Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
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.
| 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.
| 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
| 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.
| Very Large Telescope three micron spectra of dust-enshrouded red giants in the Large Magellanic Cloud
We present ESO/VLT spectra in the 2.9-4.1 μm range for a large sampleof infrared stars in the Large Magellanic Cloud (LMC), selected on thebasis of MSX and 2MASS colours to be extremely dust-enshrouded AGB starcandidates. Out of 30 targets, 28 are positively identified as carbonstars, significantly adding to the known population of opticallyinvisible carbon stars in the LMC. We also present spectra for sixIR-bright stars in or near three clusters in the LMC, identifying fourof them as carbon stars and two as oxygen-rich supergiants. We analysethe molecular bands of C2H2 at 3.1 and 3.8 μm, HCN at 3.57 μm, andsharp absorption features in the 3.70-3.78 μm region that weattribute to C2H2. There is evidence for a generally high abundance ofC2H2 in LMC carbon stars, suggestive of high carbon-to-oxygen abundanceratios at the low metallicity in the LMC. The low initial metallicity isalso likely to have resulted in less abundant HCN and CS. The sample ofIR carbon stars exhibits a range in C2H2:HCN abundance ratio. We do notfind strong correlations between the properties of the molecularatmosphere and circumstellar dust envelope, but the observed differencesin the strengths and shapes of the absorption bands can be explained bydifferences in excitation temperature. High mass-loss rates and strongpulsation would then be seen to be associated with a large scale heightof the molecular atmosphere.
| 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.
| 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.
| Asymptotic giant branch superwind speed at low metallicity
We present the results of a survey for OH maser emission at 1612 MHzfrom dust-enshrouded asymptotic giant branch (AGB) stars and supergiantsin the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud(SMC), with the Parkes radio telescope, aimed at deriving the speed ofthe superwind from the double-peaked OH maser profiles. Out of eighttargets in the LMC we detected five, of which three are new detections -no maser emission was detected in the two SMC targets. We detected forthe first time the redshifted components of the OH maser profile in theextreme red supergiant IRAS 04553-6825, confirming the suspicion thatits wind speed had been severely underestimated. Despite a much improvedspectrum for IRAS 04407-7000, which was known to exhibit a single-peakedOH maser, no complementary peak could be detected. The new detection inIRAS 05003-6712 was also single-peaked, but for two other newdetections, IRAS 04498-6842 and IRAS 05558-7000, wind speeds could bedetermined from their double-peaked maser profiles. The complete sampleof known OH/IR stars in the LMC is compared with a sample of OH/IR starsin the galactic centre. The LMC sources generally show a pronouncedasymmetry between the bright blueshifted maser emission and weakerredshifted emission, which we attribute to the greater contribution ofamplification of radiation coming directly from the star itself, as theLMC sources are both more luminous and less dusty than their galacticcentre counterparts. We confirm that the OH maser strength is a measureof the dust (rather than gas) mass-loss rate. At a given luminosity orpulsation period, the wind speed in LMC sources is lower than ingalactic centre sources, and the observed trends confirm simpleradiation-driven wind theory if the dust-to-gas ratio is approximatelyproportional to the metallicity.
| 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.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Surface brightness profiles and structural parameters for 53 rich stellar clusters in the Large Magellanic Cloud
We have compiled a pseudo-snapshot data set of two-colour observationsfrom the Hubble Space Telescope archive for a sample of 53 rich LMCclusters with ages of 106-1010 yr. We presentsurface brightness profiles for the entire sample, and derive structuralparameters for each cluster, including core radii, and luminosity andmass estimates. Because we expect the results presented here to form thebasis for several further projects, we describe in detail the datareduction and surface brightness profile construction processes, andcompare our results with those of previous ground-based studies. Thesurface brightness profiles show a large amount of detail, includingirregularities in the profiles of young clusters (such as bumps, dipsand sharp shoulders), and evidence for both double clusters andpost-core-collapse (PCC) clusters. In particular, we find power-lawprofiles in the inner regions of several candidate PCC clusters, withslopes of approximately -0.7, but showing considerable variation. Weestimate that 20 +/- 7 per cent of the old cluster population of theLarge Magellanic Cloud (LMC) has entered PCC evolution, a similarfraction to that for the Galactic globular cluster system. In addition,we examine the profile of R136 in detail and show that it is probablynot a PCC cluster. We also observe a trend in core radius with age thathas been discovered and discussed in several previous publications bydifferent authors. Our diagram has better resolution, however, andappears to show a bifurcation at several hundred Myr. We argue that thisobserved relationship reflects true physical evolution in LMC clusters,with some experiencing small-scale core expansion owing to mass loss,and others large-scale expansion owing to some unidentifiedcharacteristic or physical process.
| A statistical study of binary and multiple clusters in the LMC
Based on the Bica et al. (\cite{bica}) catalogue, we studied the starcluster system of the LMC and provide a new catalogue of all binary andmultiple cluster candidates found. As a selection criterion we used amaximum separation of 1farcm4 corresponding to 20 pc (assuming adistance modulus of 18.5 mag). We performed Monte Carlo simulations andproduced artificial cluster distributions that we compared with the realone in order to check how many of the found cluster pairs and groups canbe expected statistically due to chance superposition on the plane ofthe sky. We found that, depending on the cluster density, between 56%(bar region) and 12% (outer LMC) of the detected pairs can be explainedstatistically. We studied in detail the properties of the multiplecluster candidates. The binary cluster candidates seem to show atendency to form with components of similar size. When possible, westudied the age structure of the cluster groups and found that themultiple clusters are predominantly young with only a few cluster groupsolder than 300 Myr. The spatial distribution of the cluster pairs andgroups coincides with the distribution of clusters in general; however,old groups or groups with large internal age differences are mainlylocated in the densely populated bar region. Thus, they can easily beexplained as chance superpositions. Our findings show that a formationscenario through tidal capture is not only unlikely due to the lowprobability of close encounters of star clusters, and thus the evenlower probability of tidal capture, but the few groups with largeinternal age differences can easily be explained with projectioneffects. We favour a formation scenario as suggested by Fujimoto &Kumai (\cite{fk}) in which the components of a binary cluster formedtogether and thus should be coeval or have small age differencescompatible with cluster formation time scales. Table 6 is only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/547
| Circumstellar masers in the Magellanic Clouds
Results are presented of a search for 22 GHz H_2O616->523, 43 GHz SiOv=1(J=1->0),86 GHz SiOv=1(J=2->1) and 129 GHzSiOv=1(J=3->2) maser emission from bright IRAS pointsources in the Small and Large Magellanic Clouds - mostly circumstellarenvelopes around obscured red supergiants and Asymptotic Giant Branchstars (OH/IR stars). The aim of this effort was to test whether thekinematics of the mass loss from these stars depends on metallicity.H_2O maser emission was detected in the red supergiants IRAS 04553-6825and IRAS 05280-6910, and tentatively in the luminous IR object IRAS05216-6753 and the AGB star IRAS 05329-6708, all in the LMC.SiOv=1(J=2->1) maser emission was detected in IRAS04553-6825. The double-peaked H_2O maser line profiles of IRAS04553-6825 and IRAS 05280-6910, in combination with the OH (and SiO)maser line profiles, yield the acceleration of the outflows from thesestars. The outflow velocity increases between the H_2O masing zone nearthe dust-formation region and the more distant OH masing zone from v ~18 to 26 km s-1 for IRAS 04553-6825 and from v ~ 6 to 17 kms-1 for IRAS 05280-6910. The total sample of LMC targets isanalysed in comparison with circumstellar masers in the Galactic Centre.The photon fluxes of circumstellar masers in the LMC are found to bevery similar to those in the Galactic Centre. The expansion velocitiesin the LMC appear to be ~ 20% lower than for similarly bright OH masersin the Galactic Centre, but the data are still consistent with nodifference in expansion velocity. OH/IR stars in the LMC appear to haveslower accelerating envelopes than OH/IR stars in the Galactic Centre.The masers in the LMC have blue-asymmetric emission profiles. This maybe due to the amplification of stellar and/or free-free radiation,rather than the amplification of dust emission, and may be morepronounced in low metallicity envelopes. The SiO maser strengthincreases with the photometric amplitude at 2.2 mu m but is independentof the photometric amplitude at 10 mu m. This suggests a strongconnection between shocks in the dust-free SiO masing zone and the dustformation process. The LMC masers obey the same trend as the GalacticCentre masers. Appendices describe H_2O maser emission from themoderately mass-losing AGB star R Dor in the Milky Way, optical echellespectroscopy of IRAS 04553-6825, and the properties of circumstellarmasers in the Galactic Centre.
| Supersonic Water Masers in 30 Doradus
We report on extremely high velocity molecular gas, up to -80 kms-1 relative to the ambient medium, in the giant starformation complex 30 Doradus in the Large Magellanic Cloud (LMC), asobserved in new 22 GHz H2O 616-->523maser emission spectra obtained with the Mopra radio telescope. Themasers may trace the velocities of protostars, and the observedmorphology and kinematics indicate that current star formation occursnear the interfaces of colliding stellar wind-blown bubbles. The largespace velocities of the protostars and associated gas could result inefficient mixing of the LMC. A similar mechanism in the Milky Way couldseed the Galactic halo with relatively young stars and gas.
| Evolutionary Synthesis Modeling of Red Supergiant Features in the Near-Infrared
We present evolutionary synthesis models applied to near-infraredspectral features observed in the spectra of young Magellanic Cloudclusters and starburst galaxies. The temporal evolution of the first andsecond overtones of CO at 2.29 μm (2-0 band head) and 1.62 μm (6-3band head) and of the U-B, B-V, and J-K colors are investigated. We findthat the current evolutionary tracks of massive stars with subsolarchemical composition in the red supergiant phase are not reliable forany synthesis of the temporal evolution of infrared stellar features.The high sensitivity of the selected infrared features to theatmospheric parameters of cool stars allows us to place constraints onthe temperature and the fraction of time spent in the red part of theHertzsprung-Russell diagram by massive stars during their core heliumburning phase. We derive a set of empirically calibratedspectrophotometric models by adjusting the red supergiant parameters sothat the properties of the observed templates are reproduced.
| 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 evolution of theV-Kcolours of single stellar populations
Models of evolutionary population synthesis of galaxies rely on theproperties of the so-called single stellar populations (SSP). In thispaper, we discuss how the integrated near-infrared colours, andespecially V-K, of SSPs evolve with age and metallicity. Some of theuncertainties associated with the properties of the underlying stellarmodels are thoroughly discussed. Our models include all the relevantstellar evolutionary phases, with particular attention being dedicatedto the asymptotic giant branch (AGB), which plays a fundamental role inthe evolution of the near-infrared part of the spectrum. First, wepresent the effects that different formulations for the mass-loss ratesproduce on the final remnant mass (i.e., on the initial-final massrelation), and hence on the AGB-termination luminosity and the relativecontribution of these stars to the integrated light. The results for theevolution of the V-K colour are very different depending on the choiceof the mass-loss prescription; the same is true also for the B-V colourin the case of low-metallicity SSPs. Secondly, we describe the changesoccurring in the integrated colours at the onset of the AGB and redgiant (RGB) branches. According to the classical formalism for the AGBevolution, the onset of this evolutionary phase is marked by a colourjump to the red, the amplitude of which is shown here to be highlydependent on the metallicity and mass-loss rates adopted in the models.We then consider the effect of the overluminosity with respect to thestandard core mass-luminosity relation that occurs in the most massiveAGB stars. Different simplified formulations for this effect are testedin the models; they cause a smoothing of the colour evolution in the agerange at which the AGB starts to develop, rather than a splitting of thecolour jump into two separate events. On the other hand, we find that atemporary red phase takes place ~1.5x10^8 yr after the RGB develops.Thanks to the transient nature of this feature, the onset of the RGB isprobably not able to cause marked features in the spectral evolution ofgalaxies. We then discuss the possible reasons for the transition of V-Kcolours (from ~1.5 to 3) that takes place in LMC clusters of SWB typeIV. A revision of the ages attributed to the single clusters revealsthat the transition may not be as fast as originally suggested. Thecomparison of the data with the models indicates that the transitionresults mainly from the development of the AGB. A gradual (or delayed)transition of the colours, as predicted by models which include theoverluminosity of the most massive AGB stars, seems to describe the databetter than the sudden colour jump predicted by classical models.
| IR spectra of young Magellanic Cloud clusters and starburst galaxies: constraints on the temperature of red supergiants and new estimates of metallicity in young stellar populations
Infrared spectra of young stellar clusters in the Magellanic Clouds areused to derive information on the red supergiants dominating their 1.6mu m emission, and to obtain a new and independent estimate of theirmetallicities. The most striking result is that red supergiants with lowmetallicity appear to be much cooler than predicted by evolutionarymodels, and this most probably reflects uncertainties in the calibrationof the mixing-length parameter in the outermost layers of the stellarenvelopes. The metallicity [Fe/H] can be estimated from the W_lambda(1.62) index which is here calibrated using synthetic stellar spectra,and the new scale is also applied to eight starburst galaxies. Theresulting values of [Fe/H] range between -1.3 for the SMC cluster NGC330(in excellent agreement with previous estimates) to -0.2 for the LMCcluster NGC1994. Starburst galaxies have metallicities ranging between-1.0 (NGC6240) and -0.5 (NGC7552). The spectra are also used to estimatethe Carbon depletion which in MC clusters is found compatible with a`standard' value of [C/Fe] =~ -0.3. Interestingly, our spectra showpossible evidence of significant variations of Carbon depletion in somestarburst galaxies. Finally, the Silicon relative abundance is estimatedfrom the W_lambda (1.59) index. In MC clusters we find [Si/Fe]~+0.5,i.e. values similar to those of old clusters in our galaxy andcompatible with primordial Si-enhancement by type II supernovae. Basedon observations collected at the European Southern Observatory, LaSilla, Chile
| Obscured AGB stars in the Magellanic Clouds. I. IRAS candidates
We have selected 198 IRAS sources in the Large Magellanic Cloud, and 11in the Small Magellanic Cloud, which are the best candidates to bemass--loosing AGB stars (or possibly post--AGB stars). We used thecatalogues of \cite[Schwering \& Israel (1990)]{ref42} and\cite[Reid et al. (1990)]{ref36}. They are based on the IRAS pointedobservations and have lower detection limits than the Point SourceCatalogue. We also made cross-identifications between IRAS sources andoptical catalogues. Our resulting catalogue is divided in 7 tables.Table \ref{tab1} lists optically known red supergiants and AGB stars forwhich we found an IRAS counterpart (7 and 52 stars in the SMC and LMC,respectively). Table \ref{tab2} lists ``obscured'' (or ``cocoon'') AGBstars or late-type supergiants which have been identified as such inprevious works through their IRAS counterpart and JHKLM photometry (2SMC and 34 LMC sources; no optical counterparts). Table \ref{tab3} listsknown planetary nebulae with an IRAS counterpart (4 SMC and 19 LMC PNe).Table \ref{tab4} lists unidentified IRAS sources that we believe to begood AGB or post--AGB or PNe candidates (11 SMC and 198 LMC sources).Table~\ref{tab5} lists unidentified IRAS sources which could be any typeof object (23 SMC and 121 LMC sources). Table \ref{tab6} lists IRASsources associated with foreground stars (29 SMC and 135 LMC stars).Table \ref{tab7} lists ruled out IRAS sources associated with HIIregions, hot stars, etc... We show that the sample of IRAS AGB stars inthe Magellanic Clouds is very incomplete. Only AGB stars more luminousthan typically 10^4 L_\odot and with a mass-loss rate larger thantypically 5 10^{-6} M_\odot/yr could be detected by the IRAS satellite.As a consequence, one expects to find very few carbon stars in the IRASsample. We also expect that most AGB stars with intermediate mass--lossrates have not been discovered yet, neither in optical surveys, nor inthe IRAS survey. Tables 1 to 8 are also available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Mid-infrared properties of globular clusters using the IRAS data base
We present an analysis of the mid-IR properties of 18 globular clusters(GCs) [15 in the Galaxy and three in the Large Magellanic Cloud (LMC)]using the IRAS photometric data at 12 and 25 mum. Eight of the nineGalactic GCs with central escape velocities greater than 50 km s^-1 haveIRAS sources within a radius of 60 arcsec from the centre, in agreementwith the expectation that interstellar gas and dust should indeed bepresent in the central regions of the most massive clusters owing tomass-loss processes occurring in the late stages of the stellarevolution. No other significant correlation is found between IRAS sourceincidence and any intrinsic GC parameters. Warm dust (T~300K) isdetectable mostly around unresolved giant stars, but in three massiveGCs it is also present as diffuse emission. However, most of the dustmight be cold (T<50K) and it was thus notdetected by IRAS because of its limited sensitivity at 60 and 100 mum.The inferred mass-loss rates and statistical considerations arecompatible with a non-steady mass-loss process with several episodes ofejection lasting a few times 10^5 yr.
| Ultraviolet ages of young clusters in the Magellanic Clouds.
Following a previous investigation on the integrated UV colours ofstellar clusters (Barbero et al. 1990), we study the calibration of theultraviolet colour index C(15-31) in terms of cluster age, usingobservations by the International Ultraviolet Explorer of 29 young andpopulous clusters of the Large Magellanic Cloud (LMC), and of the SmallMagellanic Cloud (SMC). The study is limited to the range of ages5x10^6^ to 8x10^8^yr, which is free from contamination by HorizontalBranch stars. It is shown that in this range of ages the theoreticalsequence C(15-31) vs. age agrees well with the one derived by combiningthe observed colour index C(15-31) with the ages determined viaisochrone fitting to the colour-magnitude diagrams while systematicdifferences, which are discussed on here, exist with respect to the agecalibration by Meurer, Cacciari and Freeman (1990). The present agecalibration C(15-31) vs. log(t), provided in an analytical form, isfinally used to determine the ages of the 29 clusters in our sample,including 13 objects for which no determination was available via theisochrone fitting method.
| 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.
| Circumstellar envelopes and Asymptotic Giant Branch stars
Red giants are sometimes surrounded by envelopes, the result of theejection of stellar matter at a large rate (˙ M>10-7Msolar/yr) and at a low velocity (10 km/s). Inthis review the envelopes are discussed and the relation between starsand envelope: what stars combine with what envelopes? The envelope emitsradiation by various processes and has been detected at all wavelengthsbetween the visual and the microwave range. I review the observations ofcontinuum radiation emitted by dust particles and of rotationaltransitions of molecules, where these molecules have been excited bythermal or by non-thermal (``maser'') processes. I discuss mainly theoxygen-rich stars, those of spectral type M, and only briefly theclosely related carbon-rich stars. By and large the density in theenvelope is well described by spherically symmetric outflow at aconstant velocity; on the time scale needed to flow from stellar surfaceto the outermost layers, i.e. 105 yr, the loss of mass issometimes interrupted suddenly after which the envelope becomes``detached'' from the star. The temperature decreases when movingoutward; heat input is by friction between dust particles and gas andcooling occurs by line radiation by various molecules, especially byH2O. The molecular composition is determined by formation inan equilibrium process deep in the atmosphere and by destruction in theouter parts of the outflow by interstellar UV radiation (H2,CO, H2O) or by depletion due to condensation on dust grains(SiO); dust particles of silicate material solidify where the radiationtemperature is decreased to about 1000 K, and this is at a few stellarradii. The various continuum spectra produced by the dust particles indifferent stars are well modelled by a simple model of the density anddust temperature distribution plus the assumption that the particlesconsist of ``dirty silicate'', i.e. silicate with Fe and Al ions added.A large range of optical depths, τ 9.7μ m , isobserved: from 0.01 to 10. In envelopes with large optical depth thestar itself can no longer be detected directly. Model calculations alsoshow that the momentum in the outflow, i.e. ˙ Mv out isprovided by radiation pressure on the dust particles followed by thecomplete transfer of this momentum to the gas. The mass-loss rateitself, ˙ M , is not determined by radiation pressure but by dynamicprocesses in the region below the dust condensation layer. Whenτ9.7 μ m is sufficiently large its measurement, thatof the stellar luminosity, L* and that of the outflowvelocity, vout, permit the determination of ˙ M , i.e.the total outflow rate, without making assumptions about the abundanceof the dust particles or of the molecular gases. Detached envelopes havebeen seen in a few cases. Thermal molecular radiation is faint comparedto the maser emission but has been measured in distant stars, e.g. instars near the galactic center. Different molecules outline different``spheres'' around the star. The largest sphere (a radius of 0.1 pc) isoutlined by an emission line belonging to the CO(v=0, J=1-> 0)transition. Higher rotational transitions of CO give smaller diameters.A comparison of CO (J=2-> 1) and (J=1-> 0) fluxes in stars withvery thick envelopes leads to the conclusion that an abrupt decrease inthe mass-loss rate occurred some ten thousand years ago. Three moleculesproduce each several maser lines: SiO, H2O and OH. Severalnew H2O lines have recently been discovered; theirexploration has hardly been started. The high intensity of the maserlines makes interferometry possible and hence detailed mapping. The SiOlines are formed deep in the envelope, below the dust condensationlayer. OH maser lines are produced farthest out, H2O lines inbetween. The excitation mechanisms for most maser lines is understoodglobally, but detailed models are lacking, largely because the problemis non-linear and the solution of the radiative transfer equationrequires a highly anisotropic geometry. The geometrical and kinematicalproperties of the 1612 MHz OH maser, which in many objects is verystrong, are explained by a thin shell of OH; because the angulardiameter of the shell can be measured directly and the linear diametercan be determined from the difference in the time of maximum flux ofblue and red maser peaks, the distance of the shell and of the star canbe measured. The presence or absence of individual maser lines appearsto depend on the value of τ {9.7μ m} and is welldescribed by a sequence called ``Lewis' chronology''. The central staris a long-period variable with a period of 300 days or longer and with alarge luminosity amplitude (Δ mbol >0.7m). Evidence is given that each star has the maximumluminosity it will reach during its evolution and that it is athermally-pulsing Asymptotic-Giant-Branch star (TP-AGB) with amain-sequence mass between 1 and 6 Msolar . Stars of the samemain-sequence mass, Mms, have different mass-loss rates, insome cases by a factor of 10. The mass-loss rate probably increases withtime, and the highest mass-loss rates are reached toward the end of theevolution. Stars with higher Mms ultimately reach highermass-loss rates. The calibration of the main-sequence mass is reviewed.Most Mira variables with mass loss have a mass between 1.0 and 1.2Msolar . OH/IR stars with periods over 1000 days have nocounterparts among the carbon stars and thus have Mms >4.5Msolar. Stars as discussed in this review have been foundonly in the thin galactic disk and in the bulge. Finally I reviewseveral recently proposed scenarios for TP-AGB evolution in which massloss is taken into account. These scenarios represent the observationsquite well; their major short-coming is the lack of an explanation whythe central stars are always large-amplitude, long-period variables andwhy such stars are the ones with high mass-loss rates.
| Blue-violet spectral evolution of young Magellanic Cloud clusters
We study the integrated spectral evolution in the blue-violet range of97 blue star clusters in the Magellanic Clouds, from those associatedwith gas emission to those as old as a few hundred Myr. Some clustersare dominated by the flux of those massive stars that pass throughevolutionary stages such as Wolf-Rayet, Luminous Blue Variable, Be, andsupergiant stars of different temperatures. The relationships amongspectral features such as absorption and emission lines, Balmerdiscontinuity and Balmer continuum are used to study the spectralevolution of the clusters. Finally, we sort into groups spectra ofsimilar evolutionary stages, creating a template spectral library withpossible applications in stellar populations syntheses of star-forminggalaxies and in the spectral simulation of bursts of star formation withdifferent mean ages and durations.
| Ultraviolet spectral evolution of star clusters in the IUE library.
The ultraviolet integrated spectra of star clusters and H II regions inthe IUE library have been classified into groups based on their spectralappearance, as well as on age and metallicity information from otherstudies. We have coadded the spectra in these groups according to theirS/N ratio, creating a library of template spectra for futureapplications in population syntheses in galaxies. We define spectralwindows for equivalent width measurements and for continuum tracings.These measurements in the spectra of the templates are studied as afunction of age and metallicity. We indicate the windows with a strongmetallicity dependence, at different age stages.
| Ultraviolet interstellar absorption lines in the LMC: Searching for hidden SNRs
Strong x-ray emission detected in Large Magellanic Cloud (LMC)superbubbles has been explained as the result of interior supernovaremnants (SNRs) hitting the dense superbubble shell. Such SNRs cannot befound using conventional criteria. We thus investigate the possibilityof using the interstellar absorption properties in the ultraviolet (UV)as a diagnostic of hidden SNR shocks. The International UltravioletExplorer (IUE) archives provide the database for this pilot study. Theycontain high-dispersion spectra of several stars in x-ray brightsuperbubbles. To distinguish the effects of SNR shocks from those oflocal stellar winds and a global hot halo around the LMC, we includedcontrol objects in different environments. We find that almost allinterstellar absorption properties can be explained by the interstellarenvironment associated with the objects. Summarizing the two mostimportant results of this study: (1) a large velocity shift between thehigh-ionization species (C IV and Si IV) and the low-ionization species(S II, Si II, and C II*) is a diagnostic of hidden SNR shocks; however,the absence of a velocity shift does not preclude the existence of SNRshocks; (2) there is no evidence that the LMC is uniformly surrounded byhot gas; hot gas is preferentially found associated with largeinterstellar structures like superbubbles and supergiant shells, whichmay extend to large distances from the plane.
| Near-IR Spectroscopy Indices and the Stellar Content of Galaxies
Not Available
| OH/IR stars in the Magellanic Clouds
A group of IRAS sources in the LMC and SMC have been monitored in thenear infrared and pulsation period of over 1000 days have beendetermined for some of them. OH maser emission has been detected fromsome of the LMC objects. The OH line profiles indicate that stellar windvelocities in LMC stars are lower than in Galactic stars, a resultattributed to the lower metallicity in the LMC. Evidence is presentedthat reducing metallicity lowers mass loss rates from AGB stars providedthey do not turn into carbon stars. It is suggested that carbon starformation allows high mass loss rate to be maintained in AGB stars oflow metallicity. There is no convincing evidence that AGB stars evolvesignificantly beyond the classical AGB limit of M(bol) about -7.1.
| A survey of circumstellar CO emission from a sample of IRAS point sources
The first results from a survey of circumstellar CO(1-0) emission arepresented. The sources were selected from the IRAS point source catalogaccording to the IRAS color criteria described in van der Veen andHabing (1988). The sources have good quality fluxes at 12, 25, and 60microns, flux densities larger than 20 Jy at 25 microns, and aresituated more than 5 deg away from the Galactic plane. The survey isundertaken to study the relationship between mass loss rates, dustproperties, and the evolution along the AGB. The sample consists of 787sources and contains both oxygen and carbon-rich stars, including Miravariables, OH/IR objects, protoplanetary nebulae, planetary nebulae, and60-micron excess sources. So far, 519 objects, situated on both thenorthern and the southern sky, have been observed; 163 sources werefound to have circumstellar CO emission, and in 58 of these CO emissionhas not previously been detected.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
Kataloglar ve belirtme:
|
