmodestly or non-rotating objects. The set of programme stars wassufficiently large to establish the mass loss rates of OB stars in thisZ ˜ 1/2 Zȯ environment sufficiently accurate toallow for a quantitative comparison with similar objects in the Galaxyand the SMC. The mass loss properties are found to be intermediate tomassive stars in the Galaxy and SMC. Comparing the derived modified windmomenta D_mom as a function of luminosity with predictions for LMCmetallicities by Vink et al. (2001) yields good agreement in the entireluminosity range that was investigated, i.e. 5.0 < logL/Lȯ< 6.1.Appendix A is only available in electronic form at http://www.aanda.org
| A hypervelocity star from the Large Magellanic Cloud
We study the acceleration of the star HE0437-5439to hypervelocity anddiscuss its possible origin in the Large Magellanic Cloud (LMC). Thestar has a radial velocity of 723kms-1 and is located at adistance of 61 kpc from the Sun. With a mass of about8Msolar, the traveltime from the Galactic Centre is about 100Myr, much longer than its main-sequence lifetime. Given the relativelysmall distance to the LMC (18 kpc), we consider it likely thatHE0437-5439originated in the Cloud rather than in the Galactic Centrelike the other hypervelocity stars. The minimum ejection velocityrequired to travel from the LMC to its current location within itslifetime is about 500kms-1. Such a high velocity can only beobtained in a dynamical encounter with a massive black hole. We performthree-body scattering simulations in which a stellar binary encounters amassive black hole, and find that a black hole more massive than103Msolar is necessary to explain the highvelocity of HE0437-5439. We look for possible parent clusters forHE0437-5439, and find that NGC2100 and 2004 are young enough to hoststars coeval to HE0437-5439and dense enough to produce anintermediate-mass black hole able to eject an 8-Msolar starwith hypervelocity.
| Effects of metallicity, star-formation conditions, and evolution in B and Be stars. II. Small Magellanic Cloud, field of NGC 330
Aims.We search for the effects of metallicity on B and Be stars in theSmall and Large Magellanic Clouds (SMC and LMC) and in the Milky Way(MW), by extending our previous analysis of B and Be star populations inthe LMC to the SMC. The rotational velocities of massive stars and theevolutionary status of Be stars are examined with respect to theirenvironments. Methods: Spectroscopic observations of hot starsbelonging to the young cluster SMC-NGC 330 and its surrounding regionwere obtained with the VLT-GIRAFFE facilities in MEDUSA mode. Wedetermined fundamental parameters for B and Be stars with the GIRFITcode, taking the effect of fast rotation and the age of observedclusters into account. We compared the mean V sin i obtained by spectraltype- and mass-selection for field and cluster B and Be stars in the SMCwith the one in the LMC and MW. Results: We find that (i) B and Bestars rotate faster in the SMC than in the LMC and in the LMC than inthe MW; (ii) at a given metallicity, Be stars begin their main sequencelife with a higher initial rotational velocity than B stars.Consequently, only a fraction of the B stars that reach the ZAMS with asufficiently high initial rotational velocity can become Be stars; (iii)the distributions of initial rotational velocities at the ZAMS for Bestars in the SMC, LMC, and MW are mass- and metallicity-dependent; (iv)the angular velocities of B and Be stars are higher in the SMC than inthe LMC and MW; (v) in the SMC and LMC, massive Be stars appear in thesecond part of the main sequence, in contrast to massive Be stars in theMW.Tables 1-6, 8 and 11 are only available in electronic form athttp://www.aanda.org
| 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
| The Role of Evolutionary Age and Metallicity in the Formation of Classical Be Circumstellar Disks. I. New Candidate Be Stars in the LMC, SMC, and Milky Way
We present B, V, R, and Hα photometry of eight clusters in theSmall Magellanic Cloud, five in the Large Magellanic Cloud, and threeGalactic clusters and use two-color diagrams (2-CDs) to identifycandidate Be star populations in these clusters. We find evidence thatthe Be phenomenon is enhanced in low-metallicity environments, based onthe observed fractional early-type candidate Be star content of clustersof age 10-25 Myr. Numerous candidate Be stars of spectral types B0-B5were identified in clusters of age 5-8 Myr, challenging the suggestionof Fabregat & Torrejon that classical Be stars should only be foundin clusters at least 10 Myr old. These results suggest that asignificant number of B-type stars must emerge onto the zero-age mainsequence as rapid rotators. We also detect an enhancement in thefractional content of early-type candidate Be stars in clusters of age10-25 Myr, suggesting that the Be phenomenon does become more prevalentwith evolutionary age. We briefly discuss the mechanisms that mightcontribute to such an evolutionary effect. A discussion of thelimitations of using the 2-CD technique to investigate the roleevolutionary age and/or metallicity play in the development of the Bephenomenon is offered, and we provide evidence that other B-type objectsof very different nature, such as candidate Herbig Ae/Be stars, maycontaminate the claimed detections of Be stars via 2-CDs.
| 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.
| A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters
The (rest-frame) near-IR domain contains important stellar populationdiagnostics and is often used to estimate masses of galaxies at low, aswell as high, redshifts. However, many stellar population models arestill relatively poorly calibrated in this part of the spectrum. Toallow an improvement of this calibration we present a new database ofintegrated near-IR JHKs magnitudes for 75 star clusters inthe Magellanic Clouds, using the Two Micron All Sky Survey (2MASS). Themajority of the clusters in our sample have robust age and metallicityestimates from color-magnitude diagrams available in the literature, andpopulate a range of ages from 10 Myr to 15 Gyr and a range in [Fe/H]from -2.17 to +0.01 dex. A comparison with matched star clusters in the2MASS Extended Source Catalog (XSC) reveals that the XSC only provides agood fit to the unresolved component of the cluster stellar population.We also compare our results with the often-cited single-channel JHKphotometry of Persson and coworkers and find significant differences,especially for their 30" diameter apertures, up to ~2.5 mag in the Kband, more than 1 mag in J-K, and up to 0.5 mag in H-K. Usingsimulations to center apertures based on maximum light throughput (asperformed by Persson et al.), we show that these differences can beattributed to near-IR-bright cluster stars (e.g., carbon stars) locatedaway from the true center of the star clusters. The wide age andmetallicity coverage of our integrated JHKs photometry sampleconstitute a fundamental data set for testing population synthesis modelpredictions and for direct comparison with near-IR observations ofdistant stellar populations.
| Effects of metallicity, star-formation conditions, and evolution in B and Be stars. I. Large Magellanic Cloud, field of NGC 2004
Aims.To statistically study the effects of the metallicity,star-formation conditions, and evolution on the behaviour of massivestars and, more particularly, of B and Be stars, we observed largesamples of stars in the Magellanic Clouds for the first time. In thisarticle we present the first part of this study. Methods:.Spectroscopic observations of hot stars belonging to the young clusterLMC-NGC 2004 and its surrounding region were carried out with theVLT-GIRAFFE facilities in MEDUSA mode. We determined the fundamentalparameters (T_eff, log~g, V sin i, and radial velocity) for all B and Bestars in the sample thanks to a code developed in our group. The effectof fast rotation (stellar flattening and gravitational darkening) aretaken into account in this study. We also determined the age of observedclusters. We then compared the mean V sin i obtained for field andcluster B and Be stars in the Large Magellanic Cloud (LMC) with the onesin the Milky Way (MW). Results: .We find, in particular, that Bestars rotate faster in the LMC than in the MW, in the field as well asin clusters. We discuss the relations between V sin i, metallicity,star-formation conditions, and stellar evolution by comparing the LMCwith the MW. We conclude that Be stars began their main sequence lifewith an initial rotational velocity higher than the one for B stars. Itis probable that only part of the B stars, those with a sufficientinitial rotational velocity, can become Be stars. This result mayexplain the differences in the proportion of Be stars in clusters withsimilar ages.
| Equilibrium Star Cluster Formation
We argue that rich star clusters take at least several local dynamicaltimes to form and so are quasi-equilibrium structures during theirassembly. Observations supporting this conclusion include morphologiesof star-forming clumps, momentum flux of protostellar outflows fromforming clusters, age spreads of stars in the Orion Nebula cluster (ONC)and other clusters, and the age of a dynamical ejection event from theONC. We show that these long formation timescales are consistent withthe expected star formation rate in turbulent gas, as recently evaluatedby Krumholz & McKee. Finally, we discuss the implications of thesetimescales for star formation efficiencies, the disruption of gas bystellar feedback, mass segregation of stars, and the longevity ofturbulence in molecular clumps.
| A study of the B and Be star population in the field of the LMC open cluster NGC 2004 with VLT-FLAMES
Observations of hot stars belonging to the young cluster LMC-NGC 2004and its surrounding region have been obtained with the VLT-GIRAFFEfacilities in MEDUSA mode. 25 Be stars were discovered; the proportionof Be stars compared to B-type stars is found to be of the same order inthe LMC and in the Galaxy fields. 23 hot stars were discovered asspectroscopic binaries (SB1 and SB2), 5 of these are found to beeclipsing systems from the MACHO database, with periods of a few days.About 75% of the spectra in our sample are polluted by hydrogen(Hα and Hγ), [ion{S}{ii}] and [ion{N}{ii}] nebular lines.These lines are typical of ion{H}{ii} regions. They could be associatedwith patchy nebulosities with a bi-modal distribution in radialvelocity, with higher values (+335 km s-1) preferentiallyseen inside the southern part of the known bubble LMC4 observed inion{H}{i} at 21 cm.
| The VLT-FLAMES survey of massive stars.
Not Available
| 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.
| Pristine CNO Abundances from Magellanic Cloud B Stars. II. Fast Rotators in the Large Magellanic Cloud Cluster NGC 2004
We present spectroscopic abundance analyses of three main-sequence Bstars in the young Large Magellanic Cloud cluster NGC 2004. All threetargets have projected rotational velocities around 130 kms-1. Techniques are presented that allow the derivation ofstellar parameters and chemical abundances in spite of these high vsinivalues. Together with previous analyses of stars in this cluster, wefind no evidence among the main-sequence stars for effects due torotational mixing up to vsini~130 km s-1. Unless theequatorial rotational velocities are significantly larger than the vsinivalues, this finding is probably in line with theoretical expectations.NGC 2004/B30, a star of uncertain evolutionary status located in theblue Hertzsprung gap, clearly shows signs of mixing in its atmosphere.To verify the effects due to rotational mixing will therefore requirehomogeneous analysis of statistically significant samples oflow-metallicity main-sequence B stars over a wide range of rotationalvelocities.Based on observations carried out at the European Southern Observatory(ESO), Paranal, Chile, under program ID 66.D-0214(A).
| Dust-enshrouded giants in clusters in the Magellanic Clouds
We present the results of an investigation of post-Main Sequence massloss from stars in clusters in the Magellanic Clouds, based around animaging survey in the L'-band (3.8 μm) performed with the VLT at ESO.The data are complemented with JHKs (ESO and 2MASS) andmid-IR photometry (TIMMI2 at ESO, ISOCAM on-board ISO, and data fromIRAS and MSX). The goal is to determine the influence of initialmetallicity and initial mass on the mass loss and evolution during thelatest stages of stellar evolution. Dust-enshrouded giants areidentified by their reddened near-IR colours and thermal-IR dust excessemission. Most of these objects are Asymptotic Giant Branch (AGB) carbonstars in intermediate-age clusters, with progenitor masses between 1.3and ~5 M_ȯ. Red supergiants with circumstellar dust envelopes arefound in young clusters, and have progenitor masses between 13 and 20M_ȯ. Post-AGB objects (e.g., Planetary Nebulae) and massive starswith detached envelopes and/or hot central stars are found in severalclusters. We model the spectral energy distributions of the cluster IRobjects, in order to estimate their bolometric luminosities andmass-loss rates. The IR objects are the most luminous cluster objects,and have luminosities as expected for their initial mass andmetallicity. They experience mass-loss rates in the range from a few10-6 up to 10-4 M_ȯ yr-1 (ormore), with most of the spread being due to evolutionary effects andonly a weak dependence on progenitor mass and/or initial metallicity.About half of the mass lost by 1.3-3 M_ȯ stars is shed during thesuperwind phase, which lasts of order 105 yr. Objects withdetached shells are found to have experienced the highest mass-lossrates, and are therefore interpreted as post-superwind objects. We alsopropose a simple method to measure the cluster mass from L'-band images.
| HST/ACS observations of the old and metal-poor Sagittarius dwarf irregular galaxy
We have obtained deep color-magnitude diagrams (CMDs) of the Sagittariusdwarf irregular galaxy with the Advanced Camera for Surveys (ACS) onboard the Hubble Space Telescope. The new diagrams show for the firsttime the star formation history of SagDIG. The young main-sequence istraced down to mF606W ≃27.5, well separated from theolder He burning blue loop stars. The wide color range spanned by theblue and red super-giants is consistent with Padua evolutionary modelswith Z0.0004. A well-defined red giant branch indicates that thegalaxy is dominated by a stellar population older than 1 Gyr, for whichwe have estimated a metallicity in the range [ Fe/H] = -2.2 to -1.9depending on the adopted reddening. The identification of several Cstars on a luminous asymptotic giant branch, as well as the presence ofa conspicuous red clump with an elongated structure, are indicative ofan extended star formation epoch between 1 and 10 Gyr ago. Perhaps mostimportantly, we have identified a small population of stars on agenuinely old red horizontal branch at mF606W=25.70±0.05, which sets the first epoch of star formation inSagDIG back to 9{-}10 Gyr ago. This shows that SagDIG has been able tomaintain a very low mean metallicity over a 10 Gyr life time, and thatall Local Group dwarf galaxies share an ancient (≥10 Gyr) initialepisode of star formation. Thus, at the moment, I Zw 18 remains the onlyexception to this general trend, favoring the so-called "young galaxy"scenario where a very low metallicity galaxy is not necessarily old.However, a re-analysis of ACS observations of I Zw 18 suggests that anolder RGB population may be present also in I Zw 18. In order to addressthe interplay between stars and Inter-Stellar Medium, we compare thedistribution of atomic hydrogen with that of stellar populations ofvarious ages. The youngest stars are found in clumps located: (i) nearto, but not coincident with, the highest column densities of H I, and(ii) in tails reminiscent of propagating star formation. We find,however, little correlation between stellar populations older than 1Gyr and the H I. In particular we fail to find any generation of starsthat preferentially lie within the large H I hole. We therefore excludethe possibility that the ring-like H I distribution is the result ofmultiple supernova events. Alternative scenarios have to be explored forthe ring-like H I morphology of SagDIG, which amongst dwarf irregulargalaxies, is not unusual.
| The VLT-FLAMES survey of massive stars: Observations in the Galactic clusters NGC 3293, NGC 4755 and NGC 6611
We introduce a new survey of massive stars in the Galaxy and theMagellanic Clouds using the Fibre Large Array Multi-Element Spectrograph(FLAMES) instrument at the Very Large Telescope (VLT). Here we presentobservations of 269 Galactic stars with the FLAMES-Giraffe Spectrograph(R ≃ 25 000), in fields centered on the open clusters NGC 3293,NGC 4755 and NGC 6611. These data are supplemented by a further 50targets observed with the Fibre-Fed Extended Range Optical Spectrograph(FEROS, R = 48 000). Following a description of our scientificmotivations and target selection criteria, the data reduction methodsare described; of critical importance the FLAMES reduction pipeline isfound to yield spectra that are in excellent agreement with lessautomated methods. Spectral classifications and radial velocitymeasurements are presented for each star, with particular attention paidto morphological peculiarities and evidence of binarity. Theseobservations represent a significant increase in the known spectralcontent of NGC 3293 and NGC 4755, and will serve as standards againstwhich our subsequent FLAMES observations in the Magellanic Clouds willbe compared.
| The Variation of Integrated Star Initial Mass Functions among Galaxies
The integrated galaxial initial mass function (IGIMF) is the relevantdistribution function containing the information on the distribution ofstellar remnants, the number of supernovae, and the chemical enrichmenthistory of a galaxy. Since most stars form in embedded star clusterswith different masses, the IGIMF becomes an integral of the assumed(universal or invariant) stellar IMF over the embedded star cluster massfunction (ECMF). For a range of reasonable assumptions about the IMF andthe ECMF we find the IGIMF to be steeper (containing fewer massive starsper star) than the stellar IMF, but below a few solar masses it isinvariant and identical to the stellar IMF for all galaxies. However,the steepening sensitively depends on the form of the ECMF in thelow-mass regime. Furthermore, observations indicate a relation betweenthe star formation rate of a galaxy and the most massive young stellarcluster in it. The assumption that this cluster mass marks the upper endof a young-cluster mass function leads to a connection of the starformation rate and the slope of the IGIMF above a few solar masses. TheIGIMF varies with the star formation history of a galaxy. Notably, largevariations of the IGIMF are evident for dE, dIrr, and LSB galaxies witha small to modest stellar mass. We find that for any galaxy the numberof supernovae per star (NSNS) is suppressed relative to that expectedfor a Salpeter IMF. Dwarf galaxies have a smaller NSNS than massivegalaxies. For dwarf galaxies the NSNS varies substantially depending onthe galaxy assembly history and the assumptions made about the low-massend of the ECMF. The findings presented here may be of some consequencefor the cosmological evolution of the number of supernovae per low-massstar and the chemical enrichment of galaxies of different mass.
| Mass Segregation and the Initial Mass Function of Super Star Cluster M82-F
We investigate the initial mass function and mass segregation in superstar cluster M82-F with high-resolution Keck NIRSPEC echellespectroscopy. Cross-correlation with template supergiant spectraprovides the velocity dispersion of the cluster, enabling measurement ofthe kinematic (virial) mass of the cluster when combined with sizes fromNICMOS and Advanced Camera for Surveys (ACS) images. We find a mass of6.6+/-0.9×105Msolar based on near-IR lightand 7.0+/-1.2×105Msolar based on opticallight. Using PSF-fitting photometry, we derive the cluster'slight-to-mass (L/M) ratio in both near-IR and optical light and compareto population-synthesis models. The ratios are inconsistent with anormal stellar initial mass function for the adopted age of 40-60 Myr,suggesting a deficiency of low-mass stars within the volume sampled.King model light profile fits to new Hubble Space Telescope ACS imagesof M82-F, in combination with fits to archival near-IR images, indicatemass segregation in the cluster. As a result, the virial mass representsa lower limit on the mass of the cluster.Based on observations made at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and the National Aeronauticsand Space Administration. The Observatory was made possible by thegenerous financial support of the W. M. Keck Foundation.
| ISOCAM Observations of Globular Clusters in the Magellanic Clouds: The Data
Seventeen globular clusters in the Large and Small Magellanic Cloudswere observed in the mid-infrared wavelength region with the ISOCAMinstrument on board the Infrared Space Observatory (ISO). Observationswere made using the broadband filters LW1, LW2, and LW10, correspondingto the effective wavelengths of 4.5, 6.7, and 12 μm, respectively. Wepresent the photometry of point sources in each cluster, as well astheir precise positions and finding charts.Based on observations with ISO, an ESA project with instruments fundedby ESA Member states (especially the PI countries: France, Germany, theNetherlands and the United Kingdom) and with participation of ISAS andNASA.
| Ages and metallicities of star clusters: New calibrations and diagnostic diagrams from visible integrated spectra
We present homogeneous scales of ages and metallicities for starclusters from very young objects, through intermediate-age ones up tothe oldest known clusters. All the selected clusters have integratedspectra in the visible range, as well as reliable determinations oftheir ages and metallicities. From these spectra equivalent widths (EWs)of K Ca II, G band (CH) and Mg I metallic, and Hδ, Hγ andHβ Balmer lines have been measured homogeneously. The analysis ofthese EWs shows that the EW sums of the metallic and Balmer H lines,separately, are good indicators of cluster age for objects younger than10 Gyr, and that the former is also sensitive to cluster metallicity forages greater than 10 Gyr. We propose an iterative procedure forestimating cluster ages by employing two new diagnostic diagrams and agecalibrations based on the above EW sums. For clusters older than 10 Gyr,we also provide a calibration to derive their overall metal contents.
| Infrared Surface Brightness Fluctuations of Magellanic Star Clusters
We present surface brightness fluctuations (SBFs) in the near-IR for 191Magellanic star clusters available in the Second Incremental and All SkyData releases of the Two Micron All Sky Survey (2MASS) and compare themwith SBFs of Fornax Cluster galaxies and with predictions from stellarpopulation models as well. We also construct color-magnitude diagrams(CMDs) for these clusters using the 2MASS Point Source Catalog (PSC).Our goals are twofold. The first is to provide an empirical calibrationof near-IR SBFs, given that existing stellar population synthesis modelsare particularly discrepant in the near-IR. Second, whereas mostprevious SBF studies have focused on old, metal-rich populations, thisis the first application to a system with such a wide range of ages(~106 to more than 1010 yr, i.e., 4 orders ofmagnitude), at the same time that the clusters have a very narrow rangeof metallicities (Z~0.0006-0.01, i.e., 1 order of magnitude only). Sincestellar population synthesis models predict a more complex sensitivityof SBFs to metallicity and age in the near-IR than in the optical, thisanalysis offers a unique way of disentangling the effects of age andmetallicity. We find a satisfactory agreement between models and data.We also confirm that near-IR fluctuations and fluctuation colors aremostly driven by age in the Magellanic cluster populations and that inthis respect they constitute a sequence in which the Fornax Clustergalaxies fit adequately. Fluctuations are powered by red supergiantswith high-mass precursors in young populations and by intermediate-massstars populating the asymptotic giant branch in intermediate-agepopulations. For old populations, the trend with age of both fluctuationmagnitudes and colors can be explained straightforwardly by evolution inthe structure and morphology of the red giant branch. Moreover,fluctuation colors display a tendency to redden with age that can befitted by a straight line. For the star clusters only,(H-Ks)=(0.21+/-0.03)log(age)-(1.29+/-0.22) once galaxies areincluded, (H-Ks)=(0.20+/-0.02)log(age)-(1.25+/-0.16).Finally, we use for the first time a Poissonian approach to establishthe error bars of fluctuation measurements, instead of the customaryMonte Carlo simulations.This research has made use of the NASA/ IPAC Infrared Science Archive,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under contract with the National Aeronautics and SpaceAdministration.
| Mass segregation in young Magellanic Cloud star clusters: Four clusters observed with HST
We present the results of our investigation on the phenomenon of masssegregation in young star clusters in the Magellanic Clouds. HST/WFPC2observations on NGC 1818, NGC 2004 and NGC 2100 in the Large MagellanicCloud and NGC 330 in the Small Magellanic Cloud have been used for theapplication of diagnostic tools for mass segregation: i) the radialdensity profiles of the clusters for various mass groups and ii) theirmass functions (MFs) at various radii around their centres. All fourclusters are found to be mass segregated, but each one in a differentmanner. Specifically not all the clusters in the sample show the samedependence of their density profiles on the selected magnitude range,with NGC 1818 giving evidence of a strong relation and NGC 330 showingonly a hint of the phenomenon. NGC 2004 did not show any significantsignature of mass segregation in its density profiles either. The MFsradial dependence provides clear proof of the phenomenon for NGC 1818,NGC 2100 and NGC 2004, while for NGC 330 it gives only indications. Aninvestigation of the constraints introduced by the application of bothdiagnostic tools is presented. We also discuss the problems related tothe construction of a reliable MF for a cluster and their impact on theinvestigation of the phenomenon of mass segregation. We find that theMFs of these clusters as they were constructed with two methods arecomparable to Salpeter's IMF. A discussion is given on the dynamicalstatus of the clusters and a test is applied on the equipartition amongseveral mass groups in them. Both showed that the observed masssegregation in the clusters is of primordial nature.
| The Stellar Content of the Southern Tail of NGC 4038/4039 and a Revised Distance
We have used the Hubble Space Telescope and Wide Field Planetary Camera2 to image the putative tidal dwarf galaxy located at the tip of theSouthern tidal tail of NGC 4038/4039, the Antennae. We resolveindividual stars and identify two stellar populations. Hundreds ofmassive stars are present, concentrated into tight OB associations onscales of 200 pc, with ages ranging from 2 to 100 Myr. An older stellarpopulation is distributed roughly following the outer contours of theneutral hydrogen in the tidal tail; we associate these stars withmaterial ejected from the outer disks of the two spirals. The olderstellar population has a red giant branch tip at I=26.5+/-0.2 from whichwe derive a distance modulus (m-M)0=30.7+/-0.25. The implieddistance of 13.8+/-1.7 Mpc is significantly smaller than commonly quoteddistances for NGC 4038/4039. In contrast to the previously studied coreof the merger, we find no super-star clusters (SSCs). One might concludethat SSCs require the higher pressures found in the central regions inorder to form, while spontaneous star formation in the tail produces thekind of OB star associations seen in dwarf irregular galaxies. Theyoungest population in the putative tidal dwarf has a total stellar massof ~2×105 Msolar, while the old populationhas a stellar mass of ~7×107 Msolar. If oursmaller distance modulus is correct, it has far-reaching consequencesfor this prototypical merger. Specifically, the luminous to dynamicalmass limits for the tidal dwarf candidates are significantly less than1, the central SSCs have sizes typical of Galactic globular clusters,rather than being 1.5 times as large, and the unusually luminous X-raypopulation becomes both less luminous and less populous.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. These observations were made in connection withproposal GO-6669.
| Rotation of Early B-type Stars in the Large Magellanic Cloud: The Role of Evolution and Metallicity
I present measurements of the projected rotational velocities of asample of 100 early B-type main-sequence stars in the Large MagellanicCloud (LMC). This is the first extragalactic study of the distributionof stellar rotational velocities. The sample is drawn from two sources:from the vicinity of the main-sequence turnoff of young clusters (ages1-3×107yr) and from the general field. I find that thecluster population exhibits significantly more rapid rotation than thatseen in the field. I have drawn analogous Galactic cluster and fieldsamples from the literature. Comparison of these samples reveals thesame effect. I propose that the observed difference between cluster andfield populations can be explained by a scenario of evolutionaryenhancement of the surface angular momentum over the main-sequencelifetime. A comparison is made between the cluster and field populationsof the LMC and the Galaxy in order to explore the effects ofmetallicity. This shows that the stars of the LMC are more rapidrotators than their Galactic counterparts.
| Cluster Mass Functions in the Large and Small Magellanic Clouds: Fading and Size-of-Sample Effects
The properties of ~939 star clusters in the Large and Small MagellanicClouds were determined from ground-based CCD images in UBVR passbands.The areal coverage was extensive, corresponding to 11.0 kpc2in the LMC and 8.3 kpc2 in the SMC. After corrections forreddening, the colors and magnitudes of the clusters were converted toages and masses, and the resulting mass distributions were searched forthe effects of fading, evaporation, and size-of-sample bias. The datashow a clear signature of cluster fading below the detection threshold.The initial cluster mass function (ICMF) was determined by fitting themass and age distributions with cluster population models. These modelssuggest a new method to determine the ICMF that is nearly independent offading or disruption and is based on the slope of a correlation betweenage and the maximum cluster mass in equally spaced intervals of log age.For a nearly uniform star formation rate, this correlation has a slopeequal to 1/(α-1) for an ICMF of dn(M)/dM~M-α. Wedetermine that α is between 2 and 2.4 for the LMC and SMC usingthis method plus another method in which models are fitted to the massdistribution integrated over age and to the age distribution integratedover mass. The maximum mass method also suggests that the clusterformation rate in the LMC age gap between 3 and 13 Gyr is about a factorof 10 below that in the period from 0.1 to 1 Gyr. The oldest clusterscorrespond in age and mass to halo globular clusters in the Milky Way.They do not fit the trends for lower mass clusters but appear to be aseparate population that either had a very high star formation rate andbecame depleted by evaporation or formed with only high masses.
| Near-infrared color evolution of LMC clusters
We present here the digital aperture photometry for 28 LMC clusterswhose ages are between 5 Myr and 12 Gyr. This photometry is based on ourimaging observations in JHK and contains integrated magnitudes andcolors as a function of aperture radius. In contrast to optical colors,our near-infrared colors do not show any strong dependence on clusterages.Tables 2 and 3 and Fig. 2 are only available in electronic form athttp://www.edpsciences.org
| 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 present-day chemical composition of the SMC from UVES spectra of the sharp-lined, B-type dwarf AV 304
High-resolution spectroscopic VLT/UVES observations are presented forthe B-type main-sequence star, AV 304, in the Small Magellanic Cloud(SMC). These spectra have been analysed using LTE model-atmospheretechniques, to derive stellar atmospheric parameters and chemicalcompositions. As AV 304 is located within the hydrogen burningmain-sequence band, its chemical composition should reflect that of theSMC interstellar medium (ISM). A detailed line-by-line differentialanalysis has been undertaken relative to a Galactic comparison star. Ageneral metal deficiency for the alpha -process elements O, Si & Sof -0.43+/-0.05 dex is found for AV 304, with iron having a similarunderabundance. Oxygen may be relatively overabundant by ~ 0.1 dex andcarbon and aluminium underabundant by ~ 0.2 dex. A large nitrogenunderabundance (of -1.2 dex relative to hydrogen and -0.7 dex relativeto iron) is found. This is interpreted in terms of the CNO bi-cyclehaving been suppressed in the SMC. Furthermore, the large nitrogendeficiency is in excellent agreement with that found for SMC H IIregions. Indeed, this represents a first for stellar astrophysics -confirming the low base-line nitrogen composition of the SMC ISM (viz.12+log(N/H) ~ 6.66 +/- 0.10 dex).
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
