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|Evolved Stars in the Galactic Globular Cluster M55 (NGC 6809)|
We have compiled the asymptotic giant, horizontal, and upper red giantbranch (AGB, HB, and RGB) stars in the globular cluster M55 (NGC 6809).Using the star counts and the R-parameter we compute the initial heliumabundance Y=0.274+/-0.016. TheR2=NAGB/NHB ratio (0.156+/-0.023) isunusually high for a globular cluster, being almost 2 σ away fromthe predicted values, and theR1=NAGB/NRGB ratio (0.272+/-0.047) isthe highest recorded for a massive globular cluster. We argue that M55'sparticular HB morphology and metallicity have produced long-lived HBstars that are not too blue to avoid producing AGB stars. This resulthints that we are able to map evolutionary effects on the HB. Finally,although we find no evidence of variations in HB morphology withdistance from the center of the cluster, the red HB stars aresignificantly less concentrated than the majority of HB stars, and thebluest HB stars are more centrally concentrated.
|CCD Photometry of the Globular Cluster M15: RR Lyrae Fourier Decomposition and Physical Parameters|
Results of CCD photometry using V and R filters are reported for 33 RRLyrae stars in M15. The periodicities of some variables have beenrevised and new ephemerides are given. The Blazhko effect, previouslyreported in V12, was not detected. Applying the approach of Fourierdecomposition of the light curves, the physical parameters of the typeRRab and RRc variables were estimated. The cluster is Oosterhoff type IIand the values for the iron content and distance are:[Fe/H]=-1.98+/-0.24 and d=8.67+/-0.41 kpc, respectively. The mean valuesof the physical parameters determined for the RR Lyrae stars place thecluster precisely into the sequences Oosterhoff type --metallicity andmetallicity-- effective temperature, valid for globular clusters.Evidences of evolution from the ZAHB are found for the RRc but not forthe RRab stars.
|The Clusters Ages Experiment (CASE). II. The Eclipsing Blue Straggler OGLEGC 228 in the Globular Cluster 47 Tuc|
We use photometric and spectroscopic observations of the eclipsingbinary OGLEGC 228 (V228) to derive the masses, radii, and luminositiesof the component stars. Based on measured systemic velocity, propermotion, and distance, the system is a blue straggler member of theglobular cluster 47 Tuc. Our analysis shows that V228 is a semidetachedAlgol. We obtain M=1.512+/-0.022 Msolar, R=1.357+/-0.019Rsolar, and L=7.02+/-0.050 Lsolar for the hotterand more luminous primary component and M=0.200+/-0.007Msolar, R=1.238+/-0.013 Rsolar, and L=1.57+/-0.09Lsolar for the Roche lobe-filling secondary.This paper utilizes data obtained with the 6.5 m Magellan Telescopeslocated at Las Campanas Observatory, Chile.
|CN Abundance Inhomogeneities in the Globular Cluster Messier 13 (NGC 6205): Results Based on Merged Data Sets from the Literature|
The globular cluster Messier 13 (NGC 6205) has been shown by a number ofdifferent studies to be chemically inhomogeneous with regard to elementsranging from C through Al. A database of λ3883 CN-band indices,carbon, oxygen, and sodium abundances for red giants in M13 has beencompiled and homogenized from a variety of literature sources. The dataare used to document the distribution of CN band strength among both redgiant branch (RGB) and asymptotic giant branch (AGB) stars brighter thanMV=+0.8, as well as the relationships between the CNinhomogeneities and the dispersions in other elements. The CNdistribution among the M13 RGB stars is bimodal, although a fewintermediate-CN red giants are present in the cluster. The AGB starsshow uniformly weak CN bands. The spread in CN band strength, onceempirically corrected for effective temperature and gravity differencesamong the red giants, is very well correlated with the sodium abundanceand is anticorrelated with oxygen. Above MV=+0.8, the carbonabundance of red giants decreases with increasing luminosity, as firstfound by N. Suntzeff. There is a rather modest anticorrelation betweenCN band strength and [C/Fe] among RGB stars of comparable magnitude. Theabundance patterns within M13 can be interpreted as a primordial (orpre-RGB phase) abundance spread, coupled with the later effects of deepmixing within the red giants. In the CN-weak giants and those CN-strongstars with [O/Fe]~-0.2+/-0.2 dex on the upper RGB, deep mixing seems tohave been limited largely to the dredge-up of C-->N-processedmaterial. By contrast, there are some M13 stars, most notably a group ofvery oxygen-deficient giants near the RGB tip, but perhaps also afraction of CN-strong giants with lesser oxygen depletions, in whichsome measure of O-->N-processed material also appears to have beenbrought to the surface.
|A Panchromatic Study of the Globular Cluster NGC 1904. I. The Blue Straggler Population|
By combining high-resolution (HST/WFPC2) and wide-field ground-based(2.2 m ESO/WFI) and space (GALEX) observations, we have collected amultiwavelength photometric database (ranging from the far-UV to thenear infrared) of the galactic globular cluster NGC 1904 (M79). Thesample covers the entire cluster extension, from the very centralregions up to the tidal radius. In the present paper, such a data set isused to study the BSS population and its radial distribution. A totalnumber of 39 bright (m218<=19.5) BSSs have been detected,and they have been found to be highly segregated in the cluster core. Nosignificant upturn in the BSS frequency has been observed in theoutskirts of NGC 1904, in contrast to other clusters (M3, 47 Tuc, NGC6752, M5) studied with the same technique. Such evidence, coupled withthe large radius of avoidance estimated for NGC 1904(ravoid~30 core radii), indicates that the vast majority ofthe cluster heavy stars (binaries) has already sunk to the core.Accordingly, extensive dynamical simulations suggest that BSSs formed bymass transfer activity in primordial binaries evolving in isolation inthe cluster outskirts represent only a negligible (0%-10%) fraction ofthe overall population.Based on observations with the NASA/ESA HST, obtained at the SpaceTelescope Science Institute, which is operated by AURA, Inc., under NASAcontract NAS5-26555. Also based on GALEX observations (program GI-056)and WFI observations collected at the European Southern Observatory, LaSilla, Chile, within the observing programs 62.L-0354 and 64.L-0439.
|A Double-Mode RR Lyrae Star with a Strong Fundamental-Mode Component|
NSVS 5222076, a 13th magnitude star in the Northern Sky VariabilitySurvey, was identified by L. Oaster as a possible new double-mode RRLyrae star. We confirm the double-mode nature of NSVS 5222076,supplementing the survey data with new V-band photometry. NSVS 5222076has a fundamental-mode period (P0) of 0.4940 days and afirst-overtone period (P1) of about 0.3668 days, giving aperiod ratio of P1/P0=0.743. In most double-modeRR Lyrae stars, the amplitude of the first-overtone mode's pulsation isgreater than that of the fundamental-mode pulsation. That is not truefor this star. Its fundamental-mode light curve has an amplitude twiceas large as that of the first-overtone mode, a ratio very rarely seeneven among the double-mode RR Lyrae stars that have relatively strongfundamental-mode pulsation. Data from the literature are used to discussthe location in the Petersen diagram of double-mode RR Lyrae starshaving strong fundamental-mode pulsation. Such stars tend to occurtoward the short-period end of the Petersen diagram, and NSVS 5222076 isno exception to this rule.
|The Multiplicity of the Subgiant Branch of ω Centauri: Evidence for Prolonged Star Formation|
We combine spectroscopic and photometric data for subgiant stars ofω Cen to extract results that neither data set could have providedon its own. GIRAFFE@VLT spectra of 80 stars at R=6400 give metallicitiesfor all of them and abundances of C, N, Ca, Ti, and Ba for a subset ofthem. The photometric data, which have unusually high accuracy, comefrom a ~10×10 arcmin2 mosaic of HST ACS images centeredon the cluster center and on multicolor images of a ~34×33arcmin2 field, taken with the WFI@ESO2.2m camera. Stars with[Fe/H]<-1.25 have a large magnitude spread on the flat part of theSGB. We interpret this as empirical evidence for an age spread, and fromtheoretical isochrones we derive a relative age for each star. Withinthe SGB region we identify four distinct stellar groups: (1) an old,metal-poor group ([Fe/H]~-1.7) (2) an old, metal-rich group([Fe/H]~-1.1) (3) a young (up to 4-5 Gyr younger than the old component)metal-poor group ([Fe/H]~-1.7) (4) a young, intermediate-metallicity([Fe/H]~-1.4) group, on average 1-2 Gyr younger than the old metal-poorpopulation, and with an age spread that we cannot properly quantify withthe present sample. In addition, many SGB stars are spread between theintermediate-metallicity and metal-rich branches. We tentatively proposeconnections between the SGB stars and both the multiple main-sequenceand the red giant branches. Finally, we discuss the implications of themultiple stellar populations on the formation and evolution of ωCen. The spread in age within each population establishes that theoriginal system must have had a composite nature.Based on FLAMES+GIRAFFE@VLT observations under the DDT program272.D-5065(A) and on observations with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by AURA, Inc., under NASA contract NAS 5-26555.
|Optical/near-infrared colours of early-type galaxies and constraints on their star formation histories|
We introduce and discuss the properties of a theoretical (B-K)(J-K)integrated colour diagram for single-age, single-metallicity stellarpopulations. We show how this combination of integrated colours is ableto largely disentangle the well-known age-metallicity degeneracy whenthe age of the population is greater than ~300Myr, and thus providesvaluable estimates of both age and metallicity of unresolved stellarsystems. We discuss in detail the effect on this colour-colour diagramof α-enhanced metal abundance ratios (typical of the oldestpopulations in the Galaxy), the presence of blue horizontal branch starsunaccounted for in the theoretical calibration and of statistical colourfluctuations in low-mass stellar systems. In the case of populationswith multiple stellar generations, the luminosity-weighted mean ageobtained from this diagram is shown to be heavily biased towards theyoungest stellar components. We then apply this method to several datasets for which optical and near-infrared photometry are available in theliterature. We find that Large Magellanic Cloud and M31 clusters havecolours which are consistent with the predictions of the models, butthese do not provide a sensitive test due to the fluctuations which arepredicted by our modelling of the Poisson statistics in such low-masssystems. For the two Local Group dwarf galaxies NGC 185 and 6822, themean ages derived from the integrated colours are consistent with thestar formation histories inferred independently from photometricobservations of their resolved stellar populations.The methods developed here are applied to samples of nearby early-typegalaxies with high-quality aperture photometry in the literature. Asample of bright field and Virgo cluster elliptical galaxies is found toexhibit a range of luminosity-weighted mean ages from 3 to 14Gyr, with amean of ~8Gyr, independent of environment, and mean metallicities at orjust above the solar value. Colour gradients are found in all of thegalaxies studied, in the sense that central regions are redder. Apartfrom two radio galaxies, where the extreme central colours are clearlydriven by the active galactic nucleus, and one galaxy which also shows aradial age gradient, these colour changes appear consistent withmetallicity changes at a constant mean age. Finally, aperture data forfive Virgo early-type dwarf galaxies show that these galaxies appear tobe shifted to lower mean metallicities and lower mean ages (range1-6Gyr) than their higher luminosity counterparts.
|Blazhko effect and double-mode RR Lyrae stars .|
There are two kinds of multi-periodic RR Lyrae stars. The Blazhko effectis a modulation of the primary light cycle that takes place on a typicaltimescale of tens of days. Double-mode RR Lyrae stars pulsatesimultaneously in the fundamental and first overtone radial modes. Thestate of our knowledge of these multi-periodic variables is reviewed.
|Photometry of the Globular Cluster NGC 5466: Red Giants and Blue Stragglers|
We present wide-field BVI photometry for about 11,500 stars in thelow-metallicity cluster NGC 5466. We have detected the red giant branchbump for the first time, although it is at least 0.2 mag fainter thanexpected relative to the turnoff. The number of red giants (relative tomain-sequence turnoff stars) is in excellent agreement with stellarmodels from the Yonsei-Yale and Teramo groups, and slightly highcompared to Victoria-Regina models. This adds to evidence that anabnormally large ratio of red giant to main-sequence stars is notcorrelated with cluster metallicity. We discuss theoretical predictionsfrom different research groups and find that the inclusion or exclusionof helium diffusion and strong limit Coulomb interactions may be partlyresponsible. We also examine indicators of dynamical history: the massfunction exponent and the blue straggler frequency. NGC 5466 has a veryshallow mass function, consistent with large mass loss and recentlydiscovered tidal tails. The blue straggler sample is significantly morecentrally concentrated than the HB or RGB stars. We see no evidence ofan upturn in the blue straggler frequency at large distances from thecenter. Dynamical friction timescales indicate that the stragglersshould be more concentrated if the cluster's present density structurehas existed for most of its history. NGC 5466 also has an unusually lowcentral density compared to clusters of similar luminosity. In spite ofthis, the specific frequency of blue stragglers puts it right on thefrequency-cluster MV relation observed for other clusters.
|The Blue Straggler Population of the Globular Cluster M5|
By combining high-resolution HST and wide-field ground-basedobservations, in ultraviolet and optical bands, we study the bluestraggler star (BSS) population of the galactic globular cluster M5 (NGC5904) from its very central regions up to its periphery. The BSSdistribution is highly peaked in the cluster center, decreases atintermediate radii and rises again outward. Such a bimodal distributionis similar to those previously observed in other globular clusters (M3,47 Tucanae, NGC 6752). As for these clusters, dynamical simulationssuggest that, while the majority of BSSs in M5 could be originated bystellar collisions, a significant fraction (20%-40%) of BSSs generatedby mass transfer processes in primordial binaries is required toreproduce the observed radial distribution. A candidate BSS has beendetected beyond the cluster tidal radius. If confirmed, this couldrepresent an interesting case of an ``evaporating'' BSS.Based on observations with the NASA/ESA HST, obtained at the SpaceTelescope Science Institute, which is operated by AURA, Inc., under NASAcontract NAS5-26555. Also based on WFI observations collected at theEuropean Southern Observatory, La Silla, Chile, within the observingprograms 62.L-0354 and 64.L-0439.
|On the mode switching of RRd stars in the globular cluster M3 .|
M3 is so far the only cluster in which RRd variable stars have beenreported to switch from one dominant mode to another while remaining RRdvariables. Investigating synthetic light curves sampled according to thepublished photometric data sets, we have concluded that theobservational material is not sufficient to decide whether any modeswitchings have happened or not. As it is demonstrated the previouslypublished mode switchings were - most likely - artifacts of the badsampling.
|V79 in the globular cluster Messier 3 returns to fundamental mode|
V79 was known as an RRab type star or fundamental mode pulsator with theperiod of about 0.483 day during the century after its discovery byS.I.Bailey in 1895. In 1996 it was unexpectedly found to pulsate indouble mode (J. Kaluzhny et al., 1998, MNRAS V.296, p.347). The firstovertone with the period of 0.359 day became the dominant mode. Theswitch of mode occured in April 1992. This phenomenon was unprecedentedfor the class of RR Lyrae stars.
|Completing the census of (bright) variable stars in galactic globular clusters .|
We present a long-term project aimed at completing the census of(bright) variable stars in Galactic globular clusters. While our mainaim is to obtain a reliable assessment of the populations of RR Lyraeand type II Cepheid stars in the Galactic globular cluster system, dueattention is also being paid to other types of variables, including SXPhoenicis stars, long-period variables, and eclipsing binaries.
|The Dynamical Implications of Multiple Stellar Formation Events in Galactic Globular Clusters|
Various Galactic globular clusters display abundance anomalies thataffect the morphology of their color-magnitude diagrams. In this paperwe consider the possibility of helium enhancement in the anomaloushorizontal branch of NGC 2808. We examine the dynamics of aself-enrichment scenario in which an initial generation of stars with atop-heavy initial mass function enriches the interstellar medium withhelium via the low-velocity ejecta of its asymptotic giant branch stars.This enriched medium then produces a second generation of stars whichare themselves helium-enriched. We use a direct N-body approach toperform five simulations and conclude that such two-generation clustersare both possible and would not differ significantly from theirsingle-generation counterparts on the basis of dynamics. We find,however, that the stellar populations of such clusters would differ fromsingle-generation clusters with a standard initial mass function and inparticular would be enhanced in white dwarf stars. We conclude, at leastfrom the standpoint of dynamics, that two-generation globular clustersare feasible.
|Multiple helium abundances in globular clusters stars: consequences for the horizontal branch and RR Lyrae|
Most inhomogeneities in the chemical composition of GC stars are due toprimordial enrichment. The model today most credited is that the windslost by high mass Asymptotic Giant Branch (AGB) stars, evolving during thefirst ≲ 200Myr of the Clusters life, directly form a second generation of stars with abundance anomalies. The best indirect hint towards this suggestion is the recognition that some peculiarities in the Horizontal Branch (HB) starsdistribution (blue tails, gaps, anomalous luminosity slope of the flatpart of the HB) can be attributed to the larger helium abundance in thematter, processed through Hot Bottom Burning, from which these stars areborn. The model has been reinforced by finding a peculiar main sequencedistribution in the cluster NGC 2808, which also has a bimodal HBdistribution and an extended blue tail: the excess of blue objects onthe main sequence has been interpreted as stars with very high helium.We remark that the RR Lyr distribution may be affected by the heliumspread, and this can be at the basis of the very long periods of theRRab variables of the metal rich clusters NGC 6388 and NGC 6441, longerthan for the very metal poor Oosterhoff II clusters. These periods implythat the RR Lyr are brighter than expected for their metallicities,consistent with a larger helium abundance.
|BVI Photometry and the Luminosity Functions of the Globular Cluster M92|
We present new BVI ground-based photometry and VI space-based photometryfor the globular cluster M92 (NGC 6341) and examine luminosity functionsin B, V, and I containing over 50,000 stars ranging from the tip of thered giant branch to several magnitudes below the main-sequence turnoff.Once corrected for completeness, the observed luminosity functions agreevery well with theoretical models and do not show stellar excesses inany region of the luminosity function. Using reduced-χ2fitting, the new M92 luminosity function is shown to be an excellentmatch to the previously published luminosity function for M30. Thesepoints combine to establish that the ``subgiant excess'' found inpreviously published luminosity functions of Galactic globular clustersis due to deficiencies in the stellar models used at that time. Usingup-to-date stellar models results in good agreement between observationsand theory. Several statistical methods are presented to best determinethe age of M92. These methods prove to be insensitive to the exactchoice of metallicity within the published range. Using [Fe/H]=-2.17 tomatch recent studies we find an age of 14.2+/-1.2 Gyr for the cluster.
|Discovery of the first SX Phoenicis-type pulsating component in the semi-detached Algol-type eclipsing binary in M71 .|
Through the time series CCD photometry to search for pulsating bluestraggler stars (BSSs), we discovered a pulsating feature with shortperiods about 0.03 day and small amplitude about 0.024 mag in theAlgol-type eclipsing binary QU Sge. The variable has an orbital periodof 3.790818 day and the primary minimum depth of Delta V = 1.333 mag.Eclipsing light curve solution shows that QU Sge has a semi-detachedbinary configuration with the secondary component totally filling itsRoche lobe.
|RR Lyrae variables in GGCs: distribution of periods and synthetic models. .|
We present some applications of our Synthetic Horizontal Branches (SHB)simulations, aimed to reproduce the peculiar period distributions of RRLyrae belonging to the Galactic Globular Clusters M3 and M5. We showsome evidence, supporting the importance of SHBs in obtaining parameterssuch as the mass distribution inside the instability strip.
|Where the Blue Stragglers Roam: Searching for a Link between Formation and Environment|
The formation of blue stragglers is still not completely understood,particularly the relationship between formation environment andmechanism. We use a large, homogeneous sample of blue stragglers in thecores of 57 globular clusters to investigate the relationships betweenblue straggler populations and their environments. We use a consistentdefinition of ``blue straggler'' based on position in thecolor-magnitude diagram and normalize the population relative to thenumber of red giant branch stars in the core. We find that thepreviously determined anticorrelation between blue straggler frequencyand total cluster mass is present in the purely core population. We findsome weak anticorrelations with central velocity dispersion and withhalf-mass relaxation time. The blue straggler frequency does not showany trend with any other cluster parameter. Even though collisions maybe expected to be a dominant blue straggler formation process inglobular cluster cores, we find no correlation between the frequency ofblue stragglers and the collision rate in the core. We also investigatedthe blue straggler luminosity function shape and found no relationshipbetween any cluster parameter and the distribution of blue stragglers inthe color-magnitude diagram. Our results are inconsistent with somerecent models of blue straggler formation that include collisionalformation mechanisms and may suggest that almost all observed bluestragglers are formed in binary systems.
|Blue Straggler Stars in Galactic Open Clusters and the Simple Stellar Population Model|
The presence of blue straggler stars (BSs) as secure members of Galacticopen clusters (OCs) poses a major challenge to the conventional pictureof simple stellar population (SSP) models. These are based on thestellar evolution theory of single stars, whereas the major formationmechanisms of BSs are all correlated with stellar interactions. We haveillustrated this in a previous study based on a small sample of old (age>=1 Gyr) Galactic OCs. However, for the purpose of demonstrating thecontributions of BSs to the conventional SSP models statistically andsystematically, a large database with sufficient coverage of age andmetallicity is definitely needed. The working sample now includes 100Galactic OCs with ages ranging from 0.1 to 10 Gyr. The contributions ofBSs to the integrated light of their host clusters are calculated on anindividual cluster basis. The general existence of BSs in our starcluster sample dramatically alters the predictions of conventional SSPmodels in terms of their integrated properties. Neglecting theconsequences of nonstandard evolutionary products, such as BSs, instellar populations, very large uncertainties can be made in analyzingtheir integrated spectral energy distributions at unresolvableconditions. The current work strongly suggests that when evolutionarypopulation synthesis technique is used to study the properties ofunresolved stellar populations in galaxies, the contributions of BSsshould be taken into account.
|Low mass stars|
Low-mass stars dominate the population of clusters and old galaxies andserve as important tools for understanding the history of galaxies. Iwill review some recent developments and open questions concerning ourunderstanding of these comparatively simple stellar objects.
|Different Characteristics of the Bright Branches of the Globular Clusters M15 and M92|
We carried out relatively wide-field BVI CCD photometric observations ofthe globular clusters M15 (NGC 7078) and M92 (NGC 6341) using the 1.8 mtelescope of the Bohyun Optical Astronomy Observatory. We presentcolor-magnitude diagrams (V vs. B-V, V vs. V-I, and V vs. B-I) of M15and M92. We found asymptotic giant branch (AGB) bumps atVbumpAGB=15.20+/-0.05 mag andVbumpAGB=14.50+/-0.05 mag for M15 and M92,respectively. We identified the red giant branch (RGB) bumps of the twoclusters. We have estimated the population ratios R and R2for M15 and M92 in two cases: when only normal horizontal-branch (HB)stars are used and when all the HB stars are used. We have compared theobserved RGB luminosity functions of M15 and M92 with the theoreticalRGB luminosity functions of Bergbusch & VandenBerg and found nosignificant ``extra stars'' in the comparisons. This implies that the HBmorphology difference between M15 and M92 is not certain due to theresults of deep mixing in the RGB sequence.
|Beyond the big Galaxy: the structure of the stellar system 1900 - 1952|
|The Abundance of Deuterium in the Warm Neutral Medium of the Lower Galactic Halo|
We use high-resolution ultraviolet spectra to obtain Milky Wayinterstellar column densities of H I, D I, O I, S II, Fe II, and P IItoward the QSO HE 0226-4110 in the Galactic directionl=253.4deg and b=-65.77deg. We obtainD/H=21+8-6 ppm from an analysis of the strongdamped Lyα line of H I and the weak higher Lyman series absorptionof D I. Correcting for a small amount of foreground contamination from Dand H in the Local Bubble we obtain D/H=22+8-6 forthe warm neutral medium of the lower Galactic halo. The medium sampledhas [O/H]=0.12+0.41-0.20 and[Fe/H]=-1.01+0.10-0.09. This suggests theabundances in the gas in the halo toward HE 0226-4110 are not affectedby the infall of low-metallicity gas and that the gas originates in thedisk and is elevated into the halo by energetic processes that erode butdo not totally destroy the dust grains. We compare our result tomeasured values of D/H in other astrophysical sites. The value wemeasure in the halo gas is consistent with the hypothesis that for manyGalactic disk lines of sight D is incorporated into dust. The highaverage value of D/H=23.1+/-2.4(1 σ) ppm measured along five sightlines through disk gas in the solar neighborhood is similar to D/H inthe lower Galactic halo. These disk and halo observations imply theabundance of deuterium in the Galaxy has only been reduced by a factorof 1.12+/-0.13 since its formation. In contrast, current galacticchemical evolution models predict the astration reduction factor shouldbe in the range from 1.39 to 1.83.
|The Nature of the Hot Stars in the Bulge of Globular Cluster NGC 6388|
The metal-rich bulge globular clusters NGC 6388 and NGC 6441 showdistinct blue horizontal-branch tails in their color-magnitude diagrams.They are thus strong cases of the well known Second Parameter Problem.In addition, the horizontal branches in these globular clusters show anupward tilt toward bluer colors, which cannot be explained by canonicalevolutionary models. We will discuss several scenarios which have beenproposed to explain these two features and present observations obtainedto test these scenarios.
|The Global Kinematics of the Globular Cluster M92|
We report the determination of high-accuracy radial velocities for 306members of the globular cluster M92 using the Hydra multiobjectspectrograph on the WIYN telescope. We have concentrated on starsoutside of the central region of the cluster, located up to 14.4' fromthe cluster center. Candidate members were selected for spectroscopybased on a photometric metallicity index determined from three-bandWashington photometry, also obtained with the WIYN telescope. The medianerror in the velocities is 0.35 km s-1. We find theheliocentric radial velocity of the cluster to be -121.2+/-0.3 kms-1. We have used an improved Bayesian analysis to determinethe velocity dispersion profile of M92. The most probable profile is acored power law with a scale radius of 2', a velocity dispersion at 1'of 6.3 km s-1, and an outer power law with a slope of -0.6.We have also reanalyzed the M15 radial velocities of Drukier et al. andfind that a pure power law with a 1' velocity dispersion of 8 kms-1 and a slope of -0.5 and the combination of a power lawwith a slope of -0.4 and a scale of 7.5 km s-1 inside 9' anda dispersion of 4 km s-1 outside are equally likely. In bothclusters there is evidence that the samples include escaping stars. Wepresent results from a GRAPE-based N-body simulation of an isolatedcluster that demonstrates this effect. We suggest additional tests todetermine the relative importance of tidal heating and stellar ejectionfor establishing the velocity field in globular cluster halos.
|Discovery of Carbon/Oxygen-depleted Blue Straggler Stars in 47 Tucanae: The Chemical Signature of a Mass Transfer Formation Process|
We use high-resolution spectra obtained with the ESO Very LargeTelescope to measure surface abundance patterns of 43 blue stragglerstars (BSSs) in 47 Tuc. We discovered that a subpopulation of BSSs showsa significant depletion of carbon and oxygen with respect to thedominant population. This evidence would suggest the presence of CNOburning products on the BSS surface coming from a deeply peeled parentstar, as expected in the case of a mass transfer process. This is thefirst detection of a chemical signature clearly pointing to a specificBSS formation process in a globular cluster.Based on observations collected at the ESO-VLT (Cerro Paranal, Chile)under program 072.D-0337.
|Na-O anticorrelation and horizontal branches. IV. Detection of He-rich and He-poor stellar populations in the globular cluster NGC 6218|
We used the multifiber spectrograph FLAMES on the ESO Very LargeTelescope UT2 to derive atmospheric parameters, metallicities andabundances of O and Na for 79 red giant stars in the Galactic globularcluster NGC 6218 (M 12). We analyzed stars in the magnitude range fromabout 1 mag below the bump to the tip of the Red Giant Branch. Theaverage metallicity we derive is [Fe/H]=-1.31± 0.004±0.028 dex (random and systematic errors, respectively), with a verysmall star-to-star scatter (rms=0.033 dex), from moderatelyhigh-resolution Giraffe spectra. This is the first extensivespectroscopic abundance analysis in this cluster. Our results indicatethat NGC 6218 is very homogeneous as far as heavy elements areconcerned. On the other hand, light elements involved in the well knownproton-capture reactions of H-burning at high temperature, such as O andNa, show large variations, anticorrelated with each other, at allluminosities along the red giant branch. The conclusion is that the Na-Oanticorrelation must be established in early times at the clusterformation. We interpret the variation of Na found near the RGB-bump asthe effect of two distinct populations having different bumpluminosities, as predicted for different He content. To our knowledge,NGC 6218 is the first GC where such a signature has beenspectroscopically detected, when combined with consistent andhomogeneous data obtained for NGC 6752 to gain in statisticalsignificance.Based on observations collected at ESO telescopes under programme073.D-0211. Full Tables 2, 3, and 5 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/939
|Dynamical Formation of Close Binaries in Globular Clusters: Cataclysmic Variables|
We answer the long-standing question of which production mechanism isresponsible for the cataclysmic variables (CVs) in globular clusters.Arguments have been given that range from mostly primordial presence toa significant contribution of later dynamical formation in close stellarencounters. We conclude, based on a thorough analysis of a homogeneousChandra data set, that the majority of CVs in dense globular clustershave a dynamical origin.
|[Fe/H] relations for c-type RR Lyrae variables based upon Fourier coefficients|
[Fe/H]-φ31-P relations are found for c-type RR Lyraestars in globular clusters. The relations are analogous to that found byJurcsik & Kovács for field ab-type RR Lyrae stars, where alonger period correlates with lower metallicity values for similarvalues of the Fourier coefficient φ31. The relationsobtained here are used to determine the metallicity of field c-type RRLyrae stars, those within ωCen, the Large Magellanic Cloud andtoward the galactic bulge. The results are found to compare favourablyto metallicity values obtained elsewhere.
|A Genuine Intermediate-Age Globular Cluster in M33|
We present deep integrated-light spectroscopy of nine M33 globularclusters taken with the Hectospec instrument at the MMT Observatory.Based on our spectroscopy and previous deep color-magnitude diagramsobtained with HST WFPC2, we present evidence for the presence of agenuine intermediate-age globular cluster in M33. The analysis of Lickline indices indicates that all globular clusters are metal-poor([Z/H]<~-1.0) and that cluster M33-C38 is ~5-8 Gyr younger than therest of the sample M33 star clusters. We find no evidence for apopulation of blue horizontal-branch stars in the CMD of M33-C38, whichrules out the possibility of an artificially young spectroscopic age dueto the presence of hot stars. We infer an initial mass of(0.8-1.2)×105 Msolar for M33-C38, whichimplies that intermediate-age clusters with masses similar to those ofGalactic globular clusters were able to form and survive in M33,although it is not yet clear with which dynamical component of M33-thindisk, thick disk, halo-the cluster is associated.Based on observations obtained with the Hectospec instrument at the MMTObservatory. The MMT Observatory is a joint venture of the SmithsonianInstitute and the University of Arizona. Also based on observations madewith the NASA/ESA Hubble Space Telescope, obtained from the data archiveat the Space Telescope Science Institute. STScI is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
|Why Haven't Loose Globular Clusters Collapsed Yet?|
We report on the discovery of a surprising observed correlation betweenthe slope of the low-mass stellar global mass function (GMF) of globularclusters (GCs) and their central concentration parameterc=log(rt/rc), i.e., the logarithmic ratio of tidaland core radii. This result is based on the analysis of a sample of 20Galactic GCs with solid GMF measurements from deep HST or VLT data. Allthe high-concentration clusters in the sample have a steep GMF, mostlikely reflecting their initial mass function. Conversely,low-concentration clusters tend to have a flatter GMF, implying thatthey have lost many stars via evaporation or tidal stripping. No GCs arefound with a flat GMF and high central concentration. This findingappears counterintuitive, since the same two-body relaxation mechanismthat causes stars to evaporate and the cluster to eventually dissolveshould also lead to higher central density and possibly core collapse.Therefore, more concentrated clusters should have lost proportionatelymore stars and have a shallower GMF than low-concentration clusters,contrary to what is observed. It is possible that severely depleted GCshave also undergone core collapse and have already recovered a normalradial density profile. It is, however, more likely that GCs with a flatGMF have a much denser and smaller core than that suggested by theirsurface brightness profile and may well be undergoing collapse atpresent. In either case, we may have so far seriously underestimated thenumber of post-core collapse clusters, and many may be lurking in theMilky Way.
|Mergers of Close Primordial Binaries|
We study the production of main-sequence mergers of tidally synchronizedprimordial short-period binaries. The principal ingredients of ourcalculation are the angular momentum loss rates inferred from thespin-down of open cluster stars and the distribution of binaryproperties in young open clusters. We compare our results with theexpected number of systems that experience mass transfer in thepost-main-sequence phases of evolution and compute the uncertainties inthe theoretical predictions. We estimate that main-sequence mergers canaccount for the observed number of single blue stragglers in M67.Applied to the blue straggler population, this implies that such mergersare responsible for about one-quarter of the population of halo bluemetal-poor stars and at least one-third of the blue stragglers in openclusters for systems older than 1 Gyr. The observed trends as a functionof age are consistent with a saturated angular momentum loss rate forrapidly rotating tidally synchronized systems. The predicted number ofblue stragglers from main-sequence mergers alone is comparable to thenumber observed in globular clusters, indicating that the net effect ofdynamical interactions in dense stellar environments is to reduce ratherthan increase the blue straggler population. A population of subturnoffmergers of order 3%-4% of the upper main sequence population is alsopredicted for stars older than 4 Gyr, which is roughly comparable to thesmall population of highly Li-depleted halo dwarfs. Other observationaltests are discussed.
|Integrated colours of Milky Way globular clusters and horizontal branch morphology|
Broadband colours are often used as metallicity proxies in the study ofextragalactic globular clusters. A common concern is the effect ofvariations in horizontal branch (HB) morphology - the second-parametereffect - on such colours. We have used U BV I, Washington, and DDOphotometry for a compilation of over 80 Milky Way globular clusters toaddress this question. Our method is to fit linear relations betweencolour and [Fe/H], and study the correlations between the residualsabout these fits and two quantitative measures of HB morphology. Whilethere is a significant HB effect seen in U-B, for the commonly usedcolours B-V, V-I, and C-T_1, the deviations from the baselinecolour-[Fe/H] relations are less strongly related to HB morphology.There may be weak signatures in B-V and C-T_1, but these are at thelimit of observational uncertainties. The results may favour the use ofB-I in studies of extragalactic globular clusters, especially when itshigh [Fe/H]-sensitivity is considered.
|Detection of a 60°-long Dwarf Galaxy Debris Stream|
We report on a 60°-long stream of stars, extending from Ursa Majorto Sextans, in the Sloan Digital Sky Survey. The stream is approximately2° wide and is clearly distinct from the northern tidal arm of theSagittarius dwarf galaxy. The apparent width of the stream indicates aprogenitor with a size and mass similar to that of a dwarf galaxy. Thestream is about 21 kpc distant and appears to be oriented almostperpendicular to our line of sight. The visible portion of the streamdoes not pass near any known dwarf galaxies, although we cannot rule outthat the stream may form the inner part of a known dwarf galaxy's orbit.The most likely explanation is that the stream constitutes the remainsof a dwarf galaxy that has been completely disrupted at some point inthe past. We also briefly report on the discovery of a diminutiveGalactic satellite that lies near the projected path of the new streambut is unlikely to be related to it.
|New catalogue of blue stragglers in open clusters|
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (220.127.116.11) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
|On the Primordial Scenario for Abundance Variations within Globular Clusters: The Isochrone Test|
Self-enrichment processes occurring in the early stages of a globularcluster lifetime are generally invoked to explain the observed CNONaMgAlabundance anticorrelations within individual Galactic globular clusters.We have tested, with fully consistent stellar evolution calculations,whether theoretical isochrones for stars born with the observedabundance anticorrelations satisfy the observational evidence thatobjects with different degrees of these anomalies lie on essentiallyidentical sequences in the color-magnitude diagram (CMD). To thispurpose, we have computed for the first time low-mass stellar models andisochrones with an initial metal mixture that includes the extremevalues of the observed abundance anticorrelations and varying initial Hemass fractions. Comparisons with ``normal'' α-enhanced isochronesand suitable Monte Carlo simulations that include photometric errorsshow that a significant broadening of the CMD sequences occurs only ifthe helium enhancement is extremely large (in this study, when Y=0.35)in the stars showing anomalous abundances. Stellar luminosity functionsup to the red giant branch tip are also very weakly affected, apartfrom-depending on the He content of the polluting material-the red giantbranch bump region. We also study the distribution of stars along thezero-age horizontal branch and derive general constraints on therelative location of objects with and without abundance anomalies alongthe observed horizontal branches of globular clusters.
|Computation of the Fourier parameters of RR Lyrae stars by template fitting|
Aims.Due to the importance of accurate Fourier parameters, we devise amethod that is more appropriate for deriving these parameters forlow-quality data than the traditional Fourier fitting. Methods: Basedon the accurate light curves of 248 fundamental mode RR Lyrae stars, wetest the power of a full-fetched implementation of the template methodin the computation of the Fourier decomposition. The applicability ofthe method is also demonstrated on data sets of filter passbandsdifferent from those of the template set. Results: We examine in moredetail the question of the estimation of Fourier-based iron abundance[Fe/H] and average brightness. We get, for example, for light curvessampled randomly in 30 data points with σ=0.03 mag observationalnoise, that optimized direct Fourier fits yield σ([Fe/H])=0.33,whereas the template fits result in σ([Fe/H])=0.18. Tests made onthe RR Lyrae database of the Large Magellanic Cloud of the OpticalGravitational Lensing Experiment support the applicability of the methodon real photometric time series. These tests also show that the dominantpart of error in estimating the average brightness comes from othersources, most probably from crowding effects, even for under-sampledlight curves.
|A New Milky Way Dwarf Satellite in Canes Venatici|
In this Letter, we announce the discovery of a new dwarf satellite ofthe Milky Way, located in the constellation Canes Venatici. It was foundas a stellar overdensity in the north Galactic cap using Sloan DigitalSky Survey Data Release 5 (SDSS DR5). The satellite's color-magnitudediagram shows a well-defined red giant branch as well as a horizontalbranch. As judged from the tip of the red giant branch, it lies at adistance of ~220 kpc. Based on the SDSS data, we estimate an absolutemagnitude of MV~-7.9, a central surface brightness ofμ0,V~28 mag arcsec-2, and a half-light radiusof ~8.5 arcmin (~550 pc at the measured distance). The outer regions ofCanes Venatici appear extended and distorted. The discovery of such afaint galaxy in proximity to the Milky Way strongly suggests that moresuch objects remain to be found.
|Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters|
We present a homogeneous set of stellar atmospheric parameters(Teff, logg, [Fe/H]) for MILES, a new spectral stellarlibrary covering the range λλ 3525-7500Å at2.3Å (FWHM) spectral resolution. The library consists of 985 starsspanning a large range in atmospheric parameters, from super-metal-rich,cool stars to hot, metal-poor stars. The spectral resolution, spectraltype coverage and number of stars represent a substantial improvementover previous libraries used in population synthesis models. Theatmospheric parameters that we present here are the result of aprevious, extensive compilation from the literature. In order toconstruct a homogeneous data set of atmospheric parameters we have takenthe sample of stars of Soubiran, Katz & Cayrel, which has very welldetermined fundamental parameters, as the standard reference system forour field stars, and have calibrated and bootstrapped the data fromother papers against it. The atmospheric parameters for our clusterstars have also been revised and updated according to recent metallicityscales, colour-temperature relations and improved set of isochrones.
|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.
|Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular Clusters|
We have mosaicked Two Micron All Sky Survey (2MASS) images to derivesurface brightness profiles in J, H, and Ks for 104 Galacticglobular clusters. We fit these with King profiles and show that thecore radii are identical to within the errors for each of these IRcolors and are identical to the core radii at V in essentially allcases. We derive integrated-light colors V-J, V-H, V-Ks, J-H,and J-Ks for these globular clusters. Each color shows areasonably tight relation between the dereddened colors and metallicity.Fits to these are given for each color. The IR - IR colors have verysmall errors, due largely to the all-sky photometric calibration of the2MASS survey, while the V-IR colors have substantially largeruncertainties. We find fairly good agreement with measurements ofintegrated-light colors for a smaller sample of Galactic globularclusters by M. Aaronson, M. Malkan, and D. Kleinmann from 1977. Ourresults provide a calibration for the integrated light of distantsingle-burst old stellar populations from very low to solarmetallicities. A comparison of our dereddened measured colors withpredictions from several models of the integrated light of single-burstold populations shows good agreement in the low-metallicity domain forV-Ks colors but also shows an offset at a fixed [Fe/H] of~0.1 mag in J-Ks, which we ascribe to photometric systemtransformation issues. Some of the models fail to reproduce the behaviorof the integrated-light colors of the Galactic globular clusters nearsolar metallicity.
|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.
|Colour-colour diagrams and extragalactic globular cluster ages. Systematic uncertainties using the (V - K) - (V - I) diagram|
Context: Age and metallicity estimates for extragalactic globularclusters, from integrated colour-colour diagrams, are examined. Aims:We investigate biases in cluster ages and [Fe/H] estimated from the(V-K)-(V-I) diagram, arising from inconsistent Horizontal Branchmorphology, metal mixture, treatment of core convection between observedclusters and the theoretical colour grid employed for age andmetallicity determinations. We also study the role played by statisticalfluctuations of the observed colours, caused by the low total mass oftypical globulars. Methods: Synthetic samples of globular clustersystems are created, by means of Monte-Carlo techniques. Each sampleaccounts for a different possible source of bias, among the onesaddressed in this investigation. Cumulative age and [Fe/H] distributionsare then retrieved by comparisons with a reference theoreticalcolour-colour grid, and analyzed. Results: Horizontal Branch morphologyis potentially the largest source of uncertainty. A single-age systemharbouring a large fraction of clusters with an HB morphologysystematically bluer than the one accounted for in the theoreticalcolour grid, can simulate a bimodal population with an age difference aslarge as ~8 Gyr. When only the redder clusters are considered, thisuncertainty is almost negligible, unless there is an extreme mass lossalong the Red Giant Branch phase. The metal mixture affects mainly theredder clusters; the effect of colour fluctuations becomes negligiblefor the redder clusters, or when the integrated MV isbrighter than ~-8.5 mag. The treatment of core convection is relevantfor ages below ~4 Gyr. The retrieved cumulative [Fe/H] distributions areoverall only mildly affected. Colour fluctuations and convective coreextension have the largest effect. When 1σ photometric errorsreach 0.10 mag, all biases found in our analysis are erased, and bimodalage populations with age differences of up to ~8 Gyr go undetected. Theuse of both (U-I)-(V-K) and (V-I)-(V-K) diagrams may help disclosing thepresence of blue HB stars unaccounted for in the theoretical colourcalibration.
|Theoretical Uncertainties in Red Giant Branch Evolution: The Red Giant Branch Bump|
A Monte Carlo simulation exploring uncertainties in standard stellarevolution theory on the red giant branch of metal-poor globular clustershas been conducted. Confidence limits are derived on the absolute V-bandmagnitude of the bump in the red giant branch luminosity function(MV,b) and the excess number of stars in the bump,Rb. The analysis takes into account uncertainties in theprimordial helium abundance, abundance of α-capture elements,radiative and conductive opacities, nuclear reaction rates, neutrinoenergy losses, the treatments of diffusion and convection, the surfaceboundary conditions, and color transformations. The uncertainty intheoretical values for the red giant bump magnitude varies withmetallicity between +0.13 and -0.12 mag at [Fe/H]=-2.4 and between +0.23and -0.21 mag at [Fe/H]=-1.0. The dominant sources of uncertainty arethe abundance of the α-capture elements , the mixing length, andthe low-temperature opacities. The theoretical values of MV,bare in good agreement with observations. The uncertainty in thetheoretical value of Rb is +/-0.01 at all metallicitiesstudied. The dominant sources of uncertainty are the abundance of theα-capture elements, the mixing length, and the high-temperatureopacities. The median value of Rb varies from 0.44 at[Fe/H]=-2.4 to 0.50 at [Fe/H]=-1.0. These theoretical values forRb are in agreement with observations.
|Variable Stars in the Newly Discovered Milky Way Satellite in Bootes|
We present V, I light curves for 12 variable stars identified in thenewly discovered satellite of the Milky Way in the Bootes constellation.Our sample includes 11 RR Lyrae stars (five first overtone, fivefundamental mode, and one double-mode pulsator) and one long-periodvariable close to the galaxy red giant branch tip. The RR Lyrae starstrace very well the average V luminosity of the galaxy horizontalbranch, leading to a true distance modulus for the galaxy ofμ0=19.11+/-0.08 mag for an assumed metal abundance of[Fe/H]=-2.5 and for E(B-V)=0.02 mag. Average periods are=0.69 days and =0.37 days forab- and c-type RR Lyrae stars, respectively, making Bootes the secondpure Oosterhoff type II (OoII) dSph after Ursa Minor. The location ofthe double-mode RR Lyrae (RRd) in the Petersen diagram is consistentwith RRd stars in OoII clusters and corresponds to an intrinsicluminosity of logL/logLsolar=1.72 (for Z=10-4 andM=0.80 Msolar) according to nonlinear convective pulsationmodels.Based on data collected at the 1.52 m telescope of the INAF-OsservatorioAstronomico di Bologna, Loiano, Italy, at the INAF-Telescopio NazionaleGalileo, La Palma, Canary Islands, and at the 2.3 m telescope at theWyoming Infrared Observatory (WIRO) at Mount Jelm, WY.
|From Canonical to Enhanced Extra Mixing in Low-Mass Red Giants: Tidally Locked Binaries|
Stellar models that incorporate simple diffusion or shear-induced mixingare used to describe canonical extra mixing in low-mass red giants oflow and solar metallicity. These models are able to simultaneouslyexplain the observed Li and CN abundance changes along the upper redgiant branch (RGB) in field low-metallicity stars and match photometry,rotation, and 12C/13C ratios for stars in the oldopen cluster M67. The shear mixing model requires that main-sequence(MS) progenitors of upper RGB stars possessed rapidly rotating radiativecores and that specific angular momentum was conserved in each of theirmass shells during their evolution. We surmise that solar-type starswill not experience canonical extra mixing on the RGB because their moreefficient MS spin-down resulted in solid-body rotation, as revealed byhelioseismological data for the Sun. Thus, RGB stars in the old,high-metallicity cluster NGC 6791 should show no evidence for mixing intheir 12C/13C ratios. We develop the idea thatcanonical extra mixing in a giant component of a binary system may beswitched to its enhanced mode with much faster and somewhat deepermixing as a result of the giant's tidal spin-up. This scenario canexplain photometric and composition peculiarities of RS CVn binaries.The tidally enforced enhanced extra mixing might contribute to thestar-to-star abundance variations of O, Na, and Al in globular clusters.This idea may be tested with observations of12C/13C ratios and CN abundances in RS CVnbinaries.
|The Chemical Evolution of Helium in Globular Clusters: Implications for the Self-Pollution Scenario|
We investigate the suggestion that there are stellar populations in someglobular clusters with enhanced helium (Y~0.28-0.40) compared to theprimordial value. We assume that a previous generation of massiveasymptotic giant branch (AGB) stars have polluted the cluster. Twoindependent sets of AGB yields are used to follow the evolution ofhelium and CNO using a Salpeter initial mass function (IMF) and twotop-heavy IMFs. In no case are we able to produce the postulated largeY~0.35 without violating the observational constraint that the CNOcontent is nearly constant.
|New Metallicities of RR Lyrae Stars in ω Centauri: Evidence for a Non-He-enhanced Metal-intermediate Population|
We present new spectroscopic metal abundances for 74 RR Lyrae (RRL)stars in ω Cen obtained with FLAMES. The well-known metallicityspread is visible among the RRL variables. The metal-intermediate (MI)RRL stars ([Fe/H]~-1.2) are fainter than the bulk of the dominantmetal-poor population ([Fe/H]~-1.7), in good agreement with thecorresponding zero-age horizontal-branch models with cosmological heliumabundance Y=0.246. This result conflicts with the hypothesis that theprogenitors of the MI RRL stars correspond to the anomalous bluemain-sequence stars, which share a similar metallicity but whoseproperties are currently explained by assuming for them a large heliumenhancement. Therefore, in this scenario, the coexistence within thecluster of two different populations with similar metallicities([Fe/H]~-1.2) and different helium abundances has to be considered.Based on FLAMES/GIRAFFE observations collected with the Very LargeTelescope at the European Southern Observatory, Cerro Paranal, Chile,within the observing programs 74.B-0170(A).
|The Orbits of 48 Globular Clusters in a Milky Way-like Barred Galaxy|
The effect of a barred potential (such as the one of the Milky Way) onthe Galactic orbits of 48 globular clusters for which absolute propermotions are known is studied. The orbital characteristics are comparedwith those obtained for the case of an axisymmetric Galactic potential.Tidal radii are computed and discussed for both the better knownaxisymmetric case and that including a bar. The destruction rates due tobulge and disk shocking are calculated and compared in both Galacticpotentials.
|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.
|The Detection of a 45° Tidal Stream Associated with the Globular Cluster NGC 5466|
We report on the detection in Sloan Digital Sky Survey data of a 45°tidal stream of stars, extending from Bootes to Ursa Major, which weassociate with the halo globular cluster NGC 5466. Using an optimalcontrast, matched-filter technique, we find a long, almost linearstellar stream with an average width of 1.4d. The stream is an order ofmagnitude more tenuous than the stream associated with Palomar 5. Thestream's orientation on the sky is consistent to a greater or lesserextent with existing proper-motion measurements for the cluster.
|A Moderate Sample Size, Multielement Analysis of the Globular Cluster M12 (NGC 6218)|
We present chemical abundances of several proton-capture, α-,Fe-peak, and neutron-capture elements and radial velocities for 21 redgiant branch (RGB) and asymptotic giant branch members of the Galacticglobular cluster M12. Abundances are based on equivalent widthmeasurements and synthetic spectral analyses of moderate-resolutionspectra (R~15,000) obtained with the 3.5 m WIYN telescope and Hydramultifiber spectrograph. The stars observed range from the RGB tip(M0v=-2.47) down to about 0.50 mag above the levelof the horizontal branch (M0v=+0.11). Ourspectroscopic analysis suggests that M12 is a moderately metal-poorcluster with [Fe/H]=-1.54 (σ=0.09). While the Na abundancesexhibit a range of 0.90 dex, Mg and Al abundances are enhanced by 0.37and 0.54 dex and are nearly constant at all RGB luminosities, incontrast to the blue horizontal-branch cluster M13. The α- andFe-peak elements indicate that M12 has undergone a similar chemicalenrichment history to that of globular clusters and field stars ofcomparable metallicity, with <[α/Fe]>=+0.33 (σ=0.11).M12 also appears to be slightly r-process-rich, with<[Eu/Ba,La]>=+0.22 (σ=0.18).
|Empirical color transformations between SDSS photometry and other photometric systems|
Aims.We present empirical color transformations between the SloanDigital Sky Survey (SDSS) ugriz photometry and the Johnson-Cousins UBVRIsystem and Becker's RGU system, respectively. Owing to the magnitude ofdata that is becoming available in the SDSS photometric system it isparticularly important to be able to convert between this new system andtraditional photometric systems. Unlike earlier publishedtransformations we based our calculations on stars actually measured bythe SDSS with the SDSS 2.5-m telescope. The photometric database of theSDSS provides in a sense a single-epoch set of "tertiary standards"covering more than one quarter of the sky. Our transformations shouldfacilitate their use to easily and reliably derive the correspondingapproximate Johnson-Cousins or RGU magnitudes. Methods: .The SDSSsurvey covers a number of areas that were previously established asstandard fields in the Johnson-Cousins system, in particular, fieldsestablished by Landolt and by Stetson. We used these overlapping fieldsto create well-photometered star samples on which our calculatedtransformations are based. For the RGU photometry we used fieldsobserved in the framework of the new Basel high-latitude field starsurvey. Results: .We calculated empirical color transformationsbetween SDSS photometry and Johnson-Cousins UBVRI and Becker's RGUsystem. For all transformations we found linear relations to besufficient. Furthermore we showed that the transformations between theJohnson-Cousins and the SDSS system have a slight dependence onmetallicity.
|The Pure Noncollisional Blue Straggler Population in the Giant Stellar System ω Centauri|
We have used high spatial resolution data from the Hubble SpaceTelescope (HST) and wide-field ground-based observations to search forblue straggler stars (BSSs) over the entire radial extent of the largestellar system ω Centauri. We have detected the largest populationof BSSs ever observed in any stellar system. Even though the sample isrestricted to the brightest portion of the BSS sequence, more than 300candidates have been identified. BSSs are thought to be produced by theevolution of binary systems (formed either by stellar collisions or massexchange in binary stars). Since systems like Galactic globular clusters(GGCs) and ω Cen evolve dynamically on timescales significantlyshorter than their ages, binaries should have settled toward the center,showing a more concentrated radial distribution than the ordinary, lessmassive single stars. Indeed, in all GGCs that have been surveyed forBSSs, the BSS distribution is peaked at the center. Conversely, inω Cen we find that the BSSs share the same radial distribution asthe adopted reference populations. This is the cleanest evidence everfound that such a stellar system is not fully relaxed even in thecentral region. We further argue that the absence of centralconcentration in the BSS distribution rules out a collisional origin.Thus, the ω Cen BSSs are the purest and largest population ofnoncollisional BSSs ever observed. Our results allow the first empiricalquantitative estimate of the production rate of BSSs via this channel.BSSs in ω Cen may represent the best local template for modelingthe BSS populations in distant galaxies where they cannot beindividually observed.Based on observations with the NASA/ESA HST, obtained at the SpaceTelescope Science Institute, which is operated by AURA, Inc., under NASAcontract NAS5-26555. Also based on WFI observations collected at theEuropean Southern Observatory, La Silla, Chile, within the observingprograms 62.L-0354 and 64.L-0439.
|The Discovery of Tidal Tails around the Globular Cluster NGC 5466|
We report the discovery of tidal tails around the high-latitude Galacticglobular cluster NGC 5466 in Sloan Digital Sky Survey data. Neuralnetworks are used to reconstruct the probability distribution of clusterstars in ugriz space. The tails are clearly visible once extragalacticcontaminants and field stars have been eliminated. They extend ~4°on the sky, corresponding to ~1 kpc in projected length. The orientationof the tails is in good agreement with the cluster's Galactic orbit, asjudged from the proper-motion data.
|The radial distribution of blue straggler stars and the nature of their progenitors|
The origin of blue straggler stars (BSS) in globular clusters (GCs) isstill not fully understood: they can form from stellar collisions, orthrough mass transfer in isolated, primordial binaries (PBs). In thispaper we use the radial distribution of BSS observed in four GCs (M3,47Tuc, NGC6752 and ωCen) to investigate which formation processprevails. We find that both channels co-exist in all the considered GCs.The fraction of mass-transfer (collisional) BSS with respect to thetotal number of BSS is around ~0.4-0.5 (~0.5-0.6) in M3, 47Tuc andNGC6752. The case of ωCen is peculiar with an underproduction ofcollisional BSS. The relative lack of collisional BSS in ωCen canbe understood if mass segregation has not yet driven to the core asizeable number of PBs, which dominate stellar collisions through three-and four-body processes. The spatial distribution of BSS provides stronghints to their origin: the BSS in the cluster outskirts form almostexclusively from mass transfer in PBs, whereas the BSS found close tothe cluster core most likely have a collisional origin.
|A Method for Deriving Accurate Gas-Phase Abundances for the Multiphase Interstellar Galactic Halo|
We describe a new method for accurately determining total gas-phaseabundances for the Galactic halo interstellar medium with minimalionization uncertainties. For sight lines toward globular clusterscontaining both ultraviolet-bright stars and radio pulsars, it ispossible to measure column densities of H I and several ionizationstates of selected metals using ultraviolet absorption line measurementsand of H II using radio dispersion measurements. By measuring theionized hydrogen column, we minimize ionization uncertainties thatplague abundance measurements of Galactic halo gas. We apply this methodfor the first time to the sight line toward the globular cluster Messier3 [(l,b)=(42.2d,+78.7d), d=10.2 kpc, z=10.0 kpc] using Far UltravioletSpectroscopic Explorer and Hubble Space Telescope ultravioletspectroscopy of the post-asymptotic giant branch star von Zeipel 1128and radio observations by Ransom et al. of recently discoveredmillisecond pulsars. The fraction of hydrogen associated with ionizedgas along this sight line is 45%+/-5%, with the warm (T~104K) and hot (T>~105 K) ionized phases present in roughly a5:1 ratio. This is the highest measured fraction of ionized hydrogenalong a high-latitude pulsar sight line. We derive total gas-phaseabundances logN(S)/N(H)=-4.87+/-0.03 and logN(Fe)/N(H)=-5.27+/-0.05. Ourderived sulfur abundance is in excellent agreement with recent solarsystem determinations of Asplund, Grevesse, & Sauval. However, it is-0.14 dex below the solar system abundance typically adopted in studiesof the interstellar medium. The iron abundance is ~-0.7 dex below thesolar system abundance, consistent with the significant incorporation ofiron into interstellar grains. Abundance estimates derived by simplycomparing S II and Fe II to H I are +0.17 and +0.11 dex higher,respectively, than the abundance estimates derived from our refinedapproach. Ionization corrections to the gas-phase abundances measured inthe standard way are, therefore, significant compared with themeasurement uncertainties along this sight line. The systematicuncertainties associated with the uncertain contribution to the electroncolumn density from ionized helium could raise these abundances by<~+0.03 dex (+7%). Uncertainties in the amount of very hot gas(T~106 K) along the line of sight could also affect thesedeterminations.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 NAS5-26555.
|The RR Lyrae period-K-luminosity relation for globular clusters: an observational approach|
The period-metallicity-K-band luminosity (PLK) relation forRR Lyrae stars in 15 Galactic globular clusters and in the LargeMagellanic Cloud (LMC) globular cluster Reticulum has been derived. Itis based on accurate near-infrared (K) photometry combined withTwo-Micron All-Sky Survey (2MASS) and other literature data. ThePLK relation has been calibrated and compared with theprevious empirical and theoretical determinations in literature. Thezero point of the absolute calibration has been obtained from the Kmagnitude of RR Lyr whose distance modulus has been measured viatrigonometric parallax with Hubble Space Telescope (HST). Using thisrelation, we obtain a distance modulus to the LMC of (m - M)0= 18.54 +/- 0.15 mag, in good agreement with recent determinations basedon the analysis of Cepheid variable stars.Based on observations collected at the European Southern Observatorywithin the observing programs 49.5-0021, 51.5-0024, 59.E-0340,64.N-0038, 68.D-0287 and at the Telescopio Nazionale Galileo.E-mail: firstname.lastname@example.org (AS)
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