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|The tidal tails of NGC 5466|
The study of substructure in the stellar halo of the Milky Way has madea lot of progress in recent years, especially with the advent of surveyslike the Sloan Digital Sky Survey (SDSS). Here, we study the newlydiscovered tidal tails of the Galactic globular cluster NGC 5466. Bymeans of numerical simulations, we reproduce the shape, direction andsurface density of the tidal tails, as well as the structural andkinematical properties of the present-day NGC 5466. Although its tailsare very extended in SDSS data (>~45°), NGC 5466 is only losingmass slowly at the present epoch and so can survive for probably afurther Hubble time. The effects of tides at perigalacticon anddisc-crossing are the dominant causes of the slow dissolution of NGC5466, accounting for >~60 per cent of the mass loss over the courseof its evolution. The morphology of the tails provides a constraint onthe proper motion - the observationally determined proper motion has tobe refined (within the stated error margin) to match the location of thetidal tails.
|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.
|A δ Scuti Distance to the Large Magellanic Cloud|
We present results from a well-studied δ Scuti star discovered inthe Large Magellanic Cloud (LMC). The absolute magnitude of the variablewas determined from the period-luminosity (P-L) relation for Galacticδ Scuti stars and from theoretical modeling of the observed B,V,Ilight curves with nonlinear pulsation models. The two methods givedistance moduli for the LMC of 18.46+/-0.19 and 18.48+/-0.15,respectively, for a consistent value of the stellar reddening ofE(B-V)=0.08+/-0.02. We have also analyzed 24 δ Scuti candidatesdiscovered in the OGLE II survey of the LMC, and seven variablesidentified in the open cluster LW 55 and in the galaxy disk by Kaluznyand coworkers . We find that the LMC δ Scuti stars define a P-Lrelation whose slope is very similar to that defined by the Galacticδ Scuti variables, and yield a distance modulus for the LMC of18.50+/-0.22 mag. We compare the results obtained from the δ Scutivariables with those derived from the LMC RR Lyrae stars and Cepheids.The corresponding distance moduli are as follows: δ Scuti stars,18.48+/-0.02 mag (standard deviation of the weighted average of thethree above solutions); RR Lyrae stars, 18.49+/-0.06 mag; and Cepheids,18.53+/-0.02 mag. We have assumed an average color excess ofE(B-V)=0.08+/-0.02 mag for both δ Scuti stars and Cepheids. Withinthe observational uncertainties, the three groups of pulsating starsyield very similar distance moduli. These moduli are all consistent withthe ``long'' astronomical distance scale for the Large Magellanic Cloud.Based on data collected at the European Southern Observatory, proposalnumbers 62.N-0802 and 66.A-0485.
|A Wide-Field Kinematic Survey for Tidal Tails around Five Globular Clusters|
Using the AAOmega instrument of the Anglo-Australian Telescope, we haveobtained medium-resolution near-infrared spectra of 10,500 stars in2° fields centered on the galactic globular clusters 47 Tuc, NGC288, M12, M30, and M55. Radial velocities and equivalent widths of theinfrared Ca II triplet lines have been determined to constrain clustermembership, which in turn has been used to study the angular extent ofthe clusters. From the analysis of 140-1000 member stars in eachcluster, we do not find extended structures that go beyond the tidalradii. For three cluster we estimate a 1% upper limit of extratidal redgiant branch stars. We detect systemic rotation in 47 Tuc and M55.
|Tidal Tails around Globular Clusters: Are They a Good Tracer of Cluster Orbits?|
We present the results of detailed N-body simulations of clusters movingin a realistic Milky Way (MW) potential. The strong interaction with thebulge and the disk of the Galaxy leads to the formation of tidal tails,emanating from opposite sides of the cluster. Some characteristicfeatures in the morphology and orientation of these streams arerecognized and interpreted. The tails have a complex morphology,particularly when the cluster approaches its apogalacticon, showingmultiple ``arms'' in remarkable similarity to the structures observedaround NGC 288 and Willman 1. Actually, the tails are generally goodtracers of the cluster path quite far from the cluster center (>7-8tidal radii), while on the smaller scale they are mainly pointing in thedirection of the Galaxy center. In particular, the orientation of theinner part of the tails is highly correlated with the cluster orbitalphase and the local orbital angular acceleration. This implies that, ingeneral, the orbital path cannot be estimated directly from theorientation of the tails, unless a sufficient large field around thecluster is available.
|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).
|An Orphan in the ``Field of Streams''|
We use Sloan Digital Sky Survey Data Release 5 photometry andspectroscopy to study a tidal stream that extends over ~50° in thenorth Galactic cap. From the analysis of the path of the stream and thecolors and magnitudes of its stars, the stream is~20+7-5 kpc away at its nearest detection (thecelestial equator). We detect a distance gradient: the stream is fartheraway from us at higher declination. The contents of the stream are madeup from a predominantly old and metal-poor population that is similar tothe globular clusters M13 and M92. The integrated absolute magnitude ofthe stream stars is estimated to be Mr~-7.5. There istentative evidence for a velocity signature, with the stream moving at~-40 km s-1 at low declinations and ~+100 km s-1at high declinations. The stream lies on the same great circle asComplex A, a roughly linear association of H I high-velocity cloudsstretching over ~30° on the sky, and as Ursa Major II, a recentlydiscovered dwarf spheroidal galaxy. Lying close to the same great circleare a number of anomalous, young, and metal-poor globular clusters,including Palomar 1 and Ruprecht 106.
|The anomalous Cepheid XZ Ceti|
Aims. XZ Ceti is the only known anomalous Cepheid in the Galactic field.Being the nearest and brightest such variable star, a detailed study ofXZ Ceti may shed light on the behaviour of anomalous Cepheids whoserepresentatives have been mostly detected in external galaxies. Methods: CCD photometric and radial velocity observations have beenobtained. The actual period and amplitude of pulsation were determinedby Fourier analysis. The long timescale behaviour of the pulsationperiod was studied by the method of the O-C diagram using the archivalHarvard photographic plates and published photometric data. Results: XZ Ceti differs from the ordinary classical Cepheids in severalrespects. Its most peculiar feature is cycle-to-cycle variability of thelight curve. The radial velocity phase curve is not stable either. Thepulsation period is subjected to strong changes on various timescales,including a very short one. The ratio of amplitudes determined from thephotometric and radial velocity observations indicates that this Cepheidperforms an overtone pulsation, in accord with the other known anomalousCepheid in our Galaxy, BL Boo (V19 in the globular cluster NGC 5466). Conclusions: . Continued observations are necessary to study thedeviations from regularity to determine their timescale, as well as toconfirm the binarity of XZ Ceti and to study its role in the observedpeculiar behaviour.
|The ghosts of galaxies past.|
|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.
|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.
|Discovery of a Tidal Stream Extending from NGC 5053|
Here we report the discovery of a 6° tidal stream related to NGC5053 using the Sloan Digital Sky Survey data. A matched-filter analysishas been employed to optimize the contrast between the cluster andbackground noise. The stream of tidal debris associated with NGC 5053corresponds to a projected length of approximately 1.7 kpc at thecluster's distance. The tidal detection is also seen in photometric dataobtained at McDonald Observatory. Our result adds further observationalevidence that low-concentration clusters are more likely to exhibitstellar tidal extensions.
|Substructure in Tidal Streams: Tributaries in the Anticenter Stream|
We report on the detection in Sloan Digital Sky Survey data of at leastthree, roughly parallel components in a 65°-long, stellar streamcomplex previously identified with the Monoceros Ring. The three-streamcomplex varies in width from 4° to 6° along its length andappears to be made up of two or more narrow substreams as well as abroader, diffuse component. The width and complexity of the streamindicate that the progenitor was likely a dwarf galaxy of significantsize and mass. The stream is 8.9 kpc distant and is oriented almostperpendicularly to our line of sight. The visible portion of the streamdoes not pass near any known dwarf galaxies, and a preliminary orbitdoes not point to any viable progenitor candidates. Orbits for thenarrower substreams can be modeled with velocity offsets from the broadcomponent of ~8 km s-1. We suggest that the broad componentis likely to be the remains of a dwarf galaxy, while the narrowerstreams constitute the remnants of dynamically distinct components thatmay have included a native population of globular clusters. While thecolor of the main-sequence turnoff is not unlike that for the MonocerosRing, neither the visible stream nor any reasonable projection of itsorbit passes through Monoceros or Canis Major, and we conclude that thisstream is probably unrelated to the overdensities found in theseregions.
|The period-luminosity relation for type II Cepheids in globular clusters|
We report the result of our near-infrared observations (JHKs)for type II Cepheids (including possible RV Tau stars) in galacticglobular clusters. We detected variations of 46 variables in 26 clusters(10 new discoveries in seven clusters) and present their light curves.Their periods range from 1.2 d to over 80 d. They show a well-definedperiod-luminosity relation at each wavelength. Two type II Cepheids inNGC 6441 also obey the relation if we assume the horizontal branch starsin NGC 6441 are as bright as those in metal-poor globular clusters inspite of the high metallicity of the cluster. This result supports thehigh luminosity which has been suggested for the RR Lyr variables inthis cluster. The period-luminosity relation can be reproduced using thepulsation equation assuming that all the stars have the same mass.Cluster RR Lyr variables were found to lie on an extrapolation of theperiod-luminosity relation. These results provide important constraintson the parameters of the variable stars.Using Two Micron All-Sky Survey (2MASS) data, we show that the type IICepheids in the Large Magellanic Cloud (LMC) fit our period-luminosityrelation within the expected scatter at the shorter periods. However, atlong periods (P > 40 d, i.e. in the RV Tau star range) the LMC fieldvariables are brighter by about one magnitude than those of similarperiods in galactic globular clusters. The long-period cluster starsalso differ from both these LMC stars and galactic field RV Tau stars ina colour-colour diagram. The reasons for these differences arediscussed.
|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.
|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.
|Atmospheres, Chemical Compositions, and Evolutionary Histories of Very Metal-Poor Red Horizontal-Branch Stars in the Galactic Field and in NGC 7078 (M15)|
We have conducted spectrum analyses of 24 field metal-poor([Fe/H]<-2) red horizontal-branch (RHB) stars identified in the HKobjective-prism survey and 6 such stars in the globular cluster M15,based on high-quality spectra (R~40,000, S/N~100) obtained with theMagellan Inamori Kyocera Echelle spectrograph at the Clay 6.5 mtelescope at Las Campanas Observatory. The atmospheric parameters of theRHB stars provide interesting bridges between turnoff stars of similartemperature and red giant branch (RGB) stars of similar gravity, andthey permit investigations of abundance trends [X/Fe] versus [Fe/H] in arelatively unexplored region of the temperature-gravity plane. We findthat the Teff, logg, vt, and [Fe/H] valuesdetermined from our spectra are consistent with expectations fromliterature spectroscopic studies of other evolved metal-poor stellarclasses. We show that the RHB stars have abundance distributions thatare consistent with typical halo stars of similar metallicities. Thephotometric and spectroscopic gravities of the M15 stars differ byamounts that grow with declining temperature. We use a regressionderived from these differences to calculate photometric gravities forthe field RHB stars. Then we use the locations of the field RHB starsamong the evolutionary tracks of Cassisi et al. in the logg versuslogTeff plane to estimate their masses and lifetimes as RHBstars. We use these lifetimes to estimate the size of the metal-poor HBpopulation from which they arise. Then, using counts of HB and RGB starsin metal-poor globular clusters, we conclude that the number ofmetal-poor RGB stars at high latitudes (|b|>30deg)brighter than V=15 exceeds those identified in extant objective-prismsurveys by more than an order of magnitude. Finally, we deduce theeffective temperature of the fundamental red edge of the metal-poor RRLyrae instability strip, logTeff(FRE)=3.80+/-0.01, from theinterface between the temperature distributions of metal-poor field RHBstars and the RR Lyrae stars of similar [Fe/H] in five metal-poorglobular clusters.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center, California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation. This paper includes data gatheredwith the 6.5 m Magellan telescopes located at Las Campanas Observatory,Chile.
|Physical parameters of RR Lyrae stars in the globular cluster NGC 5466: The Oosterhoff dichotomy|
The highlights of CCD photometry of RR Lyrae stars in NGC 5466 arepresented. The image subtraction method (Bramich et al. 2005) was used.The Fourier light curve decomposition, lead to an average[Fe/H]=-1.88+/-0.3 and a distance of 14.8+/-0.4 kpc for the cluster andto other physical parameters of astrophysical relevance for individualRR Lyrae stars. When a comparison of the mean physical parameters inglobular clusters, analysed by the same technique, is performed, clearinsights of the stellar evolution on the Horizontal Branch are obtained.The origin of the Oosterhoff dichotomy is clearly of evolutionarynature, being age the determining factor.
|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.
|The Field of Streams: Sagittarius and Its Siblings|
We use Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) u, g, r, i,z photometry to study Milky Way halo substructure in the area around thenorth Galactic cap. A simple color cut (g-r<0.4) reveals the tidalstream of the Sagittarius dwarf spheroidal galaxy, as well as a numberof other stellar structures in the field. Two branches (A and B) of theSagittarius stream are clearly visible in an RGB composite image createdfrom three magnitude slices, and there is also evidence for a still moredistant wrap behind the A branch. A comparison of these data withnumerical models suggests that the shape of the Galactic dark halo isclose to spherical.
|A 22° Tidal Tail for Palomar 5|
Using Data Release 4 of the Sloan Digital Sky Survey, we have applied anoptimal-contrast, matched-filter technique to trace the trailing tidaltail of the globular cluster Palomar 5 to a distance of 18.5d from thecenter of the cluster. This more than doubles the total known length ofthe tail to some 22° on the sky. Based on a simple model of theGalaxy, we find that the stream's orientation on the sky is consistentat the 1.7 σ level with existing proper-motion measurements. Wefind that a spherical Galactic halo is adequate to model the stream overits currently known length, and we are able to place new constraints onthe current space motion of the cluster.
|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.
|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.
|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.
|Central energy equipartition in multimass models of globular clusters|
In the construction of multimass King-Michie models of globularclusters, an approximated central energy equipartition between stars ofdifferent mass is usually imposed by scaling the velocity parameter ofeach mass class inversely with the stellar mass, as if the distributionfunction were isothermal. In this paper, this `isothermal approximation'has been checked and its consequences on the model parameters studied bya comparison with models including central energy equipartitioncorrectly. It is found that, under the isothermal approximation, the`temperatures' of a pair of components can differ to a non-negligibleamount for low concentration distributions. It is also found that, ingeneral, this approximation leads to a significantly reduced masssegregation in comparison with that given under the exact energyequipartition at the centre. As a representative example, an isotropicthree-component model fitting a given projected surface brightness andline-of-sight velocity dispersion profiles is discussed. In thisexample, the isothermal approximation gives a cluster envelope much moreconcentrated (central dimensionless potential W= 3.3) than under thetrue equipartition (W= 5.9 × 10-2), as well as a highermass function logarithmic slope. As a consequence, the inferred totalmass (and then the global mass-to-light ratio) is a factor of 1.4 timeslower than the correct value and the amount of mass in heavy darkremnants is 3.3 times smaller. Under energy equipartition, the fate ofstars having a mass below a certain limit is to escape from the system.This limit is derived as a function of the mass and W of the componentof giant and turn-off stars.
|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.
|Galactic Orbits of Globular Clusters in a Barred Galaxy|
We study the effect of a bar in the galactic orbits of forty-fiveglobular clusters whose absolute proper motions are known. The orbitalcharacteristics of the orbits are compared with those obtained for thecase of an axisymmetric galactic potential. Tidal radii are computed anddiscussed for both cases.
|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.
|Discovery of an SX Phoenicis Type Pulsating Component in the Algol-Type Semidetached Eclipsing Binary QU Sagittae in M71|
We report the discovery of an SX Phoenicis type pulsating component inthe Algol-type semidetached eclipsing binary QU Sge, in the metal-richglobular cluster M71. QU Sge is only about 80" from the center of M71and is located in the blue straggler region in the color-magnitudediagram of M71. It is considered to be a probable member of M71, with amembership probability greater than 60% deduced from a proper-motionstudy in the literature. From time-series CCD photometry, we find thatQU Sge has an orbital period of 3.790818 days and a primary minimumdepth of ΔV=1.333 mag. The eclipsing light curve solution showsthat QU Sge has a semidetached binary configuration with the secondarycomponent fully filling its Roche lobe. After subtracting the eclipsesfrom the light curve, we discover an SX Phoenicis type pulsationfeature. It is found to have a short period of about 0.03 days and asmall amplitude of about 0.024 mag. This is the first eclipsing binarysystem in a globular cluster to exhibit a pulsating feature. This resultsupports the model in which the origin of some blue stragglers inglobular clusters is mass transfer between two components in theprimordial binary systems.
|High-amplitude delta Scuti stars in the Galactic Bulge from the OGLE-II and MACHO data .|
Searching for main-sequence pulsators, we analyzed photometry of˜200,000 variable star candidates from the OGLE-II Galacticfields, finding 193 high-amplitude delta Scuti stars. This doubles thenumber of known stars of this type. The MACHO data, available for halfof stars, were also analyzed. In our sample of the HADS stars, we found50 multiperiodic objects, including 39 that have period ratios in therange of 0.76-0.80, an indication of the radial fundamental andfirst-overtone pulsation. We discuss the resulting Petersen diagram forthese stars in view of the period ratios predicted by models. Except forstars showing pulsations in the radial fundamental mode and firstovertone, we find the evidence for higher radial overtones andnon-radial modes in the analyzed sample of multiperiodic HADS stars.
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