目录
图像
上传图像
DSS Images Other Images
相关文章
The RR Lyrae distance scale from near-infrared photometry: current results .
We present new observational results on the RR Lyrae K-bandPeriod-Luminosity relation (PLK). Data on the Galactic globular clustersNGC 3201 and NGC 4590 (M68), and on the Large Magellanic Cloud clusterReticulum are shown. We compare the observed slopes of the PLK relationsfor these three clusters with those predicted by pulsational andevolutionary models, finding a fair agreement. Trusting on this findingwe decided to adopt these theoretical calibrations to estimate thedistance to the target clusters, finding a good agreement withoptical-based RR Lyrae distances, but with a smaller formal scatter.
| Detection of a 63° Cold Stellar Stream in the Sloan Digital Sky Survey
We report on the detection in Sloan Digital Sky Survey data of a63°-long tidal stream of stars, extending from Ursa Major to Cancer.The stream has no obvious association with the orbit of any knowncluster or galaxy. The contrast of the detected stream is greatest whenusing a star count filter that is matched to the color-magnitudedistribution of stars in M13, which suggests that the stars making upthe stream are old and metal-poor. The visible portion of the stream isvery narrow and about 8.5 kpc above the Galactic disk, suggesting thatthe progenitor is or was a globular cluster. While the surface densityof the stream varies considerably along its length, its path on the skyis very smooth and uniform, showing no evidence of perturbations bylarge mass concentrations in the nearby halo. While definitiveconstraints cannot be established without radial velocity information,the stream's projected path and estimates of its distance suggest thatwe are observing the stream near the perigalacticon of its orbit.
| A Large Stellar Evolution Database for Population Synthesis Studies. II. Stellar Models and Isochrones for an α-enhanced Metal Distribution
We present a large, new set of stellar evolution models and isochronesfor an α-enhanced metal distribution typical of Galactic halo andbulge stars; it represents a homogeneous extension of our stellar modellibrary for a scaled-solar metal distribution already presented byPietrinferni et al. The effect of the α-element enhancement hasbeen properly taken into account in the nuclear network, opacity,equation of state, and for the first time in the bolometric correctionsand color transformations. This allows us to avoid the inconsistentuse-common to all α-enhanced model libraries currentlyavailable-of scaled-solar bolometric corrections and colortransformations for α-enhanced models and isochrones. We show howbolometric corrections to magnitudes obtained for the U, B portion ofstellar spectra for Teff<=6500 K are significantlyaffected by the metal mixture, especially at the higher metallicities.Our models cover both an extended mass range (between 0.5 and 10Msolar, with a fine mass spacing) and a broad metallicityrange, including 11 values of the metal mass fraction Z, correspondingto the range -2.6<=[Fe/H]<=0.05. The initial He mass fraction isY=0.245 for the most metal-poor models, and it increases with Z,according to ΔY/ΔZ=1.4. Models with and without theinclusion of overshoot from the convective cores during the centralH-burning phase are provided, as well as models with different mass lossefficiencies. We also provide complete sets of evolutionary models forlow-mass, He-burning stellar structures cover the whole metallicityrange. This database, used in combination with our scaled-solar modellibrary, is a valuable tool for investigating both Galactic andextragalactic simple and composite stellar populations, using stellarpopulation synthesis techniques.
| An Empirical Calibration of the Mixing-Length Parameter α
We present an empirical calibration of the mixing-length free parameterα based on a homogeneous infrared database of 28 Galactic globularclusters spanning a wide metallicity range (-2.15<[Fe/H]<-0.2).Empirical estimates of the red giant effective temperatures have beenobtained from infrared colors. Suitable relations linking thesetemperatures to the cluster metallicity have been obtained and comparedto theoretical predictions. An appropriate set of models for the Sun andPopulation II giants has been computed by using both the standard solarmetallicity (Z/X)solar=0.0275 and the most recently proposedvalue (Z/X)solar=0.0177. We find that when the standard solarmetallicity is adopted, a unique value of α=2.17 can be used toreproduce both the solar radius and the Population II red gianttemperature. Conversely, when the new solar metallicity is adopted, twodifferent values of α are required: α=1.86 to fit the solarradius and α~2.0 to fit the red giant temperatures. However, itmust be noted that regardless the adopted solar reference, theα-parameter does not show any significant dependence onmetallicity.Based on observations collected at the European Southern Observatory(ESO), La Silla, Chile. Also based on observations made with the ItalianTelescopio Nazionale Galileo (TNG) operated on the island of La Palma bythe Fundacion Galileo Galilei of the INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
| Chemical Compositions of Red Giant Stars in Old Large Magellanic Cloud Globular Clusters
We have observed 10 red giant stars in four old Large Magellanic Cloudglobular clusters with the high-resolution spectrograph MIKE on theMagellan Landon Clay 6.5 m telescope. The stars in our sample have up to20 elemental abundance determinations for the α-, iron peak, andneutron-capture element groups. We have also derived abundances for thelight odd-Z elements Na and Al. We find NGC 2005 and NGC 2019 to be moremetal-rich than previous estimates from the Ca II triplet, and we derive[Fe/H] values closer to those obtained from the slope of the red giantbranch. However, we confirm previous determinations for Hodge 11 and NGC1898 to within 0.2 dex. The LMC cluster [Mg/Fe] and [Si/Fe] ratios arecomparable to the values observed in old Galactic globular clusterstars, as are the abundances [Y/Fe], [Ba/Fe], and [Eu/Fe]. The LMCclusters do not share the low-Y behavior observed in some dwarfspheroidal galaxies. [Ca/Fe], [Ti/Fe], and [V/Fe] in the LMC, however,are significantly lower than what is seen in the Galactic globularcluster system. Neither does the behavior of [Cu/Fe] as a function of[Fe/H] in our LMC clusters match the trend seen in the Galaxy, stayinginstead at a constant value of roughly -0.8. Because not all[α/Fe] ratios are suppressed, these abundance ratios cannot beattributed solely to the injection of Type Ia supernova material andinstead reflect the differences in star formation history of the LMCversus the Milky Way. An extensive numerical experimental study wasperformed, varying both input parameters and stellar atmosphere models,to verify that the unusual abundance ratios derived in this study arenot the result of the adopted atomic parameters, stellar atmospheres, orstellar parameters. We conclude that many of the abundances in the LMCglobular clusters we observed are distinct from those observed in theMilky Way, and these differences are intrinsic to the stars in thosesystems.
| Image-Subtraction Photometry of Variable Stars in the Globular Clusters NGC 6388 and NGC 6441
We have applied Alard's image-subtraction method (ISIS ver. 2.1) to theobservations of the globular clusters NGC 6388 and NGC 6441 previouslyanalyzed using standard photometric techniques (DAOPHOT and ALLFRAME).In this reanalysis of observations obtained at the Cerro TololoInter-American Observatory, besides recovering the variables previouslydetected on the basis of our ground-based images, we have also been ableto recover most of the RR Lyrae variables previously detected only inthe analysis of Hubble Space Telescope WFPC2 observations of the innerregion of NGC 6441. In addition, we report five possible new variablesnot found in the analysis of the Hubble Space Telescope observations ofNGC 6441. This dramatically illustrates the capabilities ofimage-subtraction techniques applied to ground-based data to recovervariables in extremely crowded fields. We have also detected 12 newvariables and 6 possible variables in NGC 6388 not found in our previousground-based studies. The revised mean period for RRab stars in NGC 6388is 0.676 days, while the mean period of RRab stars in NGC 6441 isunchanged at 0.759 days. These values are among the largest known forany Galactic globular cluster. Additional probable type II Cepheids wereidentified in NGC 6388, confirming its status as a metal-rich globularcluster rich in Cepheids.
| The QUEST RR Lyrae Survey. II. The Halo Overdensities in the First Catalog
The first catalog of the RR Lyrae stars (RRLSs) in the Galactic halo bythe Quasar Equatorial Survey Team (QUEST) has been searched forsignificant overdensities that may be debris from disrupted dwarfgalaxies or globular clusters. These RRLSs are contained in a band ~2.3dwide in declination that spans ~165° in right ascension and lie ~4to ~60 kpc from the Sun. Away from the major overdensities, thedistribution of these stars is adequately fitted by a smooth halo model,in which the flattening of the halo decreases with increasinggalactocentric distance (as reported by Preston et al.). This model wasused to estimate the ``background'' of RRLSs on which the halooverdensities are overlaid. A procedure was developed for recognizinggroups of stars that constitute significant overdensities with respectto this background. To test this procedure, a Monte Carlo routine wasused to make artificial RRLS surveys that follow the smooth halo modelbut with Poisson-distributed noise in the numbers of RRLSs and, withinlimits, random variations in the positions and magnitudes of theartificial stars. The 104 artificial surveys created by thisroutine were examined for significant groups in exactly the same way asthe QUEST survey. These calculations provided estimates of thefrequencies with which random fluctuations produce significant groups.In the QUEST survey there are six significant overdensities that containsix or more stars and several smaller ones. The small ones and possiblyone or two of the larger ones may be artifacts of statisticalfluctuations, and they need to be confirmed by measurements of radialvelocity and/or proper motion. The most prominent groups are thenorthern stream from the Sagittarius dwarf spheroidal galaxy and a largegroup in Virgo, formerly known as the ``12.4 hr clump,'' which Duffauand coworkers have recently shown to contain a stellar stream (the Virgostellar stream). Two other groups lie in the direction of the Monocerosstream and at approximately the right distance for membership. Anothergroup is related to the globular cluster Palomar 5.
| Atmospheres, Chemical Compositions, and Evolutionary Histories of Very Metal-Poor Red Horizontal-Branch Stars in the Galactic Field and in NGC 7078 (M15)
We have conducted spectrum analyses of 24 field metal-poor([Fe/H]<-2) red horizontal-branch (RHB) stars identified in the HKobjective-prism survey and 6 such stars in the globular cluster M15,based on high-quality spectra (R~40,000, S/N~100) obtained with theMagellan Inamori Kyocera Echelle spectrograph at the Clay 6.5 mtelescope at Las Campanas Observatory. The atmospheric parameters of theRHB stars provide interesting bridges between turnoff stars of similartemperature and red giant branch (RGB) stars of similar gravity, andthey permit investigations of abundance trends [X/Fe] versus [Fe/H] in arelatively unexplored region of the temperature-gravity plane. We findthat the Teff, logg, vt, and [Fe/H] valuesdetermined from our spectra are consistent with expectations fromliterature spectroscopic studies of other evolved metal-poor stellarclasses. We show that the RHB stars have abundance distributions thatare consistent with typical halo stars of similar metallicities. Thephotometric and spectroscopic gravities of the M15 stars differ byamounts that grow with declining temperature. We use a regressionderived from these differences to calculate photometric gravities forthe field RHB stars. Then we use the locations of the field RHB starsamong the evolutionary tracks of Cassisi et al. in the logg versuslogTeff plane to estimate their masses and lifetimes as RHBstars. We use these lifetimes to estimate the size of the metal-poor HBpopulation from which they arise. Then, using counts of HB and RGB starsin metal-poor globular clusters, we conclude that the number ofmetal-poor RGB stars at high latitudes (|b|>30deg)brighter than V=15 exceeds those identified in extant objective-prismsurveys by more than an order of magnitude. Finally, we deduce theeffective temperature of the fundamental red edge of the metal-poor RRLyrae instability strip, logTeff(FRE)=3.80+/-0.01, from theinterface between the temperature distributions of metal-poor field RHBstars and the RR Lyrae stars of similar [Fe/H] in five metal-poorglobular clusters.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center, California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation. This paper includes data gatheredwith the 6.5 m Magellan telescopes located at Las Campanas Observatory,Chile.
| Manganese Abundances in Cluster and Field Stars
We have derived Mn abundances for more than 200 stars in 19 globularclusters. In addition, Mn abundance determinations have been made for acomparable number of halo field and disk stars possessing an overlappingrange of metallicities and stellar parameters. Our primary data set wascomprised of high-resolution spectra previously acquired at theMcDonald, Lick, and Keck Observatories. To enlarge our data pool, weacquired globular and open cluster spectra from several otherinvestigators. Data were analyzed using synthetic spectra of the 6000Å Mn I triplet. Hyperfine structure parameters were included inthe synthetic spectra computations. Our analysis shows that for themetallicity range -0.7>[Fe/H]>-2.7, stars of 19 globular clustershave a mean relative abundance of <[Mn/Fe]>=-0.37+/-0.01(σ=0.10), a value in agreement with that of the field stars,<[Mn/Fe]>=-0.36+/-0.01 (σ=0.08). Despite the 2 orders ofmagnitude span in metallicity, the <[Mn/Fe]> ratio remainsconstant in both stellar populations. Our Mn abundance data indicatethat there is no appreciable variation in the relative nucleosyntheticcontribution from massive stars that undergo core-collapse supernovaeand thus no significant change of the associated initial mass functionin the specified metallicity range.
| On the Predicted and Observed Color Boundaries of the RR Lyrae Instability Strip as a Function of Metallicity
The purpose of this paper is to predict the temperature at thefundamental blue edge (FBE) of the instability strip for RR Lyrae (RRL)variables from the pulsation equation that relates temperature toperiod, luminosity, and mass. Modern data for the correlations betweenperiod, luminosity, and metallicity at the FBE for field and cluster RRLstars are used for the temperature calculation. The predictedtemperatures are changed to B-V colors using an adopted colortransformation. The predicted temperatures at the FBE become hotter as[Fe/H] changes from 0 to -1.5, and thereafter cooler as the metallicitydecreases to -2.5. The temperature range over this interval ofmetallicity is ΔlogTe=0.04, or 640 K at 6900 K. Thepredicted color variation is at the level of 0.03 mag in B-V over mostof this range. The predictions are compared with the observed RRL colorsat the FBE for both the field and cluster variables, showing generalagreement at the level of 0.02 mag in (B-V)0, which, however,is the uncertainty of the reddening corrections. The focus of theproblem is then reversed by fitting a better envelope to the observedFBE relation between color and metallicity for metallicities smallerthan -1.8, which, when inserted in the pulsation equation, gives anonlinear calibration of the absolute magnitude of the average evolvedlevel of the horizontal branch (HB) ofMV=1.109+0.600([Fe/H])+0.140([Fe/H])2, where thezero point has been set by the observed RR Lyrae stars in the LMC at0=19.064 by Clementini et al. for [Fe/H]=-1.5, andusing an adopted LMC modulus of (m-M)0=18.54 from Tammann etal. that is independent of the LMC Cepheids. This equation givesMV=0.52 at [Fe/H]=-1.5.However, the calibration fails for the extreme second-parameter clustersNGC 6388 and NGC 6441. Proof that the MV absolute magnitudesfor their RR Lyrae variables are ~0.4 mag brighter than the calibrationequation predicts is from the unusually long periods for givenamplitudes at their high metallicities of [Fe/H]~-0.5. Allsecond-parameter clusters are believed to violate the equation, but lessseverely than these two extreme examples. An additional complication inusing RRL stars as distance indicators at the 0.1 mag level is shown bythe difference of ΔlogP=0.029+/-0.007 in the position of theenvelope locus at the shortest periods for the observedperiod-metallicity correlation between the field and cluster variables.The field variables have shorter periods than cluster variables at theenvelope. This requires the cluster RRL stars to be 0.09 mag brighterthan the field variables at the same temperature and mass, or to have atemperature difference of ΔlogTe=0.008 at fixedluminosity and mass. The field and cluster variables also differ in thenear absence of cluster RR Lyrae stars in the -1.7>[Fe/H]>-2.0metallicity interval, whereas the field variables show no such gap. Atest is proposed for different origins for the field and the clustervariables by comparing the morphology of the HBs in the local dwarfspheroidal galaxies with that in the Galactic globular clusters in theinner halo and by relating the differences with the relevantsecond-parameter indicators.
| Complexity on Small Scales: The Metallicity Distribution of the Carina Dwarf Spheroidal Galaxy
The Carina dwarf spheroidal galaxy is the only galaxy of this type thatshows clearly episodic star formation separated by long pauses. Here wepresent metallicities for 437 radial velocity members of this Galacticsatellite. The metallicities and radial velocities were measured as partof a Large Programme with the Very Large Telescope at the EuropeanSouthern Observatory, Chile. We obtained medium-resolution spectroscopywith the multiobject spectrograph FLAMES. Our target red giants coverthe entire projected surface area of Carina. Our spectra are centered atthe near-infrared Ca II triplet, which is a well-established metallicityindicator for old and intermediate-age red giants. The resulting datasample provides the largest collection of spectroscopically derivedmetallicities for a Local Group dwarf spheroidal galaxy to date. Four ofour likely radial velocity members of Carina lie outside this galaxy'snominal tidal radius, supporting earlier claims of the possibleexistence of such stars beyond the main body of Carina. We find a meanmetallicity of [Fe/H]~-1.7 dex on the 1997 metallicity scale of Carrettaand Gratton for Carina. The formal FWHM of the metallicity distributionfunction is 0.92 dex, while the full range of metallicities is found tospan approximately -3.0 dex<[Fe/H]<0.0 dex. The metallicitydistribution function might be indicative of several subpopulationsdistinct in metallicity. There appears to be a mild radial gradient suchthat more metal-rich populations are more centrally concentrated,matching a similar trend for an increasing fraction of intermediate-agestars (see the 2001 work of Harbeck and coworkers). This, as well as thephotometric colors of the more metal-rich red giants, suggests thatCarina exhibits an age-metallicity relation. Indeed, the age-metallicitydegeneracy seems to conspire to form a narrow red giant branch despitethe considerable spread in metallicity and wide range of ages. Themetallicity distribution function is not well matched by a simpleclosed-box model of chemical evolution. Qualitatively better matches areobtained by chemical models that also take into account infall andoutflows. A G dwarf problem remains for all these models.Based on observations collected at the European Southern Observatory atParanal, Chile; proposal 171.B-0520(A).
| VLT/UVES spectroscopy of individual stars in three globular clusters in the Fornax dwarf spheroidal galaxy
We present a high resolution (R ~ 43 000) abundance analysis of a totalof nine stars in three of the five globular clusters associated with thenearby Fornax dwarf spheroidal galaxy. These three clusters (1, 2 and 3)trace the oldest, most metal-poor stellar populations in Fornax. Wedetermine abundances of O, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Zn, Y, Ba, Nd andEu in most of these stars, and for some stars also Mn and La. Wedemonstrate that classical indirect methods (isochrone fitting andintegrated spectra) of metallicity determination lead to values of[Fe/H] which are 0.3 to 0.5 dex too high, and that this is primarily dueto the underlying reference calibration typically used by these studies.We show that Cluster 1, with [Fe /H] = -2.5, now holds the record forthe lowest metallicity globular cluster. We also measure anover-abundance of Eu in Cluster 3 stars that has only been previouslydetected in a subgroup of stars in M 15. We find that the Fornaxglobular cluster properties are a global match to what is found in theirGalactic counterparts; including deep mixing abundance patterns in twostars. We conclude that at the epoch of formation of globular clustersboth the Milky Way and the Fornax dwarf spheroidal galaxy shared thesame initial conditions, presumably pre-enriched by the same processes,with identical nucleosynthesis patterns.
| Multivariate analysis of globular cluster horizontal branch morphology: searching for the second parameter
Aims.The interpretation of globular cluster horizontal branch (HB)morphology is a classical problem that can significantly blur ourunderstanding of stellar populations. Methods: .We present a newmultivariate analysis connecting the effective temperature extent of theHB with other cluster parameters. The work is based on Hubble SpaceTelescope photometry of 54 Galactic globular clusters. Results: .The present study reveals the important role of the total mass of theglobular cluster on its HB morphology. More massive clusters tend tohave HBs more extended to higher temperatures. For a set of three inputvariables including the temperature extension of the HB, [Fe/H] and M_V,the first two eigenvectors account for 90% of the total samplevariance. Conclusions: . Possible effects of clusterself-pollution on HB morphology, stronger in more massive clusters,could explain the results derived here.
| Age and Metallicity Estimation of Globular Clusters from Strömgren Photometry
We present a new technique for the determination of age and metallicityin composite stellar populations using Strömgren filters. Usingprincipal component (PC) analysis on multicolor models, we isolate therange of values necessary to uniquely determine age and metallicityeffects. The technique presented here can only be applied to old(τ>3 Gyr) stellar systems composed of simple stellar populations,such as globular clusters and elliptical galaxies. Calibration using newphotometry of 40 globular clusters with spectroscopic [Fe/H] values andmain-sequence-fitted ages links the PC values to the Strömgrencolors, for an accuracy of 0.2 dex in metallicity and 0.5 Gyr in age.
| The metal abundance distribution of the oldest stellar component in the Sculptor dwarf spheroidal galaxy*
Low-resolution spectroscopy obtained with FORS2 at the Very LargeTelescope (VLT) has been used for the measurement of individual metalabundances ([Fe/H]) for 110 variable stars, including 107 RR Lyrae starsand one anomalous Cepheid, and to trace the metal distribution of theoldest stellar component in the Sculptor dwarf spheroidal galaxy. The RRLyrae stars are spread over a 15 × 15 arcmin2 areaaround the galaxy centre. Their metallicities have an average value of[Fe/H]=-1.83 +/- 0.03 (rms = 0.26 dex) and cover the metallicity range-2.40 < [Fe/H] < -0.85 (on the scale of Zinn & West), butthere is only one variable that has [Fe/H] > -1.3. The star-to-starscatter is larger than typical errors on individual metallicities(+/-0.15-0.16 dex), indicating a real spread in metal abundances. Theradial velocities measured from the RR Lyrae spectra have a dispersionof 12.9kms-1. This value is consistent with the dispersionderived by Tolstoy et al. for metal-poor red giants associated with theblue horizontal branch stars in Sculptor. Along with the metallicitydistribution these results suggest that most of the RR Lyrae stars inSculptor arise from the same burst of stellar formation that producedthe metal-poor component, originating the galaxy blue horizontal branch.The metal-rich red horizontal branch population found to be centrallyconcentrated only produced a few (if any) of the RR Lyrae stars in oursample. The spectroscopic metallicities were used along with theapparent luminosities to study the luminosity-metallicity relationfollowed by the RR Lyrae stars in Sculptor, for which we derive ashallow slope of 0.09magdex-1. This result can be due to ahigh level of evolution off the zero-age horizontal branch of the RRLyrae stars in this galaxy, again in agreement with their origin fromthe blue horizontal branch population.
| The Adequacy of Stellar Evolution Models for the Interpretation of the Color-Magnitude Diagrams of Resolved Stellar Populations
Most of what we know about the stellar population of nearby, resolvedgalaxies comes from the interpretation of their color-magnitudediagrams, by comparison with stellar evolutionary models. We review howwell current stellar evolution models reproduce the properties of simplestellar populations. Emphasis is given to the regions of thecolor-magnitude diagram which are most useful for deriving age,metallicity, or distance of a population. Extensive comparison is madebetween the predictions of the most-used stellar evolution libraries, inorder to estimate how model dependent the results are. The presentreview, written from a user perspective, aims at emphasizing thestrengths and weaknesses of the models, and is intended both forobservers and theoreticians. We hope to encourage observers to providestronger observational constraints where they are needed, and tostimulate theoreticians to isolate the input physics responsible for thedifferent behavior between models and the reasons for the discrepancieswith data.
| 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.
| Metal Abundances of RR Lyrae Stars in the Metal-rich Globular Cluster NGC 6441
Low-resolution spectra have been used to measure individual metalabundances of RR Lyrae stars in NGC 6441, a Galactic globular clusterknown to have very unusual horizontal-branch morphology and periods ofthe RR Lyrae stars for its high metallicity. We find an average metalabundance of [Fe/H]=-0.69+/-0.06 (rms=0.33 dex) and [Fe/H]=-0.41+/-0.06(rms=0.36 dex) on Zinn & West and Carretta & Gratton metallicityscales, respectively, consistent with the cluster metal abundancederived by Armandroff & Zinn. Most of the metallicities wereextrapolated from calibration relations defined for [Fe/H]<=-1however, they are clearly high and contrast with the rather long periodsof the NGC 6441 variables, thus confirming that the cluster does not fitin the general Oosterhoff classification scheme. The rms scatter of theaverage is larger than observational errors (0.15-0.16 dex), possiblyindicating some spread in metallicity. However, even the metal-poorvariables, if confirmed to be cluster members, are still more metal-richthan those commonly found in the Oosterhoff type II globular clusters.Based on data collected at the Very Large Telescope of the EuropeanSouthern Observatory, Paranal, Chile, program number 71.B-0621.
| Discovery of More than 200 RR Lyrae Variables in M62: An Oosterhoff I Globular Cluster with a Predominantly Blue Horizontal Branch
We report on the discovery of a large number of RR Lyrae variable starsin the moderately metal-rich Galactic globular cluster M62 (NGC 6266),which places it among the top three most RR Lyrae-rich globular clustersknown. Likely members of the cluster in our studied field, from ourpreliminary number counts, include ~130 fundamental-mode (RRab)pulsators, with =0.548 days, and ~75first-overtone (RRc) pulsators, with =0.300 days.The average periods and the position of the RRab variables withwell-defined light curves in the Bailey diagram both suggest that thecluster is of Oosterhoff type I. However, the morphology of thecluster's horizontal branch (HB) is strikingly similar to that of theOosterhoff type II globular cluster M15 (NGC 7078), with a dominant blueHB component and a very extended blue tail. Since M15 and M62 differ inmetallicity by about 1 dex, we conclude that metallicity, at a fixed HBtype, is a key parameter determining the Oosterhoff status of a globularcluster and the position of its variables in the Bailey diagram.
| A Comparison of Elemental Abundance Ratios in Globular Clusters, Field Stars, and Dwarf Spheroidal Galaxies
We have compiled a sample of globular clusters with high-quality stellarabundances from the literature to compare to the chemistries of stars inthe Galaxy and in dwarf spheroidal galaxies. Of the 45 globular clustersexamined, 29 also have kinematic information. Most of the globularclusters belong to the Galactic halo; however, a significant number havedisk kinematics or belong to the bulge. Focusing on the [α/Fe] andlight r-process element ratios, we find that most globular cluster starsmimic field stars of similar metallicities, and neither clearlyresembles the currently available stellar abundances in dwarf galaxies(including globular clusters in the Large Magellanic Cloud). Theexceptions to these general elemental ratio comparisons are alreadyknown in the literature, e.g., ω Centauri, Palomar 12, and Terzan7 associated with the Sagittarius remnant and Ruprecht 106, which has ahigh radial velocity and low [α/Fe] ratio. A few other globularclusters show more marginal peculiarities. The most notable one is thehalo cluster M68, which has a high galactocentric rotational velocity, aslightly younger age, and a unique [Si/Ti] ratio. The [Si/Ti] ratiosdecrease with increasing [Fe/H] at intermediate metallicities, which isconsistent with very massive stars playing a larger role in the earlychemical evolution of the Galaxy. The chemical similarities betweenglobular clusters and field stars with [Fe/H]<=-1.0 suggests a sharedchemical history in a well-mixed early Galaxy. The differences in thepublished chemistries of stars in the dwarf spheroidal galaxies suggestthat neither the globular clusters, halo stars, nor thick disk stars hadtheir origins in small isolated systems like the present-day Milky Waydwarf satellites.
| CCD Photometry of Candidate RR Lyrae Stars in the Sloan Digital Sky Survey
We present unfiltered CCD photometry of a sample of 71 candidate RRLyrae stars during 2003 and 2004, out of 148 candidate stars that wereselected from Sloan Digital Sky Survey (SDSS) commissioning data byIvezić and coworkers. We obtain light curves of 69 candidatestars and present a catalog of their positions, amplitudes, periods,types, and mean magnitudes. We confirm that the 69 stars are true RRLyrae variables based on periods and light curves, for the first timefor 41 of them. We identify 57 type ab, 11 type c, and 1 type d(double-mode) in our sample. The double-mode RR Lyrae star showsproperties similar to globular cluster stars and are unlike field starsof the Galactic halo in the Petersen diagram. We also find one RR Lyraestar that is related to Pal 5 and recover another. In our sample, onlytwo candidate stars do not have a reasonable light curve. We concludethat the ratio of contamination in our sample is less than ~8%, whichshows that the criteria used by Ivezić and coworkers to select RRLyrae stars from the SDSS two-epoch data are reliable.
| The Century Survey Galactic Halo Project. II. Global Properties and the Luminosity Function of Field Blue Horizontal Branch Stars
We discuss a 175 deg2 spectroscopic survey for bluehorizontal branch (BHB) stars in the Galactic halo. We use the TwoMicron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) toselect BHB candidates, and we find that the 2MASS and SDSS colorselection is 38% and 50% efficient, respectively, for BHB stars. Oursamples include one likely runaway B7 star 6 kpc below the Galacticplane. The global properties of the BHB samples are consistent withmembership in the halo population: the median metallicity is[Fe/H]=-1.7, the velocity dispersion is 108 km s-1, and themean Galactic rotation of the BHB stars 3 kpc<|z|<15 kpc is-4+/-30 km s-1. We discuss the theoretical basis of thePreston, Shectman, and Beers MV-color relation for BHB starsand conclude that the intrinsic shape of the BHB MV-colorrelation results from the physics of stars on the horizontal branch. Wecalculate the luminosity function for the field BHB star samples usingthe maximum likelihood method of Efstathiou and coworkers, which isunbiased by density variations. The field BHB luminosity functionexhibits a steep rise at bright luminosities, a peak between0.8
| Galactic Globular Cluster Relative Ages
We present accurate relative ages for a sample of 55 Galactic globularclusters. The ages have been obtained by measuring the differencebetween the horizontal branch and the turnoff in two internallyphotometrically homogeneous databases. The mutual consistency of the twodata sets has been assessed by comparing the ages of 16 globularclusters in common between the two databases. We have also investigatedthe consistency of our relative age determination within the recentstellar model framework. All clusters with [Fe/H]<-1.7 are found tobe old and coeval, with the possible exception of two objects, which aremarginally younger. The age dispersion for the metal-poor clusters is0.6 Gyr (rms), consistent with a null age dispersion.Intermediate-metallicity clusters (-1.7<[Fe/H]<-0.8) are onaverage 1.5 Gyr younger than the metal-poor ones, with an age dispersionof 1.0 Gyr (rms) and a total age range of ~3 Gyr. About 15% of theintermediate-metallicity clusters are coeval with the oldest clusters.All the clusters with [Fe/H]>-0.8 are ~1 Gyr younger than the mostmetal-poor ones, with a relatively small age dispersion, although themetal-rich sample is still too small to allow firmer conclusions. Thereis no correlation of the cluster age with the galactocentric distance.We briefly discuss the implication of these observational results forthe formation history of the Galaxy.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, and on observations made at the European SouthernObservatory, La Silla, Chile, and with the Isaac Newton GroupTelescopes.
| A ``Pulsational'' Distance Determination for the Large Magellanic Cloud
We present results from the theoretical modeling of the observed B,Vlight curves of 14 RR Lyrae stars in the Large Magellanic Cloud. Thesample includes seven fundamental and seven first-overtone pulsatorscovering the metallicity range from -2.12 to -0.79 dex in [Fe/H], withan average value of -1.54 dex. Masses, intrinsic luminosities, effectivetemperatures, and reddenings were derived by fitting high-accuracymultiband light curves available for these RR Lyrae stars to nonlinearconvective pulsation models. Individual distance moduli were determinedfor each variable and lead to an average distance modulus for the LargeMagellanic Cloud of μ0=18.54+/-0.02 (σ=0.09 mag,standard deviation of the average), in good agreement with the ``long''astronomical distance scale.Based on observations collected at the European Southern Observatory,proposals 62.N-0802, 66.A-0485, and 68.D-0466.
| Different Characteristics of the Bright Branches of the Globular Clusters M3 and M13
We carried out wide-field BVI CCD photometric observations of theglobular clusters M3 (NGC 5272) and M13 (NGC 6205) using the BohyunOptical Astronomy Observatory 1.8 m telescope equipped with a SITe 2KCCD. We present color-magnitude diagrams (V vs. B-V, V vs. V-I, and Vvs. B-I) of M3 and M13. We have found asymptotic giant branch (AGB)bumps at VbumpAGB=14.85+/-0.05 mag for M3 and atVbumpAGB=14.25+/-0.05 mag for M13. We also foundthat AGB stars in M3 are more concentrated near the bump, while those inM13 are scattered along the AGB sequence. We identified the red giantbranch (RGB) bump of M3 at VbumpRGB=15.50+/-0.05mag and that of M13 at VbumpRGB=14.80+/-0.05 magthrough luminosity functions and slope changes of the integratedluminosity functions of M3 and M13. We have estimated the ratios R andR2 for M3 and M13 and found that the value of R for M3 islarger than that for M13, while values of R2 for M3 and M13are similar and compatible with the value expected from evolutionarytheory when only normal horizontal-branch (HB) stars are used forestimation of R and R2 for M13. However, we found that valuesof R for M3 and M13 are similar, while the value of R2 for M3is larger than that for M13 when all the HB stars are included forestimation of R and R2 for M13. We have compared the observedRGB luminosity functions of M3 and M13 with the theoretical RGBluminosity function of Bergbusch & VandenBerg at the same radialdistances from the cluster centers as used in the estimation of theratios R and R2 for M3 and M13. We found ``extra stars''belonging to M13 in the comparison of the observed RGB luminosityfunction of M13 and the theoretical RGB luminosity function of Bergbusch& VandenBerg and even in the comparison of the observed RGBluminosity functions of M3 and M13. In the original definition of theratio R of Buzzoni et al., NHB corresponds to the lifetime ofHB stars in the RR Lyrae instability strip at logTeff=3.85.Thus, the smaller R value resulting for M13 compared with that for M3 inthe case in which only normal HB stars are included in the estimation ofR and R2 for M13 may be partially caused by extra stars, andthe similar R values for M3 and M13 in the case in which all the HBstars are included in the estimation of R and R2 for M13 maybe caused by extra stars in the upper RGB of M13. If extra stars in theupper RGB of M13 are caused by an effective ``deep mixing,'' these factssupport the contention that an effective deep mixing could lead todifferent HB morphologies between M3 and M13 and subsequent sequences.
| Spectroscopy of Red Giants in the Large Magellanic Cloud Bar: Abundances, Kinematics, and the Age-Metallicity Relation
We report metallicities and radial velocities derived from spectra atthe near-infrared calcium triplet for 373 red giants in a 200arcmin2 area at the optical center of the LMC bar. These arethe first spectroscopic abundance measurements of intermediate-age andold field stars in the high surface brightness heart of the LMC. Themetallicity distribution is sharply peaked at the median value[Fe/H]=-0.40, with a small tail of stars extending down to[Fe/H]<=-2.1 10% of the red giants are observed to have[Fe/H]<=-0.7. The relative lack of metal-poor stars indicates thatthe LMC has a ``G dwarf'' problem, similar to the Milky Way. Theabundance distribution can be closely approximated by two Gaussianscontaining 89% and 11% of the stars, respectively: the first componentis centered at [Fe/H]=-0.37 with σ=0.15, and the second at[Fe/H]=-1.08 with σ=0.46. The dominant population has ametallicity distribution similar to that of the LMC's intermediate-agestar clusters. The mean heliocentric radial velocity of the sample is257 km s-1, corresponding to the same center-of-mass velocityas the disk (measured at larger radii). Because of the central locationof our field, kinematic constraints are not strong, but there is noevidence that the bar deviates from the general motion of the LMC disk.The velocity dispersion of the whole sample isσv=24.7+/-0.4 km s-1. When cut bymetallicity, the most metal-poor 5% of stars ([Fe/H]<-1.15) showσv=40.8+/-1.7 km s-1, more than twice thevalue for the most metal-rich 5%; this suggests that an old, thickerdisk or halo population is present. The age-metallicity relation (AMR)is almost flat during the period from 5 to 10 Gyr ago, with an apparentscatter of +/-0.15 dex about the mean metallicity for a given age.Comparing this to chemical evolution models from the literature, we findthat a burst of star formation 3 Gyr ago does not reproduce the observedAMR more closely than a steadily declining star formation rate. The AMRsuggests that the epoch of enhanced star formation, if any, must havecommenced earlier, ~6 Gyr ago-the exact time is model dependent. Wecompare the properties of the LMC and the Milky Way, and discuss ourresults in the context of models that attempt to use tidal interactionswith the Milky Way and Small Magellanic Cloud to explain the star andcluster formation histories of the LMC.
| Infrared Photometry of NGC 6791
We present deep JHK photometry of the old and metal-rich open clusterNGC 6791. The photometry reaches below the main-sequence turnoff toK~16.5 mag. We combine our photometry with that from Stetson et al. toprovide color-magnitude diagrams showing K versus J-K, K versus V-K, andV versus V-K. We study the slope of the red giant branch in the infraredbut find that it is not a useful metallicity indicator for the cluster,nor any metal-rich cluster that lacks a well-populated red giant branch,because it is not linear, as has often been assumed, in K versus J-K.The mean color of the red horizontal-branch/red clump stars provide anestimate of the cluster reddening, E(B-V)=0.14+/-0.04 mag for[Fe/H]=+0.4+/-0.1. The mean magnitudes of these stars also provide agood distance estimate, (m-M)0=13.07+/-0.04. Finally, we findthat the isochrones of Yi et al. provide optimal fits in V versus B-Vand V-K and K versus J-K and V-K for such values if [Fe/H] lies between+0.3 and +0.5 (with a slight preference for +0.5) and ages between 9 Gyr([Fe/H]=+0.3) and 7.5 Gyr ([Fe/H]=+0.5).Based on observations made with the Mayall 4 m Telescope of the NationalOptical Astronomy Observatory.
| Chemical Abundances for Seven Giant Stars in M68 (NGC 4590): A Globular Cluster with Abnormal Silicon and Titanium Abundances
We present a detailed chemical abundance study of seven giant stars inM68, including six red giants and one post-asymptotic giant branch (AGB)star. We find significant differences in the gravities determined usingphotometry and those obtained from ionization balance, which suggeststhat non-LTE (NLTE) affects are important for these low-gravity,metal-poor stars. We adopt an iron abundance using photometric gravitiesand Fe II lines to minimize those effects, finding [Fe/H]=-2.16+/-0.02(σ=0.04). For element-to-iron ratios, we rely on neutral linesversus Fe I and ionized lines versus Fe II (except for [O/Fe]) to alsominimize NLTE effects. We find variations in the abundances of sodiumamong the program stars. However, there is no correlation (oranticorrelation) with the oxygen abundances. Furthermore, the post-AGBstar has a normal (low) abundance of sodium. Both of these facts addfurther support to the idea that the variations seen among some lightelements within individual globular clusters arise from primordialvariations and not from deep mixing. M68, like M15, shows elevatedabundances of silicon compared with other globular clusters andcomparable-metallicity field stars. But M68 deviates even more inshowing a relative underabundance of titanium. We speculate that in M68titanium is behaving like an iron-peak element rather than its morecommonly observed adherence to enhancements seen in the so-calledα-elements such as magnesium, silicon, and calcium. We interpretthis result as implying that the chemical enrichment seen in M68 mayhave arisen from contributions from supernovae with somewhat moremassive progenitors than those that contribute to abundances normallyseen in other globular clusters. The neutron capture elements barium andeuropium vary among the stars in M15, according to earlier work bySneden et al., but [Ba/Eu] is relatively constant, suggesting that bothelements arise in the same nucleosynthesis events. M68 shares the same[Ba/Eu] ratio as the stars in M15, but the average abundance ratio ofthese elements, as well as lanthanum, are lower in M68 relative to ironthan in M15, implying a slightly weaker contribution of r-processnucleosynthesis in M68.
| On the Distribution of the Modulation Frequencies of RR Lyrae Stars
For the first time connection between the pulsation and modulationproperties of RRLyr stars has been detected. Based on the available datait is found that the possible range of the modulation frequencies, ie.,the possible maximum value of the modulation frequency depends on thepulsation frequency. Short period variables (P<0.4 d) can havemodulation period as short as some days, while longer period variables(P>0.6 d) always exhibit modulation with P_mod>20 d. We interpretthis tendency with the equality of the modulation period with thesurface rotation period, because similar distribution of the rotationalperiods is expected if an upper limit of the total angular momentum ofstars leaving the RGB exists. The distribution of the projectedrotational velocities of red and blue horizontal branch stars atdifferent temperatures shows a similar behavior as v_rot derived for RRLyr stars from their modulation periods. This common behavior givesreason to identify the modulation period with the rotational period ofthe modulated RR Lyr stars.
| A homogeneous set of globular cluster relative distances and reddenings
We present distance modulus and reddening determinations for 72 Galacticglobular clusters from the homogeneous photometric database of Piotto etal. ([CITE]), calibrated to the HST flight F439W and F555W bands. Thedistances have been determined by comparison with theoretical absolutemagnitudes of the ZAHB. For low and intermediate metallicity clusters,we have estimated the apparent Zero Age Horizontal Branch (ZAHB)magnitude from the RR Lyrae level. For metal rich clusters, the ZAHBmagnitude was obtained from the fainter envelope of the red HB.Reddenings have been estimated by comparison of the HST colour-magnitudediagrams (CMD) with ground CMDs of template clusters with low reddening.The homogeneity of both the photometric data and the adoptedmethodological approach allowed us to obtain highly accurate relativecluster distances and reddenings. Our results are also compared withrecent compilations in the literature.Based on observations with the Hubble Space Telescope. Tables 3 and 4are only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/432/851
| Globular cluster system and Milky Way properties revisited
Aims.Updated data of the 153 Galactic globular clusters are used toreaddress fundamental parameters of the Milky Way, such as the distanceof the Sun to the Galactic centre, the bulge and halo structuralparameters, and cluster destruction rates. Methods: .We build areduced sample that has been decontaminated of all the clusters youngerthan 10 Gyr and of those with retrograde orbits and/or evidence ofrelation to dwarf galaxies. The reduced sample contains 116 globularclusters that are tested for whether they were formed in the primordialcollapse. Results: .The 33 metal-rich globular clusters([Fe/H]≥-0.75) of the reduced sample basically extend to the Solarcircle and are distributed over a region with the projected axial-ratiostypical of an oblate spheroidal, Δ x:Δ y:Δz≈1.0:0.9:0.4. Those outside this region appear to be related toaccretion. The 81 metal-poor globular clusters span a nearly sphericalregion of axial-ratios ≈1.0:1.0:0.8 extending from the central partsto the outer halo, although several clusters in the external regionstill require detailed studies to unravel their origin as accretion orcollapse. A new estimate of the Sun's distance to the Galactic centre,based on the symmetries of the spatial distribution of 116 globularclusters, is provided with a considerably smaller uncertainty than inprevious determinations using globular clusters, R_O=7.2±0.3 kpc.The metal-rich and metal-poor radial-density distributions flatten forR_GC≤2 kpc and are represented well over the full Galactocentricdistance range both by a power-law with a core-like term andSérsic's law; at large distances they fall off as R-3.9. Conclusions: .Both metallicity components appearto have a common origin that is different from that of the dark matterhalo. Structural similarities between the metal-rich and metal-poorradial distributions and the stellar halo are consistent with a scenariowhere part of the reduced sample was formed in the primordial collapseand part was accreted in an early period of merging. This applies to thebulge as well, suggesting an early merger affecting the central parts ofthe Galaxy. The present decontamination procedure is not sensitive toall accretions (especially prograde) during the first Gyr, since theobserved radial density profiles still preserve traces of the earliestmerger(s). We estimate that the present globular cluster populationcorresponds to ≤23±6% of the original one. The fact that thevolume-density radial distributions of the metal-rich and metal-poorglobular clusters of the reduced sample follow both a core-likepower-law, and Sérsic's law indicates that we are dealing withspheroidal subsystems at all scales.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
INHABITED SKY
