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|The Infrared Ca II Triplet as Metallicity Indicator|
From observations of almost 500 red giant branch stars in 29 Galacticopen and globular clusters, we have investigated the behavior of theinfrared Ca II triplet (8498, 8542, and 8662 Å) in the age range13 Gyr<=age<=0.25 Gyr and the metallicity range-2.2<=[Fe/H]<=+0.47. These are the widest ranges of ages andmetallicities in which the behavior of the Ca II triplet lines has beeninvestigated in a homogeneous way. We report the first empirical studyof the variation of the Ca II triplet lines' strength, for givenmetallicities, with respect to luminosity. We find that the sequencedefined by each cluster in the luminosity-ΣCa plane is not exactlylinear. However, when only stars in a small magnitude interval areobserved, the sequences can be considered as linear. We have studied theCa II triplet lines on three metallicity scales. While a linearcorrelation between the reduced equivalent width(W'V or W'I) and metallicityis found in the Carretta & Gratton and Kraft & Ivans scales, asecond-order term needs to be added when the Zinn & West scale isadopted. We investigate the role of age from the wide range of agescovered by our sample. We find that age has a weak influence on thefinal relationship. Finally, the relationship derived here is used toestimate the metallicities of three poorly studied open clusters:Berkeley 39, Trumpler 5, and Collinder 110. For the latter, themetallicity derived here is the first spectroscopic estimate available.
|Wide-Field Survey around Local Group Dwarf Spheroidal Galaxy Leo II: Spatial Distribution of Stellar Content|
We carried out a wide-field V, I imaging survey of the Local Group dwarfspheroidal galaxy Leo II using the Subaru Prime Focus Camera on the 8.2m Subaru Telescope. The survey covered an area of 26.67×26.67arcmin2, far beyond the tidal radius of Leo II (8.63'), downto the limiting magnitude of V~=26, which is roughly 1 mag deeper thanthe turnoff point of the main-sequence stars of Leo II. Radial numberdensity profiles of bright and faint red giant branch (RGB) stars werefound to change their slopes at around the tidal radius, and extendbeyond the tidal radius with shallower slopes. A smoothed surfacebrightness map of Leo II suggests the existence of a small substructure(4×2.5 arcmin2, 270×170 pc 2 inphysical size) of globular cluster luminosity beyond the tidal radius.We investigated the properties of the stellar population by means of acolor-magnitude diagram. The horizontal branch (HB) morphology indexshows a radial gradient in which red HB stars are more concentrated thanblue HB stars, which is common to many Local Group dwarf spheroidalgalaxies. The color distribution of RGB stars around the mean RGBsequence shows a larger dispersion at the center than in the outskirts,indicating a mixture of stellar populations at the center and a morehomogeneous population in the outskirts. Based on the age estimationusing subgiant branch stars, we found that although the major starformation took place ~8 Gyr ago, a considerable stellar populationyounger than 8 Gyr is found at the center; such a younger population isinsignificant in the outskirts. The following star formation history issuggested for Leo II. Star-forming activity occurred more than ~8 Gyrago throughout the galaxy at a modest star formation rate. Thestar-forming region gradually shrank from the outside toward the center,and star-forming activity finally dropped to ~0 by ~4 Gyr ago, exceptfor the center, where a small population younger than 4 Gyr is present.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
|Timing the Eccentric Binary Millisecond Pulsar in NGC 1851|
We have used the Green Bank Telescope to observe the millisecond pulsarPSR J0514-4002A on 43 occasions spread over 2 years. This 5 ms pulsar islocated in the globular cluster NGC 1851; it belongs to a binary systemand has a highly eccentric (e=0.888) orbit. We have obtained aphase-coherent timing solution for this object, including very preciseposition, spin, and orbital parameters. The pulsar is located 4.6"(about 1.3 core radii) from the center of the cluster and is likely tolie on its more distant half. The nondetection of eclipses at superiorconjunction can be used, given the peculiar geometry of this system, torule out the possibility of an extended companion. We have measured therate of advance of periastron for this binary system to beω˙=0.01289(4)deg yr-1, which if duecompletely to general relativity implies a total system mass of2.453(14) Msolar. Given the known mass function, the pulsarmass has to be <1.5 Msolar, and the mass of the companionhas to be >0.96 Msolar, implying that it is a heavy whitedwarf. The 350 MHz flux density of this pulsar varies between 0.2 and1.4 mJy; the origin of these variations is not known.
|Kinematic Decoupling of Globular Clusters with the Extended Horizontal Branch|
About 25% of the Milky Way globular clusters (GCs) exhibit unusuallyextended color distribution of stars in the core helium-burninghorizontal-branch (HB) phase. This phenomenon is now best understood asdue to the presence of helium-enhanced second-generation subpopulations,which has raised the possibility that these peculiar GCs might have aunique origin. Here we show that these GCs with extended HB are clearlydistinct from other normal GCs in kinematics and mass. The GCs withextended HB are more massive than normal GCs and are dominated by randommotion with no correlation between kinematics and metallicity.Surprisingly, however, when they are excluded, most normal GCs in theinner halo show clear signs of dissipational collapse that apparentlyled to the formation of the disk. Normal GCs in the outer halo sharetheir kinematic properties with the extended HB GCs, which is consistentwith the accretion origin. Our result further suggests heterogeneousorigins of GCs, and we anticipate this to be a starting point for moredetailed investigations of Milky Way formation, including early mergers,collapse, and later accretion.
|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.
|The ACS Survey of Galactic Globular Clusters. I. Overview and Clusters without Previous Hubble Space Telescope Photometry|
We present the first results of a large Advanced Camera for Surveys(ACS) survey of Galactic globular clusters. This Hubble Space Telescope(HST) Treasury project is designed to obtain photometry with S/N(signal-to-noise ratio) >~10 for main-sequence stars with masses>~0.2 Msolar in a sample of globulars using the ACS WideField Channel. Here we focus on clusters without previous HST imagingdata. These include NGC 5466, NGC 6779, NGC 5053, NGC 6144, Palomar 2,E3, Lyngå 7, Palomar 1, and NGC 6366. Our color-magnitude diagrams(CMDs) extend reliably from the horizontal branch to as much as 7 magfainter than the main-sequence turnoff and represent the deepest CMDspublished to date for these clusters. Using fiducial sequences for threestandard clusters (M92, NGC 6752, and 47 Tuc) with well-knownmetallicities and distances, we perform main-sequence fitting on thetarget clusters in order to obtain estimates of their distances andreddenings. These comparisons, along with fitting the cluster mainsequences to theoretical isochrones, yield ages for the target clusters.We find that the majority of the clusters have ages that are consistentwith the standard clusters at their metallicities. The exceptions areE3, which appears ~2 Gyr younger than 47 Tuc, and Pal 1, which could beas much as 8 Gyr younger than 47 Tuc.Based on observations with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated byAURA, Inc., under NASA contract NAS5-26555, under program GO-10775 (PI:A. Sarajedini).
|Six new candidate ultracompact X-ray binaries|
Ultracompact X-ray binaries (UCXBs) appear able to sustain accretiononto the compact accretor at rates lower than in wider X-ray binaries.This may be understood by the smaller accretion disks in UCXBs: a lowerX-ray luminosity suffices to keep a disk completely ionized throughirradiation and, thus, keep the viscosity at a sufficiently high levelto allow effective transport of matter to the compact object. We employthis distinguishing factor on data from RXTE and BeppoSAX to identifysix new candidate UCXBs, thus increasing the population by one quarter.The candidates are drawn from the population of persistently accretingand type-I X-ray bursting low-mass X-ray binaries. The X-ray burstsestablish the low-mass X-ray binary nature and provide a handle on theaccretion rate. We find that the low accretion rates are supported bythe long burst recurrence times and the hard X-ray spectra of thepersistent emission as derived from the 2nd INTEGRAL catalog of softγ-ray sources. We discuss the peculiar light curves of some newUCXB candidates.
|Inside the whale: the structure and dynamics of the isolated Cetus dwarf spheroidal|
This paper presents a study of the Cetus dwarf, an isolated dwarf galaxywithin the Local Group. A matched-filter analysis of the INT/WFC imagingof this system reveals no evidence for significant tidal debris thatcould have been torn off the galaxy, bolstering the hypothesis thatCetus has never significantly interacted with either the Milky Way orM31. Additionally, Keck/Deimos spectroscopic observations identify thisgalaxy as a distinct kinematic population possessing a systematicvelocity of -87 +/- 2kms-1 and with a velocity dispersion of17 +/- 2kms-1 while tentative, these data also suggest thatCetus possesses a moderate rotational velocity of ~8kms-1.The population is confirmed to be relatively metal-poor, consistent with[Fe/H] ~ -1.9, and, assuming virial equilibrium, implies that the Cetusdwarf galaxy possesses a mass-to-light ratio of ~70. It appears,therefore, that Cetus may represent a primordial dwarf galaxy, retainingthe kinematic and structural properties lost by other members of thedwarf population of the Local Group in their interactions with the largegalaxies. An analysis of Cetus' orbit through the Local Group indicatesthat it is at apocentre; taken in conjunction with the general dwarfpopulation, this shows the mass of the Local Group to be >~2 ×1012Msolar.
|[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.
|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.
|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.
|Newly Discovered Variable Stars in the Globular Cluster NGC 1261|
We report the discovery of several RR Lyrae and SX Phe variable stars inthe innermost part of the globular cluster NGC 1261. Preliminarypulsation periods, location and B-band light curves in relative fluxunits are provided for all these stars.
|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.
|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.
|Properties of RR Lyrae stars in the inner regions of the Large Magellanic Cloud. II. The extended sample|
Context: .All galaxies that have been adequately examined so far haveshown an extended stellar halo. Aims: .To search for such a haloin the LMC we have obtained low-resolution spectra for 100 LMC RR Lyraestars, of which 87 are in the field and 13 in the clusters NGC 1835 andNGC 2019. Methods: .We measured radial velocities for 87 LMC RRLyrae stars, and metallicities for 78 RR Lyrae stars, nearly triplingthe previous sample. These targets are located in 10 fields covering awide range of distances, out to 2.5 degrees from the center of the LMC. Results: .Our main result is that the mean velocity dispersionfor the LMC RR Lyrae stars is σRV = 50 ± 2 kms-1. This quantity does not appear to vary with distance fromthe LMC center. The metallicity shows a Gaussian distribution, with mean[Fe/H] = -1.53 ± 0.02 dex, and dispersion σ_[Fe/H] =0.20± 0.02 dex in the Harris metallicity scale, confirming thatthey represent a very homogeneous metal-poor population. There is nodependence between the kinematics and metallicity of the field RR Lyraestar population. Conclusions: .Using good quality low-resolutionspectra from FORS1, FORS2 and GEMINI-GMOS we have found that field RRLyrae stars in the LMC show a large velocity dispersion and that thisindicate the presence of old and metal-poor stellar halo. All theevidence so far for the halo, however, is from the spectroscopy of theinner LMC regions, similar to the inner flattened halo in our Galaxy.Further study is necessary to confirm this important result.
|Color-Magnitude Diagrams of Resolved Stars in Virgo Cluster Dwarf Galaxies|
The Advanced Camera for Surveys (ACS) on the Hubble Space Telescope(HST) has been used to image two fields in the core of the Virgo Clusterthat contain a number of dwarf elliptical galaxies. The combined F555Wand F814W images have resolved red giant stars in these galaxies, downto 1 mag below the giant branch tip. Two of the galaxies were targetedbecause of their extremely low central surface brightnesses(Bo>27.0) thus, the successful resolution into starsconfirms the existence of such tenuous galaxies. Red giant stars werealso found that are not ostensibly associated with any galaxy.Color-magnitude diagrams in V and I have been derived for the fivedwarfs, as well as the halo of a nearby spiral galaxy and theintracluster stars in the two fields. These diagrams were used to derivedistances and metallicities via the magnitude of the red giant branchtip, and the mean color of the giant branch. The mean abundances ofstars in the dwarfs range from -1.2<[Fe/H]<-2.4, and fall alongthe relation between galaxy luminosity and metallicity found for LocalGroup and M81 group dwarf elliptical galaxies. [Fe/H] does not appear tobe well-correlated with galaxy surface brightness, as the two extremelylow surface brightness galaxies do not have extreme abundances. The meandistance modulus of the six Virgo galaxies is 31.0+/-0.05, or 16.1+/-0.4Mpc, whereas that for the intracluster stars in those fields is31.2+/-0.09 (17.4+/-0.7 Mpc).
|Global fitting of globular cluster age indicators|
Context: .Stellar models and the methods for the age determinations ofglobular clusters are still in need of improvement. Aims: .Weattempt to obtain a more objective method of age determination based oncluster diagrams, avoiding the introduction of biases due to thepreference of one single age indicator. Methods: .We compute newstellar evolutionary tracks and derive the dependence of age indicatingpoints along the tracks and isochrone - such as the turn-off or bumplocation - as a function of age and metallicity. The same criticalpoints are identified in the colour-magnitude diagrams of globularclusters from a homogeneous database. Several age indicators are thenfitted simultaneously, and the overall best-fitting isochrone isselected to determine the cluster age. We also determine thegoodness-of-fit for different sets of indicators to estimate theconfidence level of our results. Results: .We find that ourisochrones provide no acceptable fit for all age indicators. Inparticular, the location of the bump and the brightness of the tip ofthe red giant branch are problematic. On the other hand, the turn-offregion is very well reproduced, and restricting the method to indicatorsdepending on it results in trustworthy ages. Using an alternative set ofisochrones improves the situation, but neither leads to an acceptableglobal fit. Conclusions: .We conclude that evolutionary tracks oflow-mass metal-poor stars are far from reproducing all aspects ofglobular cluster colour-magnitude diagrams and that the determination ofcluster ages still depends on the favourite method or indicator chosen.
|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.
|Multivariate analysis of globular cluster horizontal branch morphology: searching for the second parameter|
Aims.The interpretation of globular cluster horizontal branch (HB)morphology is a classical problem that can significantly blur ourunderstanding of stellar populations. Methods: .We present a newmultivariate analysis connecting the effective temperature extent of theHB with other cluster parameters. The work is based on Hubble SpaceTelescope photometry of 54 Galactic globular clusters. Results: .The present study reveals the important role of the total mass of theglobular cluster on its HB morphology. More massive clusters tend tohave HBs more extended to higher temperatures. For a set of three inputvariables including the temperature extension of the HB, [Fe/H] and M_V,the first two eigenvectors account for 90% of the total samplevariance. Conclusions: . Possible effects of clusterself-pollution on HB morphology, stronger in more massive clusters,could explain the results derived here.
|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.
|DDO 44 and UGC 4998: Distances, Metallicities, and Star Formation Histories|
We have obtained deep HST imaging of two candidate dwarf systems in thenearby M81 Group, DDO 44 and UGC 4998. Both are isolated, low surfacebrightness systems, but with likely very different star formationhistories based on their mean colors and color fluctuations. Thecolor-magnitude diagrams of these galaxies have been used to estimatetheir distances using the tip of the red giant branch (RGB), and theirmean metallicities from the colors of the RGB. For DDO 44 we find adistance of D=3.01+/-0.18 Mpc-confirming it to be a member of the M81Group-and a mean metallicity of [Fe/H]=-1.54+/-0.14. We also find fromthe properties and numbers of a population of luminous asymptotic giantbranch (AGB) stars that about 20% of the luminous population of DDO 44consists of ``intermediate-age'' stars (with ages between about 2 and 8Gyr) that give rise to the observed AGB. There is no difference in thespatial distribution of the RGB and AGB stars in this galaxy. For UGC4998 we derive a metallicity of [Fe/H]=-1.58+/-0.21 and a distance ofD=8.24+/-0.43 Mpc. The latter puts UGC 4998 well behind the M81 Group,confirming results from previous measurements. We find this galaxy tocontain a population of young (<50 Myr old) stars, and there isevidence of older RGB stars. The young stars are considerably morecentrally concentrated than the older stars. The current star formationrate in UGC 4998 is ~9×10-4 Msolaryr-1 over the entire galaxy, or ~5×10-4Msolar yr-1 kpc-2. Given its H I mass(1×107 Msolar), this galaxy can continue toform stars at this rate for another ~11 Gyr.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. These observations are associated with proposalGO-8137.
|The Puzzling Properties of the Helium White Dwarf Orbiting the Millisecond Pulsar PSR J1911-5958A in NGC 6752|
We have used phase-resolved high-resolution images and low-resolutionspectra taken at the ESO Very Large Telescope to study the properties ofthe low-mass helium white dwarf companion to the millisecond pulsar PSRJ1911-5958A (COM J1911-5958A), in the halo of the Galactic globularcluster NGC 6752. The radial velocity curve confirms that COMJ1911-5958A is orbiting the pulsar and allows us to derive a systemicvelocity of the binary system nicely in agreement with that of NGC 6752.This strongly indicates that the system is a member of the cluster,despite its very offset position (~74 core radii) with respect to thecore. Constraints on the orbital inclination (>~70°) and pulsarmass (1.2-1.5 Msolar) are derived from the mass ratioMPSR/MCOM=7.49+/-0.64 and photometric propertiesof COM J1911-5958A. The light curve in the B band shows two phases ofunequal brightening (Δmag~0.3 and 0.2, respectively) located closeto quadratures and superimposed on an almost steady baseline emission:this feature is quite surprising and needs to be further investigated.Based on observations collected at the ESO Very Large Telescope (CerroParanal, Chile), programs 071.D-0232A and 073.D-0067A, and on dataretrieved from the ESO Science Archive Facility.
|Globular cluster system and Milky Way properties revisited|
Aims.Updated data of the 153 Galactic globular clusters are used toreaddress fundamental parameters of the Milky Way, such as the distanceof the Sun to the Galactic centre, the bulge and halo structuralparameters, and cluster destruction rates. Methods: .We build areduced sample that has been decontaminated of all the clusters youngerthan 10 Gyr and of those with retrograde orbits and/or evidence ofrelation to dwarf galaxies. The reduced sample contains 116 globularclusters that are tested for whether they were formed in the primordialcollapse. Results: .The 33 metal-rich globular clusters([Fe/H]≥-0.75) of the reduced sample basically extend to the Solarcircle and are distributed over a region with the projected axial-ratiostypical of an oblate spheroidal, Δ x:Δ y:Δz≈1.0:0.9:0.4. Those outside this region appear to be related toaccretion. The 81 metal-poor globular clusters span a nearly sphericalregion of axial-ratios ≈1.0:1.0:0.8 extending from the central partsto the outer halo, although several clusters in the external regionstill require detailed studies to unravel their origin as accretion orcollapse. A new estimate of the Sun's distance to the Galactic centre,based on the symmetries of the spatial distribution of 116 globularclusters, is provided with a considerably smaller uncertainty than inprevious determinations using globular clusters, R_O=7.2±0.3 kpc.The metal-rich and metal-poor radial-density distributions flatten forR_GC≤2 kpc and are represented well over the full Galactocentricdistance range both by a power-law with a core-like term andSérsic's law; at large distances they fall off as R-3.9. Conclusions: .Both metallicity components appearto have a common origin that is different from that of the dark matterhalo. Structural similarities between the metal-rich and metal-poorradial distributions and the stellar halo are consistent with a scenariowhere part of the reduced sample was formed in the primordial collapseand part was accreted in an early period of merging. This applies to thebulge as well, suggesting an early merger affecting the central parts ofthe Galaxy. The present decontamination procedure is not sensitive toall accretions (especially prograde) during the first Gyr, since theobserved radial density profiles still preserve traces of the earliestmerger(s). We estimate that the present globular cluster populationcorresponds to ≤23±6% of the original one. The fact that thevolume-density radial distributions of the metal-rich and metal-poorglobular clusters of the reduced sample follow both a core-likepower-law, and Sérsic's law indicates that we are dealing withspheroidal subsystems at all scales.
|Galactic Globular Clusters with Luminous X-Ray Binaries|
Luminous X-ray binaries (LMXBs; >1034 ergs s-1)have a neutron star or black hole primary, and in globular clusters,most of these close binaries are expected to have evolved from widerbinaries through dynamical interactions with other stars. We attempt tofind a predictor of this formation rate that is representative of theinitial properties of globular clusters rather than of the highlyevolved core quantities. Models indicate the half-light quantities bestreflect the initial conditions, so we examine whether the associateddynamical interaction rate, proportional toL1.5/r2.5h, is useful for understandingthe presence of luminous LMXBs in the Galactic globular cluster system.We find that while LMXB clusters with large values ofL1.5/r2.5h preferentially host LMXBs,the systems must also have half-mass relaxation times belowth,relax~109 yr. This relaxation time effectprobably occurs because several relaxation times are required to modifybinary separations, a timescale that must be shorter than cluster ages.The likelihood of finding an LMXB cluster is enhanced if the cluster ismetal-rich and if it is close to the bulge region. The dependence onmetallicity is most likely either due to differing initial massfunctions at the high-mass end or because bulge systems evolve morerapidly from tidal interactions with the bulge. This approach can beused to investigate globular cluster systems in external galaxies, wherecore properties are unresolved.
|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.
|Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452|
We study the red giant populations of two dE galaxies, AM 1339-445 andAM 1343-452, with the aim of investigating the number and luminosity ofany upper asymptotic giant branch (AGB) stars present. The galaxies aremembers of the Centaurus A group (D ≈ 3.8 Mpc) and are classified asoutlying (R ≈ 350 kpc) satellites of Cen A. The analysis is based onnear-IR photometry for individual red giant stars, derived from imagesobtained with ISAAC on the VLT. The photometry, along with optical dataderived from WFPC2 images retrieved from the HST science archive, enableus to investigate the stellar populations of the dEs in the vicinity ofthe red giant branch (RGB) tip. In both systems we find stars above theRGB tip, which we interpret as intermediate-age upper-AGB stars. Thepresence of such stars is indicative of extended star formation in thesedEs similar to that seen in many, but not all, dEs in the Local Group.For AM 1339-445, the brightest of the upper-AGB stars haveMbol ≈-4.5 while those in AM 1343-452 have Mbol≈ -4.8 mag. These luminosities suggest ages of approximately 6.5± 1 and 4 ± 1 Gyr as estimates for the epoch of the lastepisode of significant star formation in these systems. In both casesthe number of upper-AGB stars suggests that ~15% of the total stellarpopulation is in the form of intermediate-age stars, considerably lessthan is the case for outlying dE satellites of the Milky Way such asFornax and Leo I.
|RR Lyrae-based calibration of the Globular Cluster Luminosity Function|
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
|The Victoria-Regina Stellar Models: Evolutionary Tracks and Isochrones for a Wide Range in Mass and Metallicity that Allow for Empirically Constrained Amounts of Convective Core Overshooting|
Seventy-two grids of stellar evolutionary tracks, along with the meansto generate isochrones and luminosity/color functions from them, arepresented in this investigation. Sixty of them extend (and encompass)the sets of models reported by VandenBerg et al. for 17 [Fe/H] valuesfrom -2.31 to -0.30 and α-element abundances corresponding to[α/Fe]=0.0, 0.3, and 0.6 (at each iron abundance) to the solarmetallicity and to sufficiently high masses (up to ~2.2Msolar) that isochrones may be computed for ages as low as 1Gyr. The remaining grids contain tracks for masses from 0.4 to 4.0Msolar and 12 [Fe/H] values between -0.60 and +0.49 (assumingsolar metal-to-hydrogen number abundance ratios): in this case,isochrones may be calculated down to ~0.2 Gyr. The extent of convectivecore overshooting has been modeled using a parameterized version of theRoxburgh criterion, in which the value of the free parameter at a givenmass and its dependence on mass have been determined from analyses ofbinary star data and the observed color-magnitude diagrams for severalopen clusters. Because the calculations reported herein satisfy manyempirical constraints, they should provide useful probes into theproperties of both simple and complex stellar populations.All of the model grids may be obtained from the Canadian Astronomy DataCenter(http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/cvo/community/VictoriaReginaModels/).Included in this archive are (1) the interpolation software (FORTRAN 77)to produce isochrones, isochrone probability functions, luminosityfunctions, and color functions, along with instructions on how toimplement and use the software, (2) BVRI (VandenBerg & Clem 2003)and uvby (Clem et al. 2004) color-temperature relations, and (3)zero-age horizontal branch loci for all of the chemical compositionsconsidered.
|Hot Populations in M87 Globular Clusters|
To explore the production of UV-bright stars in old, metal-richpopulations like those in elliptical galaxies, we have obtained HubbleSpace Telescope (HST) Space Telescope Imaging Spectrograph far- andnear-UV photometry of globular clusters (GCs) in four fields in thegiant elliptical (gE) galaxy M87. To a limit of mFUV~25 wedetect a total of 66 GCs in common with the deep HST optical-band studyof Kundu et al. Despite strong overlap in V- and I-band properties, theM87 GCs have UV-optical properties that are distinct from clusters inthe Milky Way and in M31. M87 clusters, especially metal-poor ones,produce larger hot horizontal-branch populations than do Milky Wayanalogs. In color plots including the near-UV band, the M87 clustersappear to represent an extension of the Milky Way sequence. Cluster massis probably not a factor in these distinctions. The most metal-rich M87GCs in our sample are near solar metallicity and overlap the local Egalaxy sample in estimated Mg2 line indices. Nonetheless, theclusters produce much more UV light at a given Mg2, being upto 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do notappear to represent a transition between Milky Way-type clusters and Egalaxies. The differences are in the correct sense if the clusters aresignificantly older than the E galaxies.Comparisons with Galactic open clusters indicate that the hot stars lieon the extreme horizontal branch, rather than being blue stragglers, andthat the extreme horizontal branch becomes well populated for ages>~5 Gyr. Existing model grids for clusters do not match theobservations well, due to poorly understood giant branch mass loss orperhaps high helium abundances. We find that 41 of our UV detectionshave no optical-band counterparts. Most appear to be UV-brightbackground galaxies seen through M87. Eleven near-UV variable sourcesdetected at only one epoch in the central field are probably classicalnovae. Two recurrent variable sources have no obvious explanation butcould be related to activity in the relativistic jet.
|Color functions of stellar systems|
Model calculations of the photometric evolution of rather dense stellarsystems, such as globular clusters, are presented. On“luminosity-effective temperature” diagrams of thesesystems, low-mass stars are concentrated near the minimum and maximumtemperatures for a given luminosity and are deficient in theintermediate region. This sort of double-peaked distribution of thestars can be avoided in open models with ejection of excess metals intothe surrounding medium. The distributions of the stars with respect toeffective temperature on a “ luminosity-effectivetemperature” diagram are sensitive to the history of starformation in the system and to possible time variations in the initialmass function. In open systems with a single-peak distribution function,the asymmetry in the distribution varies over wide limits with the lowerbound for the initial mass function and this can be used to establishwhether the first generations of stars might have been more massive thanin the present epoch.
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