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|Surface Brightness Profiles of Galactic Globular Clusters from Hubble Space Telescope Images|
The Hubble Space Telescope (HST) allows us to study the central surfacebrightness profiles of globular clusters at unprecedented detail. Wehave mined the HST archives to obtain 38 WFPC2 images of Galacticglobular clusters with adequate exposure times and filters, which we useto measure their central structure. We outline a reliable method toobtain surface brightness profiles from integrated light that we test onan extensive set of simulated images. Most clusters have central surfacebrightness about 0.5 mag brighter than previous measurements made fromground-based data, with the largest differences around 2 mag. Includingthe uncertainties in the slope estimates, the surface brightness slopedistribution is consistent with half of the sample having flat cores andthe remaining half showing a gradual decline from 0 to -0.8[dlogΣ/dlogr)]. We deproject the surface brightness profiles in anonparametric way to obtain luminosity density profiles. Thedistribution of luminosity density logarithmic slopes shows similarfeatures, with half of the sample between -0.4 and -1.8. These resultsare in contrast to our theoretical bias that the central regions ofglobular clusters are either isothermal (i.e., flat central profiles) orvery steep (i.e., luminosity density slope approximately -1.6) forcore-collapse clusters. With only 50% of our sample having centralprofiles consistent with isothermal cores, King models appear torepresent most globular clusters in their cores poorly.
|Nearby Spiral Globular Cluster Systems. I. Luminosity Functions|
We compare the near-infrared (JHK) globular cluster luminosity functions(GCLFs) of the Milky Way, M31, and the Sculptor Group spiral galaxies.We obtained near-infrared photometry with the Persson's AuxiliaryNasmyth Infrared Camera on the Baade Telescope for 38 objects (mostlyglobular cluster candidates) in the Sculptor Group. We also havenear-infrared photometry from the Two Micron All Sky Survey (2MASS)-6Xdatabase for 360 M31 globular cluster candidates and aperture photometryfor 96 Milky Way globular cluster candidates from the 2MASS All-Sky andSecond Incremental Release databases. The M31 6X GCLFs peak at absolutereddening-corrected magnitudes of MJ0=-9.18,MH0=-9.73, and MK0=-9.98.The mean brightness of the Milky Way objects is consistent with that ofM31 after accounting for incompleteness. The average Sculptor absolutemagnitudes (correcting for relative distance from the literature andforeground reddening) are MJ0=-9.18,MH0=-9.70, and MK0=-9.80.NGC 300 alone has absolute foreground-dereddened magnitudesMJ0=-8.87, MH0=-9.39, andMK0=-9.46 using the newest Gieren et al. distance.This implies either that the NGC 300 GCLF may be intrinsically fainterthan that of the larger galaxy M31 or that NGC 300 may be slightlyfarther away than previously thought. Straightforward application of ourM31 GCLF results as a calibrator gives NGC 300 distance moduli of26.68+/-0.14 using J, 26.71+/-0.14 using H, and 26.89+/-0.14 using K.Data for this project were obtained at the Baade 6.5 m telescope, LasCampanas Observatory, Chile.
|On the origin of the radial mass density profile of the Galactic halo globular cluster system|
We investigate what may be the origin of the presently observed spatialdistribution of the mass of the Galactic Old Halo globular clustersystem. We propose its radial mass density profile to be a relic of thedistribution of the cold baryonic material in the protogalaxy. Assumingthat this one arises from the profile of the whole protogalaxy minus thecontribution of the dark matter (and a small contribution of the hot gasby which the protoglobular clouds were bound), we show that the massdistributions around the Galactic centre of this cold gas and of the OldHalo agree satisfactorily. In order to demonstrate our hypothesis evenmore conclusively, we simulate the evolution with time, up to an age of15Gyr, of a putative globular cluster system whose initial massdistribution in the Galactic halo follows the profile of the coldprotogalactic gas. We show that beyond a galactocentric distance oforder 2-3kpc, the initial shape of such a mass density profile ispreserved despite the complete destruction of some globular clusters andthe partial evaporation of some others. This result is almostindependent of the choice of the initial mass function for the globularclusters, which is still ill determined. The shape of these evolvedcluster system mass density profiles also agrees with the presentlyobserved profile of the Old Halo globular cluster system, thusstrengthening our hypothesis. Our result might suggest that theflattening shown by the Old Halo mass density profile at short distancesfrom the Galactic centre is, at least partly, of primordial origin.
|Caroline Herschel as observer|
|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.
|A Library of Integrated Spectra of Galactic Globular Clusters|
We present a new library of integrated spectra of 40 Galactic globularclusters, obtained with the Blanco 4 m telescope and the R-Cspectrograph at the Cerro Tololo Inter-American Observatory. The spectracover the range ~3350-6430 Å with ~3.1 Å (FWHM) resolution.The spectroscopic observations and data reduction were designed tointegrate the full projected area within the cluster core radii in orderto properly sample the light from stars in all relevant evolutionarystages. The S/N values of the flux-calibrated spectra range from 50 to240 Å-1 at 4000 Å and from 125 to 500Å-1 at 5000 Å. The selected targets span a widerange of cluster parameters, including metallicity, horizontal-branchmorphology, Galactic coordinates, Galactocentric distance, andconcentration. The total sample is thus fairly representative of theentire Galactic globular cluster population and should be valuable forcomparison with similar integrated spectra of unresolved stellarpopulations in remote systems. For most of the library clusters, ourspectra can be coupled with deep color-magnitude diagrams and reliablemetal abundances from the literature to enable the calibration ofstellar population synthesis models. In this paper we present a detailedaccount of the observations and data reduction. The spectral library ispublicly available in electronic format from the National OpticalAstronomical Observatory Web site.
|CCD Photometry of the RR LYRAE Stars in NGC147|
We report the results of CCD V and R photometry of the RRstars known inNGC147. The periodicities of most variables are revised and newephemerides are calculated. The Blazhko effect has been detected in V2and V6. Three previously reported variables; V5, V9, and V15 are foundto be non-variable. A new variable V18 was discovered with a period of0.49205 days and an amplitude of 0.15. Using the approach of Fourierdecomposition of the light curves, the physical parameters of the RRaband RRc variables were estimated. The cluster is of the Oosterhoff type.With the newly values [Fe/H]= -1.22Â+/-0.31 and 16.8Â+/-1.3,the cluster fits very well into the Oosterhoff type-metallicity andmetallicity-temperature sequence found in globular clusters. Acomparison with ZAHB models indicates that the RRab stars have not yetevolved off the horizontal branch, a result consistent with the meanperiod of the RRab variables and with the metallicity of the clusterderived in this work.
|Physical Characteristics of the RR Lyrae Stars in the Very Metal Poor Globular Cluster NGC 5053|
The physical characteristics of the 10 RR Lyrae stars in the verymetal-poor globular cluster NGC 5053 are derived from photometry of~1000 B and V CCD frames acquired from 1994 to 2002 with the DominionAstrophysical Observatory 1.8 m Plaskett Telescope. Revised pulsationperiods and light curves, mean magnitudes, colors, amplitudes, andFourier parameters are presented. Periods accurate to<~10-5 days are now known for all 10 RR Lyrae stars. Usingtimes of maximum light dating back to Baade's original 1923-1927observations, period change rates, dP/dt, accurate to <~0.07 daysMyr-1, have been derived for the 10 stars. Seven stars haveincreasing periods, and three have decreasing periods, with theestimated period change rates for V1, V2, V9, and V10 being very closeto zero. The mean dP/dt is equal to 0.04+/-0.04 days Myr-1and is consistent with Lee's evolutionary model predictions for acluster with horizontal-branch type ~0.5. Mean B - V colors range from0.20 to 0.40 and are more consistent with near-zero reddening thanalternative higher estimates. A reddening EB-V=0.018+/-0.003is derived from the 1998 SFD maps. Mean effective temperatures vary from6040 K (V10) to 7290 K (V6), with 2.6<=logg<=3.1. Visual andbolometric absolute magnitudes, bolometric corrections, and luminositiesare derived using Fourier methods and using intensity- andmagnitude-averaged mean magnitudes. Mean locations of the stars in theH-R diagram tend to progress from hotter, lower L stars to cooler,higher L stars and are consistent with theoretical blue and red edges ofthe instability strip. Masses estimated assuming zero reddening andDorman's oxygen-enhanced models range from 0.68 Msolar(V6) to 0.78 Msolar (V10) for the 10 stars. The meanmetal abundance for NGC 5053 derived using the Jurcsik-Kovácsmethod lies significantly higher than the range -2.3 to -2.6 dexdetermined using other, more well established methods. This findingsupports recent suggestions that metallicities derived fromFourier-based [Fe/H] calibrations need to be revised downward by atleast 0.3 dex for RR Lyrae stars with very low metal abundances.
|Globular Clusters as Candidates for Gravitational Lenses to Explain Quasar-Galaxy Associations|
We argue that globular clusters (GCs) are good candidates forgravitational lenses in explaining quasar-galaxy associations. Thecatalog of associations (Bukhmastova 2001) compiled from the LEDAcatalog of galaxies (Paturel 1997) and from the catalog of quasars(Veron-Cetty and Veron 1998) is used. Based on the new catalog, we showthat one might expect an increased number of GCs around irregulargalaxies of types 9 and 10 from the hypothesis that distant compactsources are gravitationally lensed by GCs in the halos of foregroundgalaxies. The King model is used to determine the central surfacedensities of 135 GCs in the Milky Way. The distribution of GCs incentral surface density was found to be lognormal.
|Hubble Space Telescope Snapshot Study of Variable Stars in Globular Clusters: The Inner Region of NGC 6441|
We present the results of a Hubble Space Telescope snapshot program tosurvey the inner region of the metal-rich globular cluster NGC 6441 forits variable stars. A total of 57 variable stars were found, including38 RR Lyrae stars, six Population II Cepheids, and 12 long-periodvariables. Twenty-four of the RR Lyrae stars and all of the PopulationII Cepheids were previously undiscovered in ground-based surveys. Of theRR Lyrae stars observed in this survey, 26 are pulsating in thefundamental mode with a mean period of 0.753 days and 12 arefirst-overtone-mode pulsators with a mean period of 0.365 days. Thesevalues match up very well with those found in ground-based surveys.Combining all the available data for NGC 6441, we find mean periods of0.759 and 0.375 days for the RRab and RRc stars, respectively. We alsofind that the RR Lyrae stars in this survey are located in the sameregions of a period-amplitude diagram as those found in ground-basedsurveys. The overall ratio of RRc to total RR Lyrae stars is 0.33.Although NGC 6441 is a metal-rich globular cluster and would, on thatground, be expected either to have few RR Lyrae stars or to be anOosterhoff type I system, its RR Lyrae stars more closely resemble thosein Oosterhoff type II globular clusters. However, even compared withtypical Oosterhoff type II systems, the mean period of its RRab stars isunusually long. We also derived I-band period-luminosity relations forthe RR Lyrae stars. Of the six Population II Cepheids, five are of WVirginis type and one is a BL Herculis variable star. This makes NGC6441, along with NGC 6388, the most metal-rich globular cluster known tocontain these types of variable stars. Another variable, V118, may alsobe a Population II Cepheid, given its long period and its separation inmagnitude from the RR Lyrae stars. We examine the period-luminosityrelation for these Population II Cepheids and compare it with those inother globular clusters and in the Large Magellanic Cloud. We argue thatthere does not appear to be a change in the period-luminosity relationslope between the BL Herculis and W Virginis stars, but that a change ofslope does occur when the RV Tauri stars are added to theperiod-luminosity relation.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.
|RR Lyrae variables in Galactic globular clusters. I. The observational scenario|
In this paper we revisit observational data concerning RR Lyrae stars inGalactic globular clusters, presenting frequency histograms offundamentalized periods for the 32 clusters having more than 12pulsators with well recognized period and pulsation mode. One finds thatthe range of fundamentalized periods covered by the variables in a givencluster remains fairly constant in varying the cluster metallicity allover the metallicity range spanned by the cluster sample, with the onlytwo exceptions given by M 15 and NGC 6441. We conclude that the width intemperature of the RR Lyrae instability strip appears largelyindependent of the cluster metallicity. At the same time, it appearsthat the fundamentalized periods are not affected by the predictedvariation of pulsators luminosity with metal abundance, indicating theoccurrence of a correlated variation in the pulsator mass. We discussmean periods in a selected sample of statistically significant ``RRrich" clusters with no less than 10 RRab and 5 RRc variables. One findsa clear evidence for the well known Oosterhoff dichotomy in the meanperiod of ab-type variables, together with a similarlyclear evidence for a constancy of the mean fundamentalized period in passing from Oosterhoff type II to type I clusters. Onthis basis, the origin of the Oosterhoff dichotomy is discussed,presenting evidence against a strong dependence of the RR Lyraeluminosity on the metal content. On the contrary, i) the continuity ofthe mean fundamentalized period, ii) the period frequency histograms inthe two prototypes M 3 (type I) and M 15 (type II), iii) the relativeabundance of first overtone pulsators, and iv) the observed differencebetween mean fundamental and fundamentalized periods, all agree in suggesting the dominant occurrence of avariation in the pulsation mode in a middle region of the instabilitystrip (the ``OR" zone), where variables of Oosterhoff type I and type IIclusters are pulsating in the fundamental or first overtone mode,respectively.
|Urban Astronomy: Observing the Messier Objects from the City|
|On the reliability of the semi-empirical RR Lyrae period-V-band luminosity-blue amplitude relation|
We investigate the accuracy and reliability of the semi-empiricalperiod-V-band luminosity-blue amplitude (PLA) relationship for ab-typeRR Lyrae stars originally obtained by Castellani and De Santis in themid-1990s. We infer that the zero-point of this relationship depends onthe metallicity, by studying a sample of both field and clustervariables. We also show that the use of this relationship can still beuseful for those stellar systems showing an intrinsic metallicityspread, since in this case the metallicity has a negligible effect onthe final distance modulus estimate. We compare the adoptedsemi-empirical relationship with the fully empirical one recentlyprovided by Kovács & Walker. When the zero-point of thelatter relation is fixed consistently with the former one, the twoequations are equivalent. By applying the semi-empirical period-V-bandluminosity-blue amplitude relation, as well as the technique proposedrecently by Cassisi, De Santis & Piersimoni, to the globular clusterω Cen, we show that the empirical slope of the relationshipbetween the mass of the fundamental RR Lyrae pulsators and theirmetallicity is in fair agreement with the one predicted by updatedevolutionary models for horizontal branch stars.
|Hidden subluminous stars among the FAUST UV sources towards Ophiuchus|
We present results of an analysis of a UV image in the direction ofOphiuchus, obtained with the FAUST instrument. The image contains 228 UVsources. Most of these are identified as normal early-type stars throughcorrelations with catalogued objects. For the first time in this projectwe identify UV sources as such stars by selecting suitable candidates incrowded fields as the bluest objects in colour-colour diagrams usingobservations from the Wise Observatory. These candidates are thenstudied using low-resolution spectroscopy, which allows thedetermination of spectral types to an accuracy of about one-half class,for 60 stars. Synthetic photometry of spectral data is performed inorder to predict the expected UV emission, on the basis of thephotometric information. These results are used along with the Hipparcos/Tycho information, to search for subluminous stars. The comparison ofthe predicted emission with the FAUST measured magnitudes allows us toselect 12 stars as highly probable evolved hot stars. Highsignal-to-noise spectra are obtained for nine of these stars, and Balmerline profiles are compared with the prediction of atmosphere models andwith the spectrum of real stellar atmospheres. Among the ninecandidates, six are classified as previously unrecognized sdB stars, andtwo as white dwarfs. Our result indicates that indeed more brightsubluminous stars are still unrecognized in the existing samples.
|Discovery of Likely Globular Clusters in Maffei 1|
The Hubble Space Telescope Wide Field and Planetary Camera 2 was used toimage at high resolution the core region of the nearby, heavily obscuredmassive elliptical galaxy Maffei 1. We report on the discovery of 19diffuse objects in the WFPC2 field that are likely to be globularclusters associated with Maffei 1. We present some preliminary data onthe luminosities, colors, and sizes of these candidates.
|Variable Stars in the Unusual, Metal-rich Globular Cluster NGC 6388|
We have undertaken a search for variable stars in the metal-richglobular cluster NGC 6388, using time-series BV photometry. Twenty-eightnew variables were found in this survey, increasing the total number ofvariables found near NGC 6388 to ~57. A significant number of thevariables are RR Lyrae (~14), most of which are probable clustermembers. The periods of the fundamental-mode RR Lyrae are shown to beunusually long compared with metal-rich field stars. The existence ofthese long-period RRab stars suggests that the horizontal branch of NGC6388 is unusually bright. This implies that the metallicity-luminosityrelationship for RR Lyrae stars is not universal if the RR Lyrae in NGC6388 are indeed metal-rich. We consider the alternative possibility thatthe stars in NGC 6388 may span a range in [Fe/H]. Four candidatePopulation II Cepheids were also found. If they are members of thecluster, NGC 6388 would be the most metal-rich globular cluster tocontain Population II Cepheids. The mean V magnitude of the RR Lyrae isfound to be 16.85+/-0.05, resulting in a distance of 9.0 to 10.3 kpc fora range of assumed values of for RR Lyrae. Wedetermine the reddening of the cluster to be E(B-V)=0.40+/-0.03 mag,with differential reddening across the face of the cluster. We discussthe difficulty in determining the Oosterhoff classification of NGC 6388and NGC 6441 due to the unusual nature of their RR Lyrae and addressevolutionary constraints on a recent suggestion that they are ofOosterhoff type II.
|The distance to Galactic globular clusters through RR Lyrae pulsational properties|
By adopting the same approach outlined by De Santis & Cassisi, weevaluate the absolute bolometric magnitude of the zero-age horizontalbranch (ZAHB) at the level of the RR Lyrae variable instability strip inselected Galactic globular clusters. This allows us to estimate the ZAHBabsolute visual magnitude for these clusters and to investigate itsdependence on the cluster metallicity. The derivedMV(ZAHB)-[Fe/H] relation, corrected in order to account forthe luminosity difference between the ZAHB and the mean RR Lyraemagnitude, has been compared with some of the most recent empiricaldeterminations in this field, such as the one provided byBaade-Wesselink analyses, RR Lyrae periods, Hipparcos data for fieldvariables and main-sequence fitting based on Hipparcos parallaxes forfield subdwarfs. As a result, our relation provides a clear support tothe `long' distance scale. We discuss also another method for measuringthe distance to Galactic globular clusters. This method is quite similarto the one adopted for estimating the absolute bolometric magnitude ofthe ZAHB but it relies only on the pulsational properties of the Lyraevariables in each cluster. The reliability and accuracy of this methodhave been tested by applying it to a sample of globular clusters forwhich, owing to the morphology of their horizontal branch (HB), the useof the commonly adopted ZAHB fitting is a risky procedure. We noticethat the two approaches for deriving the cluster distance modulusprovide consistent results when applied to globular clusters, the RRLyrae instability strip is well populated. As the adopted method relieson theoretical predictions on both the fundamental pulsational equationand the allowed mass range for fundamental pulsators, we give anestimate of the error affecting present results, owing to systematicuncertainties in the adopted theoretical framework.
|The Clusters AgeS Experiment (CASE): RR Lyrae variables in the globular cluster NGC 6362|
We present V- and B-band charge-coupled device (CCD) photometry of 35 RRLyr stars in the Southern hemisphere globular cluster NGC 6362. Fourierdecomposition of the light curves was used to estimate the basicproperties of these variables. From the analysis of the RRc stars weobtain a mean mass M=0.531+/-0.014Msolar, luminositylogL/Lsolar=1.656+/-0.006, effective temperatureTeff=7429+/-20 and helium abundance Y=0.292+/-0.002. The meanvalues of the absolute magnitude, metallicity (on Jurcsik's scale) andeffective temperature for 14 RRab stars with `regular' light curves are:MV=0.86+/-0.01, [Fe/H]=-0.93+/-0.04 andTeff=6555+/-25, respectively. From the B-V colours, periodsand metallicities of the RRab stars we estimate the colour excess forNGC 6362 to be equal to E(B-V)=0.08+/-0.01. Using this value we derivethe colours of the blue and red edges of the instability strip in NGC6362 to be (B-V)0BE=0.17 and (B-V)0RE=0.43. From the relations betweenthe Fourier coefficients of RRab and RRc stars and their absolutemagnitudes we estimate the apparent distance modulus to NGC 6362 to be(m-M)V=14.46+/-0.10. From the mean value oflogL/Lsolar of the RRc stars we obtain 14.59+/-0.03. TheV-band light curves of three of the RRc stars exhibit changes inamplitude of over 0.1mag on the time-scale of around one week. Near theradial first overtone frequency we find one or two peaks, which stronglysuggests that these variables belong to the newly identified group ofnon-radial pulsating RR Lyr stars.
|Near-Infrared Photometry of Globular Clusters NGC 6287 and NGC 6341 (M92): The Formation of the Galactic Halo|
We present Hubble Space Telescope (HST) Near Infrared Camera andMulti-Object Spectrometer (NICMOS) camera 3 photometry of the inner haloglobular cluster NGC 6287 and the intermediate halo globular cluster NGC6341 (M92). Our photometric measurements range from the lower red giantbranch to ~3 mag below the main-sequence turnoff, allowing us to defineaccurate mean loci of the two clusters. Our relative age estimatebetween NGC 6287 and M92 using the color difference between themain-sequence turnoff and the base of the red giant branch shows thatthey essentially have the same ages within +/-2 Gyr. The slightlydifferent chemical compositions between the two clusters do notsignificantly influence our result. Our simulations using syntheticspectra show that the temperature dependence of the interstellarreddening law for the HST NICMOS photometric system could be a moreserious problem in the relative age estimate using the color differencebetween the main-sequence turnoff and the base of the red giant branch.However, this effect adds no more than +/-0.5-1 Gyr to the uncertaintyin our relative age estimate. Our color-magnitude diagram of NGC 6287shows a well-defined blue horizontal branch population. This isconsistent with its age and metallicity. We also present the relativeinterstellar reddening toward NGC 6287 and distance modulus with respectto those of M92. We find E(B-V)=0.62 and (m-M)0=14.35 for NGC6287, which are generally in good agreement with previous estimates. Ourphotometry suggests that there exists a differential reddening acrossNGC 6287 with an amount of ΔE(B-V)=0.07-0.09 mag, consistent withthe previous study of Stetson & West. The Galactocentric distance ofNGC 6287 becomes RGC=1.6 kpc if R0=8.0 kpc. Basedon our new radial velocity measurement for NGC 6287, the cluster's(minimum) apogalacticon distance is Ra~5.5 kpc, suggestingthat NGC 6287 is an inner halo cluster. Within this framework, ourresults are consistent with the idea that the globular cluster formationmust have been triggered everywhere at the same time in our Galaxy. Inthe appendices, we discuss that the interstellar extinction law of theHST NICMOS photometric system, particularly in the F110W passband,depends on the temperature of the stars in the sense that hotter starssuffer more interstellar absorption than cooler stars. Based onobservations made with the NASA/ESA Hubble Space Telescope, obtained atthe Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
|Variable Stars in Galactic Globular Clusters|
Based on a search of the literature up to 2001 May, the number of knownvariable stars in Galactic globular clusters is approximately 3000. Ofthese, more than 2200 have known periods and the majority (approximately1800) are of the RR Lyrae type. In addition to the RR Lyrae population,there are approximately 100 eclipsing binaries, 120 SX Phoenicisvariables, 60 Cepheids (including Population II Cepheids, anomalousCepheids and RV Tauri), and 120 SR/red variables. The mean period of thefundamental mode RR Lyrae variables is 0.585 days, for the overtonevariables it is 0.342 days (0.349 days for the first-overtone pulsatorsand 0.296 days for the second-overtone pulsators) and approximately 30%are overtone pulsators. These numbers indicate that about 65% of RRLyrae variables in Galactic globular clusters belong to Oosterhoff typeI systems. The mean period of the RR Lyrae variables in the Oosterhofftype I clusters seems to be correlated with metal abundance in the sensethat the periods are longer in the more metal poor clusters. Such acorrelation does not exist for the Oosterhoff type II clusters. Most ofthe Cepheids are in clusters with blue horizontal branches.
|Near-Infrared Imaging of the Central Regions of Metal-Poor Inner Spheroid Globular Clusters|
JHK images obtained with the Canada-France-Hawaii Telescope adaptiveoptics bonnette are used to investigate the near-infrared photometricproperties of red giant branch (RGB) and horizontal-branch (HB) stars ineight metal-poor globular clusters with RGC<=2 kpc. Theslope of the RGB on the (K, J-K) CMDs confirms the metal-poor nature ofthese clusters, four of which (NGC 6287, 6293, 6333, and 6355) are foundto have metallicities that are comparable to M92. The luminosityfunctions of RGB stars in inner spheroid and outer halo clusters havesimilar slopes, although there is a tendency for core-collapsed clustersto have slightly flatter luminosity functions than noncollapsedclusters. The distribution of red HB stars on the (K, J-K) CMDs of innerspheroid clusters with [Fe/H]~-1.5 is very different from that ofclusters with [Fe/H]~-2.2, suggesting that metallicity is the mainparameter defining HB content among these objects. The RGB bump isdetected in four of the inner spheroid clusters, and this feature isused to compute distances to these objects. Finally, the specificfrequency of globular clusters in the inner Galaxy is discussed in thecontext of the early evolution of the bulge. Based on the ratio ofmetal-poor to metal-rich clusters in the inner Galaxy it is suggestedthat the metal-poor clusters formed during an early intense burst ofstar formation. It is also demonstrated that if the globular clusterformation efficiency for the inner Galaxy is similar to that measured inother spheroidal systems, then the main body of the bulge could haveformed from gas that was chemically enriched in situ; hence, materialfrom a separate pre-enriched reservoir, such as the disk or outer halo,may not be required to form the bulge.
|RR Lyrae stars in the Sagittarius dwarf galaxy: Period analysis|
We carried out a period analysis on ~ 3700 RR Lyrae stars on two Schmidtfields centred on (l,b)=(3.1deg, -7.1deg) and(6.6deg, -10.8deg) respectively, covering an areaof ~ 50 deg2. These stars are distributed almost evenlybetween the Sagittarius dwarf galaxy (Sgr) and the Milky Way. For Sgrmembers, the average periods are =0.574d and =0.322d for RRab and RRc stars respectively.This places Sgr in the long-period tail of the Oosterhoff I group. Wereport the detection of 53 double-mode RR Lyrae stars (RRd) within oursample. The magnitude of 40 of these stars is consistent with membershipin Sgr whereas 13 RRds are located within our Galaxy. We also found 13RR Lyraes (5 in Sgr and 8 in the Galaxy) exhibiting two closely spacedfrequencies, most probably related to non-radial pulsations. The perioddistribution of the RR Lyrae variables in Sgr is compared to those ofother Milky Way satellites. We find a remarkable similarity between theRR Lyrae populations in Sgr and the Large Magellanic Cloud (LMC),suggesting that these galaxies have similar horizontal branchmorphologies. This may indicate that Sgr and the LMC started theirformation under similar conditions. Using various photometricindicators, we estimate the metallicity of the RR Lyrae stars in Sgr andfind < [Fe/H]> =~ -1.6 dex with a dispersion of ~ +/-0.5 dexaround this value and a minor but significant population at ≲-2.0dex. We do not find evidence for a spatial metallicity gradient in theRR Lyrae population of Sgr. From the spatial distribution of RR Lyraes,we find that the main body of Sgr contains ~ 4200 RRab stars. Assumingthat population gradients are negligible in Sgr, we findMV(Sgr) =~ -13.9+0.4-0.6 mag for themain body. If Sgr has been stripped of 50% of its mass through Galactictides, as assumed by some models, it would imply a total absolutemagnitude of ~ -14.7 mag for this galaxy. Such a luminosity would beconsistent with the empirical metallicity/luminosity relation for dwarfspheroidal galaxies. Table 15 is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/375/909. Based onobservations obtained at the European Southern Observatory, La Silla,Chile.
|Fourier decomposition and frequency analysis of the pulsating stars with P<1 d in the OGLE database. I. Monoperiodic Delta Scuti, RRc and RRab variables. Separation criteria and particularities|
The OGLE database is revisited to investigate in more detail theproperties of the Fourier parameters. Methodological improvements led usto identify a clear separation among High-Amplitude delta Scuti (HADS),RRc and RRab stars. The bimodal distribution of the R21parameter in HADS stars is explained as a contamination effect from RRcstars: there is evidence that all stars with 0.20
0.55 d; evolutionary phases or the combination ofdifferent physical conditions (not only metallicity) are suggested toexplain this separation, observed also in the cluster RRab stars. Thestellar parameters of RRc stars in a given globular cluster showdifferent tendencies than those of RRc stars from different clusters.Tables~3, 4 and 5 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168)} or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/371/986 or via e-mail fromthe author.
|Empirical relations for cluster RR Lyrae stars revisited|
Our former study on the empirical relations between the Fourierparameters of the light curves of the fundamental mode RR Lyrae starsand their basic stellar parameters has been extended to considerablylarger data sets. The most significant contribution to the absolutemagnitude MV comes from the period P and from the firstFourier amplitude A1, but there are statistically significantcontributions also from additional higher order components, mostimportantly from A3 and in a lesser degree from the Fourierphase varphi51. When different colors are combined inreddening-free quantities, we obtain basically period-luminosity-colorrelations. Due to the log Teff(B-V,log g,[Fe/H]) relationfrom stellar atmosphere models, we would expect some dependence also onvarphi 31. Unfortunately, the data are still not extensiveand accurate enough to decipher clearly the small effect of this Fourierphase. However, with the aid of more accurate multicolor data on fieldvariables, we show that this Fourier phase should be present either inV-I or in B-V or in both. From the standard deviations of the variousregressions, an upper limit can be obtained on the overall inhomogeneityof the reddening in the individual clusters. This yields sigmaE(B-V)<~ 0.012 mag, which also implies an average minimumobservational error of sigmaV >~ 0.018 mag.
|Pulsational MV versus [Fe/H] relation(s) for globular cluster RR Lyrae variables|
We use the results from recent computations of updated non-linearconvective pulsating models to constrain the distance modulus ofGalactic globular clusters through the observed periods offirst-overtone (RRc) pulsators. The resulting relationbetween the mean absolute magnitude of RR Lyrae stars and the heavy element content [Fe/H] appearswell in the range of several previous empirical calibrations, but with anon-linear dependence on [Fe/H] so that the slope of the relationincreases when moving towards larger metallicities. On this ground, ourresults suggest that metal-poor ([Fe/H]<-1.5) and metal-rich([Fe/H]>-1.5) variables follow two different linear-[Fe/H] relations. Application to RR Lyraestars in the metal-poor globular clusters of the Large Magellanic Cloud(LMC) provides an LMC distance modulus of the order of 18.6mag, thussupporting the `long' distance scale. The comparison with recentpredictions based on updated stellar evolution theory is brieflypresented and discussed.
|A catalogue of helium abundance indicators from globular cluster photometry|
We present a survey of helium abundance indicators derived from acomprehensive study of globular cluster photometry in the literature.For each of the three indicators used, we conduct a thorough erroranalysis, and identify systematic errors in the computationalprocedures. For the population ratio RNHBNRGB, wefind that there is no evidence of a trend with metallicity, althoughthere appears to be real scatter in the values derived. Although thisindicator is the one best able to provide useful absolute heliumabundances, the mean value is Y~0.20, indicating the probable presenceof additional systematic error. For the magnitude difference from thehorizontal branch to the main sequence Δ and the RR Lyraemass-luminosity exponent A, it is only possible to determine relativehelium abundances reliably. This is due to continuing uncertainties inthe absolute metallicity scale for Δ, and uncertainty in the RRLyrae temperature scale for A. Both indicators imply that the heliumabundance is approximately constant as a function of [Fe/H]. Accordingto the A indicator, both Oosterhoff I and II group clusters haveconstant values independent of [Fe/H] and horizontal branch type. Inaddition, the two groups have slopes dlog/d[Fe/H]that are consistent with each other, but significantly smaller than theslope for the combined sample.
|Globular Cluster Subsystems in the Galaxy|
Data from the literature are used to construct a homogeneous catalog offundamental astrophysical parameters for 145 globular clusters of theMilky Way Galaxy. The catalog is used to analyze the relationshipsbetween chemical composition, horizontal-branch morphology, spatiallocation, orbital elements, age, and other physical parameters of theclusters. The overall globular-cluster population is divided by a gap inthe metallicity function at [Fe/H]=-1.0 into two discrete groups withwell-defined maxima at [Fe/H]=-1.60±0.03 and -0.60±0.04.The mean spatial-kinematic parameters and their dispersions changeabruptly when the metallicity crosses this boundary. Metal-poor clustersoccupy a more or less spherical region and are concentrated toward theGalactic center. Metal-rich clusters (the thick disk subsystem), whichare far fewer in number, are concentrated toward both the Galacticcenter and the Galactic plane. This subsystem rotates with an averagevelocity of V rot=165±28 km/s and has a very steep negativevertical metallicity gradient and a negligible radial gradient. It is,on average, the youngest group, and consists exclusively of clusterswith extremely red horizontal branches. The population ofspherical-subsystem clusters is also inhomogeneous and, in turn, breaksup into at least two groups according to horizontal-branch morphology.Clusters with extremely blue horizontal branches occupy a sphericalvolume of radius 9 kpc, have high rotational velocities (Vrot=77±33 km/s), have substantial and equal negative radial andvertical metallicity gradients, and are, on average, the oldest group(the old-halo subsystem). The vast majority of clusters withintermediate-type horizontal branches occupy a more or less sphericalvolume ≈18 kpc in radius, which is slightly flattened perpendicularto the Z direction and makes an angle of ≈30° to the X-axis. Onaverage, this population is somewhat younger than the old-halo clusters(the young-halo subsystem), and exhibits approximately the samemetallicity gradients as the old halo. As a result, since theirGalactocentric distance and distance from the Galactic plane are thesame, the young-halo clusters have metallicities that are, on average,Δ[Fe/H] ≈0.3 higher than those for old-halo clusters. Theyoung-halo subsystem, which apparently consists of objects captured bythe Galaxy at various times, contains many clusters with retrogradeorbits, so that its rotational velocity is low and has large errors, Vrot=-23±54 km/s. Typical parameters are derived for all thesubsystems, and the mean characteristics of their member globularclusters are determined. The thick disk has a different nature than boththe old and young halos. A scenario for Galactic evolution is proposedbased on the assumption that only the thick-disk and old-halo subsystemsare genetically associated with the Galaxy. The age distributions ofthese two subsystems do not overlap. It is argued that heavy-elementenrichment and the collapse of the proto-Galactic medium occurred mainlyin the period between the formation of the old-halo and thick-disksubsystems.
|Hubble Space Telescope WFPC2 Photometry of M33: Properties of the Halo Star Clusters and Surrounding Fields|
We present CCD photometry for 10 fields centered on halo star clustersin the nearby spiral galaxy M33. The data are based on Hubble SpaceTelescope Wide-Field Planetary Camera 2 observations in the F555W (~V)and F814W (~I) filters. The resultant color-magnitude diagrams (CMDs)extend ~1.5 mag below the horizontal branch (HB). From these diagrams,we draw the following conclusions: 1. Eight of the 10 M33 haloclusters possess HBs that are located entirely redward of the RR Lyraeinstability strip.2. For nine of theseclusters, we are able to use the slope of the red giant branch tomeasure metal abundances; we find their metal abundances to be in therange -1.64>=[Fe/H]>=-0.65 with a mean of<[Fe/H]>=-1.27+/-0.11. 3. Utilizingthis information, we are able to place these objects in the [Fe/H]-HBtype diagram from which we conclude that the halo clusters in M33 sufferfrom the second-parameter effect; this suggests that another parameter,in addition to metal abundance, is affecting the morphology of theirHBs.4. We present several arguments infavor of the hypothesis that this second parameter is cluster age. Ifthis is indeed the case, while two of these clusters are likely to be asold as Galactic globular clusters at their metallicity, the remainingseven clusters have a mean age of ~7 Gyr. 5.Using the two clusters in which the magnitude of the RR Lyraevariables can be inferred and adopting the RR Lyraeluminosity-metallicity relation favored by Chaboyer, we calculate adistance modulus of (m-M)0=24.84+/-0.16 for M33.6. Our CMDs of the fields onto which theclusters are projected reveal the M33 disk population to be quitecomplicated. Our main conclusion regarding the disk field stars is thatthe youngest population is concentrated toward the center ofM33. Based on observations made with the NASA/ESAHubble Space Telescope obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Incorporated, under NASA contract NAS5-26555.
|A Near-Infrared Photometric Survey of Metal-poor Inner Spheroid Globular Clusters and Nearby Bulge Fields|
Images recorded through J, H, K, 2.2 μm continuum, and CO filtershave been obtained of a sample of metal-poor ([Fe/H]<=-1.3) globularclusters in the inner spheroid of the Galaxy. The shape and color of theupper giant branch on the (K, J-K) color-magnitude diagram (CMD),combined with the K brightness of the giant branch tip, are used toestimate the metallicity, reddening, and distance of each cluster. COindices are used to identify bulge stars, which will bias metallicityand distance estimates if not culled from the data. The distances andreddenings derived from these data are consistent with published values,although there are exceptions. The reddening-corrected distance modulusof the Galactic center, based on the Carney et al. horizontal-branch(HB) brightness calibration, is estimated to be 14.9+/-0.1. The meanupper giant branch CO index shows cluster-to-cluster scatter that (1) islarger than expected from the uncertainties in the photometriccalibration and (2) is consistent with a dispersion in CNO abundancescomparable to that measured among halo stars. The luminosity functions(LFs) of upper giant branch stars in the program clusters tend to besteeper than those in the halo clusters NGC 288, NGC 362, and NGC 7089.The majority of inner spheroid clusters fall along the integrated J-Kversus metallicity relation defined by halo clusters; however, many ofthe inner spheroid clusters do not follow the relation betweenintegrated CO index and metallicity measured for halo clusters, in thatthey have CO indices that are too small. Bulge fields were also observednear most clusters. The slope of the giant branch LF does not varysignificantly between most fields, although the LFs in Baade's windowand near NGC 6273 are significantly shallower than average.Metallicities estimated from the slope of the upper giant branch on the(K, J-K) CMDs of fields within 6° of the Galactic center areconsistent with previous studies. Finally, the data suggest that the HBcontent may not be uniform throughout the bulge, in the sense that alarger than average number of red HB stars may occur in fields closestto the Galactic center.
|Photometry of the Globular Cluster M54 and the Sagittarius Dwarf Galaxy: The Age-Metallicity Relation|
We present deep VI-band photometry of the globular cluster M54, a nearbyfield in the Sagittarius dwarf galaxy, and a control field. Thecolor-magnitude diagrams reach well below the oldest main-sequenceturnoffs, thus enabling an analysis of the galaxy's age-metallicityrelation with unprecedented clarity. We also study the variable stars inthe direction of M54. From 67 RR Lyrae variables, we confirm and improveon our previous estimates of the cluster horizontal branch magnitude,foreground reddening, and horizontal branch morphology. These values areused in determining the ages of M54 and the Sagittarius fieldpopulations. We confirm our previous result that M54 is the same age asGalactic globular clusters of similar metallicity. We also derive ageson a self-consistent scale for the other three globular clusters inSagittarius. We find strong evidence for multiple episodes of starformation (or continuous star formation with a variable rate) in thefield of Sagittarius. We characterize the principal episodes with theages 11, 5, and 0.5 through 3 Gyr and with [Fe/H] values of -1.3, -0.7,and -0.4, respectively. On this scale, M54 has an age of 15 Gyr.Surprisingly, the age-metallicity relation we have derived for thegalaxy as a whole is described quite well by a closed-box chemicalevolution model. We also find that the populations associated with theSgr field are clumped spatially around M54, and we consider severalexplanations for this phenomenon. We again speculate that Sagittarius isa nucleated dwarf elliptical galaxy with M54 as its nucleus.
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