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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.
| The Michigan/MIKE Fiber System Survey of Stellar Radial Velocities in Dwarf Spheroidal Galaxies: Acquisition and Reduction of Data
We introduce a stellar velocity survey of dwarf spheroidal galaxies,undertaken using the Michigan/MIKE Fiber System (MMFS) at theMagellan/Clay 6.5 m telescope at Las Campanas Observatory. As of 2006November we have used MMFS to collect 6415 high-resolution(R=20,000-25,000) spectra from 5180 stars in four dwarf spheroidalgalaxies: Carina, Fornax, Sculptor, and Sextans. Spectra sample therange 5140-5180 Å, which includes the prominent magnesium tripletabsorption feature. We measure radial velocity (RV) to a medianprecision of 2.0 km s-1 for stars as faint as V~20.5. Fromthe spectra we also are able to measure the strength of iron andmagnesium absorption features using spectral indices that correlate witheffective temperature, surface gravity, and chemical abundance.Measurement of line strength allows us to identify interlopingforeground stars independently of velocity and to examine themetallicity distribution among dSph members. Here we present detaileddescriptions of MMFS, our target selection and spectroscopicobservations, the data reduction procedure, and error analysis. Wecompare our RV results to previously published measurements forindividual stars. In some cases we find evidence for a mild,velocity-dependent offset between the RVs that we measure using themagnesium triplet and previously published RV measurements derived fromthe infrared calcium triplet. In companion papers we will present thecomplete data sets and kinematic analyses of these new observations.This paper includes data obtained with the 6.5 m Magellan Telescopeslocated at Las Campanas Observatory, Chile.
| Stellar Populations in the Outskirts of the Small Magellanic Cloud: No Outer Edge Yet
We report the detection of intermediate-age and old stars belonging tothe SMC at 6.5 kpc from the SMC center in the southern direction. Weshow, from the analysis of three high-quality34'×33' CMDs, that the age composition ofthe stellar population is similar at galactocentric distances of ~4.7,~5.6, and ~6.5 kpc. The surface brightness profile of the SMC follows anexponential law, with no evidence of truncation, all the way out to 6.5kpc. These results, taken together, suggest that the SMC ``disk''population is dominating over a possible old Milky Way-like stellar haloand that the SMC may be significantly larger than previously thought.
| The Core Binary Fractions of Star Clusters from Realistic Simulations
We investigate the evolution of binary fractions in star clusters usingN-body models of up to 100,000 stars. Primordial binary frequencies inthese models range from 5% to 50%. Simulations are performed with theNBODY4 code and include a full mass spectrum of stars, stellarevolution, binary evolution, and the tidal field of the Galaxy. We findthat the overall binary fraction of a cluster almost always remainsclose to the primordial value, except at late times when a cluster isnear dissolution. A critical exception occurs in the central regions,where we observe a marked increase in binary fraction with time-asimulation starting with 100,000 stars and 5% binaries reached a corebinary frequency as high as 40% at the end of the core-collapse phase(occurring at 16 Gyr with ~20,000 stars remaining). Binaries aredestroyed in the core by a variety of processes as a cluster evolves,but the combination of mass segregation and creation of new binaries inexchange interactions produces the observed increase in relative number.We also find that binaries are cycled into and out of cluster cores in amanner that is analogous to convection in stars. For models of 100,000stars we show that the evolution of the core radius up to the end of theinitial phase of core collapse is not affected by the exact value of theprimordial binary frequency (for frequencies of 10% or less). We discussthe ramifications of our results for the likely primordial binarycontent of globular clusters.
| X-Ray Emission from the Planet Pulsar B1257+12
We report the detection of the millisecond pulsar B1257+12 with theChandra X-ray Observatory. In a 20 ks exposure we detected 25 photonsfrom the pulsar, with energies between 0.4 and 2.0 keV, corresponding tothe flux FX=(4.4+/-0.9)×10-15 ergss-1 cm-2 in this energy range. The X-ray spectrumcan be described by a power-law model with photon index Γ~2.8 andluminosity LX~2.5×1029 ergs s-1in the 0.3-8 keV band, for a plausible distance of 500 pc and hydrogencolumn density NH=3×1020 cm-2.Alternatively, the spectrum can be fitted by a blackbody model withkT~0.22 keV and projected emitting area ~2000 m2. If thethermal X-rays are emitted from two symmetric polar caps, the bolometricluminosity of the two caps is 2Lbol~3×1029ergs s-1. We compared our results with the data on other 30millisecond pulsars observed in X-rays and found that the apparent X-rayefficiency of PSR B1257+12, LX/E˙~3×10-5for d=500 pc, is lower than those of most millisecond pulsars. Thismight be explained by an unfavorable orientation of the X-ray pulsarbeam if the radiation is magnetospheric, or by strong asymmetry of polarcaps if the radiation is thermal (e.g., one of the polar caps is muchbrighter than the other and remains invisible for most part of thepulsar period). Alternatively, it could be attributed to absorption ofX-rays in circumpulsar matter, such as a flaring debris disk left overafter formation of the planetary system around the pulsar.
| 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.
| A new search for planet transits in NGC 6791
Context: Searching for planets in open clusters allows us to study theeffects of dynamical environment on planet formation and evolution. Aims: Considering the strong dependence of planet frequency on stellarmetallicity, we studied the metal rich old open cluster NGC6791 and searched for close-in planets using the transittechnique. Methods: A ten-night observational campaign was performedusing the Canada-France-Hawaii Telescope (3.6 m), the San PedroMártir telescope (2.1 m), and the Loiano telescope (1.5 m). Toincrease the transit detection probability we also made use of theBruntt et al. (2003, A&A, 410, 323) eight-nights observationalcampaign. Adequate photometric precision for the detection of planetarytransits was achieved. Results: Should the frequency and properties ofclose-in planets in NGC 6791 be similar to thoseorbiting field stars of similar metallicity, then detailed simulationsforesee the presence of 2-3 transiting planets. Instead, we do notconfirm the transit candidates proposed by Bruntt et al. (2003, A&A,410, 323). The probability that the null detection is simply due tochance coincidence is estimated to be 3%-10%, depending on themetallicity assumed for the cluster. Conclusions: Possible explanationsof the null-detection of transits include: (i) a lower frequency ofclose-in planets in star clusters; (ii) a smaller planetary radius forplanets orbiting super metal rich stars; or (iii) limitations in thebasic assumptions. More extensive photometry with 3-4 m class telescopesis required to allow conclusive inferences about the frequency ofplanets in NGC 6791.Based on observation obtained at theCanada-France-Hawaii Telescope (CFHT) which isoperated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the CentreNational de la Recherche Scientifique of France, and theUniversity of Hawaii and on observations obtained at San PedroMártir 2.1 m telescope (Mexico),and Loiano 1.5 m telescope (Italy).
| Population Synthesis in the Blue. IV. Accurate Model Predictions for Lick Indices and UBV Colors in Single Stellar Populations
We present a new set of model predictions for 16 Lick absorption lineindices from Hδ through Fe5335 and UBV colors for single stellarpopulations with ages ranging between 1 and 15 Gyr, [Fe/H] ranging from-1.3 to +0.3, and variable abundance ratios. The models are based onaccurate stellar parameters for the Jones library stars and a new set offitting functions describing the behavior of line indices as a functionof effective temperature, surface gravity, and iron abundance. Theabundances of several key elements in the library stars have beenobtained from the literature in order to characterize the abundancepattern of the stellar library, thus allowing us to produce modelpredictions for any set of abundance ratios desired. We develop a methodto estimate mean ages and abundances of iron, carbon, nitrogen,magnesium, and calcium that explores the sensitivity of the variousindices modeled to those parameters. The models are compared to high-S/Ndata for Galactic clusters spanning the range of ages, metallicities,and abundance patterns of interest. Essentially all line indices arematched when the known cluster parameters are adopted as input.Comparing the models to high-quality data for galaxies in the nearbyuniverse, we reproduce previous results regarding the enhancement oflight elements and the spread in the mean luminosity-weighted ages ofearly-type galaxies. When the results from the analysis of blue and redindices are contrasted, we find good consistency in the [Fe/H] that isinferred from different Fe indices. Applying our method to estimate meanages and abundances from stacked SDSS spectra of early-type galaxiesbrighter than L*, we find mean luminosity-weighed ages of theorder of ~8 Gyr and iron abundances slightly below solar. Abundanceratios, [X/Fe], tend to be higher than solar and are positivelycorrelated with galaxy luminosity. Of all elements, nitrogen is the morestrongly correlated with galaxy luminosity, which seems to indicatesecondary nitrogen enrichment. If that interpretation is correct, thisresult may impose a lower limit of 50-200 Myr to the timescale of starformation in early-type galaxies. Unlike clusters, galaxies show asystematic effect whereby higher order, bluer, Balmer lines yieldyounger ages than Hβ. This age discrepancy is stronger for lowerluminosity galaxies. We examine four possible scenarios to explain thistrend. Contamination of the bluer indices by a metal-poor stellarpopulation with a blue horizontal branch cannot account for the data.Blue stragglers and abundance-ratio effects cannot be ruled out, as theycan potentially satisfy the data, even though this can only be achievedby resorting to extreme conditions, such as extremely high [O/Fe] orspecific blue-straggler frequencies. The most likely explanation is thepresence of small amounts of a young/intermediate-age stellar populationcomponent. We simulate this effect by producing two-component models andshow that they provide a reasonably good match to the data when the massfraction of the young component is typically a few percent. Ifconfirmed, this result implies star formation has been extended inearly-type galaxies, and more so in less massive galaxies, which seemsto lend support to the ``downsizing'' scenario. Moreover, it impliesthat stellar population synthesis models are capable of constraining notonly the mean ages of stellar populations in galaxies, but also theirage spread.
| Observational Consequences of the Hypothesized Helium-rich Stellar Population in ω Centauri
In response to the proposed high-helium content stars as an explanationfor the double main sequence observed in ω Centauri, weinvestigated the consequences of such stars elsewhere on thecolor-magnitude diagram. We concentrated on the horizontal branch, wherethe effects of high helium are expected to show themselves more clearly.In the process we developed a procedure for comparing the mass losssuffered by differing stellar populations in a physically motivatedmanner. High-helium stars in the numbers proposed seem absent from thehorizontal branch of ω Cen unless their mass-loss history is verydifferent from that of the majority metal-poor stars. It is possible togenerate a double main sequence with existing ω Cen stars viaaccretion of helium-rich pollution consistent with the latest AGB ejectatheoretical yields and such polluted stars are consistent with theobserved HB morphology of ω Cen. Polluted models are consistentwith observed merging of the main sequences as opposed to our models ofhelium-rich stars. Using the (B-R)/(B+V+R) statistic, we find that thehigh-helium bMS stars require an age difference compared to the rMSstars that is too great, whereas the pollution scenario stars have nosuch conflict for inferred ω Cen mass losses.
| Star Counts in the Globular Cluster ω Centauri. I. Bright Stellar Components
We present a photometric investigation on HB, RGB, and MSTO stars inω Cen=NGC 5139. The center of the cluster was covered with amosaic of F435W, F625W, and F658N band data collected with HST ACS. Theouter reaches were covered with a mosaic of U-, B-, V-, and I-band datacollected with the 2.2 m ESO/MPI telescope. The final catalog includes~1.7 million stars. We identified more than 3200 likely HB stars, thelargest sample ever collected in a globular cluster. We found that theHB morphology changes with the radial distance from the cluster center.The relative number of extreme HB stars decreases from ~30% to ~21% whenmoving from the center toward the outer reaches of the cluster, whilethe fraction of less hot HB stars increases from ~62% to ~72%. Thecomparison between theory and observations indicates that the empiricalstar counts of HB stars are on average larger (30%-40%) than predictedby canonical evolutionary models. Moreover, the rate of HB stars is ~43%larger than the MSTO rate. We also compared theory and observations byassuming a mix of stellar populations made with 70% of canonical He(Y=0.23) stars and 30% of He-enhanced (Y=0.33, 0.42) stars. We foundthat the observed RG/MSTO ratio agrees with the predicted lifetimes ofHe-mixed stellar populations. The discrepancy between theory andobservations decreases by a factor of 2 when compared with ratespredicted by canonical He content models, but still 15%-25% (Y=0.42) and15%-20% (Y=0.33) higher than observed. Furthermore, the ratios betweenHB and MSTO star counts are ~24% (Y=0.42) and 30% (Y=0.33) larger thanpredicted lifetime ratios.During the revision of this manuscript, Vittorio Castellani passed awayon 2006 May 19. His suggestions, ideas, and personality will be greatlymissed.Based on data obtained from the ESO Science Archive Facility and theHubble Space Telescope Archive Facility.
| Using Weak-Lensing Dilution to Improve Measurements of the Luminous and Dark Matter in A1689
The E/S0 sequence of a cluster defines a boundary redward of which areliable weak-lensing signal can be obtained from background galaxies,uncontaminated by cluster members. For bluer colors, both background andcluster members are present, reducing the average distortion signal bythe proportion of unlensed cluster members. In deep Subaru and HST/ACSimages of A1689, the tangential distortion of galaxies with bluer colorsfalls rapidly toward the cluster center relative to lensing signal ofthe red background. We use this dilution effect to derive the clusterlight profile and luminosity function to large radius, with theadvantage that no subtraction of far-field background counts isrequired. The light profile declines smoothly to the limit of the data,r<2 h-1 Mpc, with a constant slope,dlog(L)/dlog(r)=-1.12+/-0.06, unlike the lensing mass profile whichsteepens continuously with radius, so that M/L peaks at an intermediateradius, ~=100 h-1 kpc. A flatter behavior is found for themore physically meaningful ratio of dark matter to stellar matter whenaccounting for the color-mass relation of cluster members. The clusterluminosity function has a flat slope, α=-1.05+/-0.05, independentof radius and with no faint upturn to Mi'<-12.We establish that the very bluest objects are negligibly contaminated bythe cluster [(V-i')AB<0.2], because theirdistortion profile rises toward the center following the red background,but offset higher by ~=20%. This larger amplitude is consistent with thegreater estimated depth of the faint blue galaxies, ~2 comparedto ~0.85 for the red background, a purely geometric effectrelated to cosmological parameters. Finally, we improve on our earliermass profile by combining both the red and blue background populations,clearly excluding low-concentration CDM profiles.
| 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.
| Stellar Populations of Globular Clusters in the Elliptical Galaxy NGC 1407
We present high-quality Keck spectroscopic data for a sample of 20globular clusters (GCs) in the massive E0 galaxy NGC 1407. A subset of20 line-strength indices of the Lick/IDS system has been measured forboth the GC system and the central integrated starlight of the galaxy.Ages, metallicities, and [α/Fe] ratios have been derived usingseveral different approaches. The majority of GCs in NGC 1407 studiedare old, follow a tight metallicity sequence reaching values slightlyabove solar, and exhibit mean [α/Fe] ratios of ~0.3 dex. Inaddition, three GCs are formally derived to be young (~4 Gyr), but weargue that they are actually old GCs hosting blue horizontal branches.We report, for the first time, evidence for the existence of twochemically distinct subpopulations of metal-rich (MR) GCs. We find thatsome MR GCs exhibit significantly larger [Mg/Fe] and [C/Fe] ratios.Different star formation timescales are proposed to explain thecorrelation between Mg and C abundances. We also find striking CNoverabundances over the entire GC metallicity range. It is interestingto note that the behavior of C and N in metal-poor GCs clearly deviatesfrom that in MR GCs. In particular, for MR GCs, N increases dramaticallywhile C essentially saturates. This may be interpreted as a consequenceof the increasing importance of the CNO cycle with increasingmetallicity.
| The ACS Survey of Galactic Globular Clusters. II. Stellar Evolution Tracks, Isochrones, Luminosity Functions, and Synthetic Horizontal-Branch Models
The ACS Survey of Galactic Globular Clusters, an HST Treasury Project,will deliver high-quality, homogeneous photometry of 65 globularclusters. This paper introduces a new collection of stellar evolutiontracks and isochrones suitable for analyzing the ACS survey data.Stellar evolution models were computed at [Fe/H]=-2.5, -2.0, -1.5, -1.0,-0.5, and 0; [α/Fe]=-0.2, 0, 0.2, 0.4, 0.6, and 0.8; and threeinitial He abundances for masses from 0.1 to 1.8 Msolar andages from 2 to 15 Gyr. Each isochrone spans a wide range in luminosity,from MV~14 up to the tip of the red giant branch. These arecomplemented by a set of He-burning tracks that extend from the zero-agehorizontal branch to the onset of thermal pulsations on the asymptoticgiant branch. In addition, a set of computer programs are provided thatmake it possible to interpolate the isochrones in [Fe/H], generateluminosity functions from the isochrones, and create synthetichorizontal-branch models. The tracks and isochrones have been convertedto the observational plane with two different color-Tefftransformations, one synthetic and one semiempirical, in ground-based B,V, and I, and F606W and F814W for both ACS WFC and WFPC2 systems. Allmodels and programs presented in this paper are available at theDartmouth Stellar Evolution Database and the Multimission Archive at theSpace Telescope Science Institute.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS 5-26555, under program GO-10775 (PI: A.Sarajedini).
| Hδ in the integrated light of galaxies: what are we actually measuring?
We present a cautionary study exploring the reliability of the H-deltaline in the integrated spectra of galaxies for determining galaxy ages.Our database consists of the observed integrated spectra of ~120early-type galaxies, of 7 metal-rich globular clusters in M31 and theGalactic globular cluster 47 Tuc, and of the open cluster M67. We havemeasured H-delta using index definitions designed to assesscontamination from the CN molecule in and around H-delta by choosingcombinations of bandpasses that both avoid and include a region of CNmolecular lines redward of H-delta. We find systematic differences inthe ages derived from H-delta measurements among the various definitionswhen extracting ages from H-delta in old stellar populations withenhanced CN bands due to non-solar abundance ratios. We propose thatneighboring CN lines have a strong effect on pseudocontinuum and centralbandpass levels. For stellar populations which have non-solar abundanceratios in C and/or N, population synthesis models that do not accountfor abundance ratio variations cannot reproduce accurately the CN 4216\AA band, which leads to a corresponding inaccuracy in reproducing thevarious H-delta indices. Hence, caution must be used when extractinggalaxy ages from the H-delta line in old stellar populations withsignificant non-solar abundance ratios.
| Washington Photometry of the Red Giant Branch in NGC 6388
We present Washington photometry of the giant branches of the relativelymetal-rich globular cluster NGC 6388 and compare it with 47 Tuc. Byexamining the spatial distribution of stellar colors across the redgiant branch and using spectroscopic data on several bright giants, wefound that NGC 6388's center is affected by differential reddening of atleast ΔE(B-V)~0.1. Most of the width of the red giant branch iscaused by that differential reddening. The southeastern outer regions ofthe cluster seem to be less affected by variable dust extinction, and wefind that NGC 6388 is slightly more metal-rich than 47 Tuc, with adistinct asymptotic giant branch. We have put an upper limit on possiblevariations in [Fe/H] for NGC 6388, finding that it must be less than~0.2 dex. It is unlikely that such a small range in metallicity could beresponsible for the blue horizontal branch and blue tail stars in acluster with a metallicity similar to the more massive 47 Tuc, whichonly exhibits a red horizontal branch.
| Predicting the frequencies of diverse exo-planetary systems
Extrasolar planetary systems range from hot Jupiters out to icy cometbelts more distant than Pluto. We explain this diversity in a modelwhere the mass of solids in the primordial circumstellar disc dictatesthe outcome. The star retains measures of the initial heavy-element(metal) abundance that can be used to map solid masses on to outcomes,and the frequencies of all classes are correctly predicted. Thediffering dependences on metallicity for forming massive planets andlow-mass cometary bodies are also explained. By extrapolation, aroundtwo-thirds of stars have enough solids to form Earth-like planets, and ahigh rate is supported by the first detections of low-mass exo-planets.
| 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.
| A study of the population of LMXBs in the bulge of M 31
Aims. We explore the population of X-ray point sources in the bulge of M31 to contrast properties of various subpopulations, such as persistentand transient sources and primordial LMXBs and dynamically formed ones. Methods: Based on the data from 26 archival Chandra observations westudy the source content and properties of various subpopulations ofX-ray sources to a maximum distance of 12´ from the centre of M31. Results: To a limiting luminosity of ˜ 1035 ergs-1 we find 263 X-ray point sources, with ˜ 1/3 ofthese being background galaxies. A study of the spatial distribution andthe luminosity function of the X-ray sources shows that the distributionof primordial LMXBs is consistent with the distribution of the K-bandlight and that their luminosity function flattens below ˜1037 erg s-1 to the dN/dL∝ L-1 lawin agreement with the behaviour found earlier for LMXBs in the Milky Wayand in Cen A. Within a radius of 12 arcmin, the luminosity function isindependent of distance to the centre of M 31, in contrast to earlierChandra studies. The LMXBs located in globular clusters and within˜ 1 arcmin from the centre of M 31 are presumably created viadynamical interactions. The dynamical origin of the r< 1 arcminsources is strongly suggested by their radial distribution which followsthe ρ^2_{*} profile rather than the K-band light distribution. Theirluminosity function shows a prominent fall-off below log(L_X)⪉ 36.5.Although the statistics is insufficient to claim a genuinelow-luminosity cut-off in the luminosity function, the best fit powerlawwith a slope of -0.6±0.2 is significantly flatter than thedN/dL∝ L-1 law. We also searched for transients andfound 28 sources that varied by a factor larger than 20. Their spatialdistribution follows the distribution of the persistent LMXBs within theaccuracy allowed by the limited number of transients.Full Table [see full text] is only available in electronic form athttp://www.aanda.org
| Near-infrared observations of the Fornax dwarf galaxy. I. The red giant branch
Aims.We present a study of the evolved stellar populations in the dwarfspheroidal galaxy Fornax based on wide-area near-infrared observations,aimed at obtaining new independent estimates of its distance andmetallicity distribution. Assessing the reliability of near-infraredmethods is most important in view of future space- and ground-based deepnear-infrared imaging of resolved stellar systems. Methods: We haveobtained JHK imaging photometry of the stellar populations in Fornax.The observations cover an 18.5×18.5 arcmin2 centralarea with a mosaic of SOFI images at the ESO NTT. Our data sample allthe red giant branch (RGB) for the whole area. Deeeper observationsreaching the red clump of helium-burning stars have also been obtainedfor a 4.5×4.5 arcmin2 region. Results: Near-infraredphotometry led to measurements of the distance to Fornax based on theK-band location of the RGB tip and the red clump. Once corrected for themean age of the stellar populations in the galaxy, the derived distancemodulus is (m-M)0 = 20.74±0.11, corresponding to adistance of 141 Kpc, in good agreement with estimates from optical data.We have obtained a photometric estimate of the mean metallicity of redgiant stars in Fornax from their (J-K) and (V-K) colors, using severalmethods. The effect of the age-metallicity degeneracy on the combinedoptical-infrared colors is shown to be less important than for opticalor infrared colors alone. By taking age effects into account, we havederived a distribution function of global metallicity [M/H] fromoptical-infrared colors of individual stars. Our photometric MetallicityDistribution Function covers the range -2.0 < [M/H] < -0.6, with amain peak at [M/H]≃ -0.9 and a long tail of metal-poor stars, andless metal-rich stars than derived by recent spectroscopy. Ifmetallicities from Ca II triplet lines are correct, this result confirmsa scenario of enhanced metal enrichment in the last 1-4 Gyr.Based on data collected at the European Southern Observatory, La Silla,Chile, Proposals No. 65.N-0167, 66.B-0247. Full Tables 2 and 3 are onlyavailable in electronic format at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/467/1025
| High-energy γ-rays from globular clusters
It is expected that specific globular clusters (GCs) can contain up to ahundred of millisecond pulsars. These pulsars can accelerate leptons atthe shock waves originated in collisions of the pulsar winds and/orinside the pulsar magnetospheres. Energetic leptons diffuse graduallythrough the GC Comptonizing stellar and microwave background radiation.We calculate the GeV-TeV γ-ray spectra for different models ofinjection of leptons and parameters of the GCs assuming reasonable, ofthe order of 1 per cent, efficiency of energy conversion from the pulsarwinds into the relativistic leptons. It is concluded that leptonsaccelerated in the GC cores should produce well localized γ-raysources which are concentric with these GCs. The results are shown forfour specific GCs (47 Tuc, Ter 5, M13 and M15), in which significantpopulation of millisecond pulsars have been already discovered. We arguethat the best candidates, which might be potentially detected by thepresent Cherenkov telescopes and the planned satellite telescopes(AGILE, GLAST), are 47 Tuc on the Southern hemisphere, and M13 on theNorthern hemisphere. We conclude that detection (or non-detection) ofGeV-TeV γ-ray emission from GCs by these instruments put importantconstraints on the models of acceleration of leptons by millisecondpulsars.
| N-body models of rotating globular clusters
In this paper we examine the dynamical evolution of rotating globularclusters with direct N-body models. Our initial models are rotating Kingmodels, and we obtain results both for equal-mass systems and forsystems composed of two mass components. Previous investigations using aFokker-Planck solver have shown that rotation has a noticeable influenceon stellar systems such as globular clusters that evolve by two-bodyrelaxation. In particular, it accelerates their dynamical evolutionthrough the gravogyro instability. We have validated the occurrence ofthe gravogyro instability with direct N-body models. In the case ofsystems composed of two mass components, mass segregation takes place, aprocess that competes with the rotation in the acceleration of the corecollapse. The `accelerating' effect of rotation was detected in ourisolated two-mass N-body models. Finally, we look at rotating N-bodymodels in a tidal field within the tidal approximation. It turns outthat rotation increases the escape rate significantly. A differencebetween retrograde- and prograde-rotating stellar clusters, with respectto the orbit of the cluster around the Galaxy, occurs. This differenceis the result of the presence of a `third integral' and chaoticscattering, respectively.
| Origin of abundance inhomogeneity in globular clusters
We numerically investigate abundance properties of the Galactic globularclusters (GCs) by adopting a new `external pollution' scenario. In thisframework, GCs are assumed to originate in forming low-mass dwarfsembedded in dark matter subhaloes at very high redshifts (z) and thus bechemically influenced by field asymptotic giant branch (AGB) stars ofthe dwarfs during early GC formation processes. GCs within a dwarfgalaxy therefore can be formed from the mixture of (i) gas ejected fromthe field AGB stars formed earlier in the dwarf and (ii) theinterstellar gas infalling to the central region of the dwarf. In thisexternal pollution scenario, the ratio of the total mass of infallinggas to that of AGB ejecta during GC formation in a dwarf (s) and thetime-scale of gas infall (σI) are the most importantkey parameters that can determine abundance properties of GCs. We mainlyinvestigate the abundance inhomogeneity among light elements (e.g. C, N,O, Na and Al) of stars in GCs by using the latest stellar yield modelsof metal-poor AGB stars with and without third dredge-up. Our principalresults for the models with no third dredge-up, which are moreconsistent with observations, are as follows. (i) Both [N/Fe] and[C/Fe] can be diverse among stars within a GC owing to chemicalpollution from field AGB stars. [N/Fe] distributions in some GCs canclearly show bimodality, whereas [C/Fe] is monomodal in most models.[N/Fe] distributions depend on s such that models with smaller s (i.e.larger mass fraction of AGB ejecta used for GC formation) show the[N/Fe] bimodality more clearly. (ii) N-rich, C-poor stars in GCs alsohave higher He abundances owing to pollution from massive AGB stars withHe-rich ejecta. The number fraction of He-rich stars (Y > 0.30) ishigher for the models with smaller s and shorter σI for3 <= s <= 24 and 105 <= σI <=107 yr. He abundances of stars correlate with [N/Fe] and[Al/Fe] and anticorrelate with [C/Fe], [O/Fe] and [Na/Fe] within GCs inour models. (iii) Although our model can much better explain theobserved C-N and Mg-Al anticorrelations than previous theoreticalmodels, it is in strong disagreement with the observed O-Naanticorrelation. (iv) This model naturally provides an explanation forthe large fraction of CN-strong stars without recourse to an implausibleinitial mass function. Based on these results for the above externalpollution scenario, we discuss the long-standing problem of theCN-bimodality prevalent in the Galactic GCs, the possible heliumabundance inhomogeneity in these systems and their horizontal branchmorphologies.
| 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.
| Overluminous Blue Horizontal-Branch Stars Formed by Low-Mass Companions
We construct a speculative scenario for rotation-induced extra heliummixing to the envelope of horizontal-branch (HB) stars. This scenariodiffers from previous ones in that the mixing occurs after the star hasleft the red giant branch (RGB). We follow the evolution of alow-metallicity star from the RGB to the HB, and examine the densityprofile and radius in the core-envelope boundary region. In thetransition from the RGB to the HB the envelope shrinks by 2 orders ofmagnitude in size and the core swells, such that any nonnegligiblerotation on the RGB will result in a strong rotational shear at thecore-envelope boundary. For a nonnegligible rotation to exist on theRGB, the star has to be spun up by a companion spiraling inside itsenvelope (a common-envelope evolution). We speculate that shearinstabilities on the HB might mix helium-rich core material to theenvelope. The shallow density profile on the HB is less likely toprevent mixing. As previously shown, extra helium mixing can account forthe overluminous blue HB stars found in some globular clusters. Althoughbeing speculative, this study supports the idea that the presence oflow-mass companions, from planets to low-mass main-sequence stars,influence the evolution of stars, and can explain some properties of thecolor-magnitude (Hertzsprung-Russel) diagram of globular clusters.Namely, low-mass companions can be an ingredient in the so-called secondparameter of globular clusters.
| Variability of 19 Millisecond Pulsars in 47 Tucanae with Chandra HRC-S
We present results from our 830 ks observation of the globular cluster47 Tucanae with the Chandra X-ray Observatory's High ResolutionCamera-S. We limit our analysis here to the 19 previously known,localized millisecond pulsars (MSPs) in the cluster. This work more thandoubles the sample of X-ray-detected MSPs observed with sensitivity torotational variability; it is also the first survey of a large group ofradio-discovered MSPs for which no previous X-ray pulsations have beendetected and is therefore an unbiased survey of the X-ray properties ofradio-discovered MSPs. We find that only 47 Tuc D, O, and R showsignificant pulsations at the >~4 σ level, but there isstatistical evidence for rotational variability in five additional MSPs.Furthermore, we constrain the pulsed magnetospheric emission of sevenmore MSPs using Monte Carlo simulations. The result is that the majorityof the 47 Tuc MSPs are characterized by low pulsed fractions, <~50%.In cases where larger pulsed fractions are measured, the folded pulseprofiles show relatively large duty cycles. When considered withprevious spectroscopic studies, this suggests that the X-ray emissionarises from the neutron star's heated polar caps and, in some cases,from intrabinary shocks, but generally not directly from the star'smagnetosphere. We discuss the impact of these results on ourunderstanding of high-energy emission from MSPs.
| A Canis Major Overdensity Imaging Survey. I. Stellar Content and Star-Count Maps: A Distinctly Elongated Body of Main-Sequence Stars
We present the first results from a large-area(~80deg×20deg), sparsely sampled, two-filter(B and R) imaging survey toward the Canis Major stellar overdensity,which is claimed to be a disrupting Milky Way satellite galaxy. Usingstellar color-magnitude diagrams reaching to B~22 mag, we provide afirst delineation of its surface density distribution usingmain-sequence stars. It is located below the Galactic midplane, and canbe discerned to at least b=-15deg. Its projected shape ishighly elongated, nearly parallel to the Galactic plane, with an axisratio of at least 5:1, substantially more so than what Martin andcoworkers originally found. We also provide a first map of a prominentoverdensity of blue, presumably younger main-sequence stars, whichextends in latitude to b~-10deg. We estimate an upper limiton the line-of-sight depth σlos of the old populationbased on the main-sequence width, obtainingσlos<1.8+/-0.3 kpc at an adoptedDsolar=7.5+/-1 kpc. For the young stellar population, we findσlos<1.5 kpc. The overall picture presented is oneof a young stellar population that is less extended, both in terms ofits line-of-sight depth and angular size, than the older population.While the data provide no firm arguments against an out-of-plane spiralarm interpretation, the data provide clear implications for others: (1)We infer from the strong elongation of the overdensity in longitude, andsimulations in the literature, that the CMa overdensity is unlikely tobe a gravitationally bound system at the present epoch, but may well bejust a recently disrupted satellite remnant. The possible ``flattening''of the young main-sequence population may, however, be a complexity forthe satellite origin. (2) Based on modeling, the line-of-sight depth ofthe main-sequence overdensity in old stars is clearly inconsistent withpublished locally axisymmetric descriptions of the warped Galactic disk,such as those considered by Momany and coworkers. Without detailedmodeling, the data set itself does not allow a distinction betweeninterpretations as substructure in the warped outer Galactic disk or adisrupted satellite.
| Old Main-Sequence Turnoff Photometry in the Small Magellanic Cloud. I. Constraints on the Star Formation History in Different Fields
We present ground-based B- and R-band color-magnitude diagrams (CMDs)reaching the oldest main-sequence (MS) turnoffs with good photometricaccuracy for 12 fields in the Small Magellanic Cloud (SMC). Our fields,located between ~1° and ~4° from the center of the galaxy, aresituated in different parts of the SMC such as the ``Wing'' area andtoward the west and south. In this paper we perform a first analysis ofthe stellar content in our SMC fields through comparison withtheoretical isochrones and color functions (CFs). We find that theunderlying spheroidally distributed population is composed of bothintermediate-age and old stars and that its age composition does notshow strong galactocentric gradients. The three fields situated towardthe east, in the Wing region, show very active current star formation.However, only in the eastern field closest to the center do we find anenhancement of recent star formation with respect to a constant SFR(t).The fields corresponding to the western side of the SMC present a muchless populated young MS, and the CF analysis indicates that the SFR(t)greatly diminished around 2 Gyr ago in these parts. Field smc0057, theclosest to the center of the galaxy and located in the southern part,shows recent star formation, while the rest of the southern fieldspresent few bright MS stars. The structure of the red clump in all theCMDs is consistent with the large amount of intermediate-age starsinferred from the CMDs and color functions. None of the SMC fieldspresented here are dominated by old stellar populations, a fact that isin agreement with the lack of a conspicuous horizontal branch in allthese SMC CMDs. This could indicate that a disk population is rulingover a possible old halo in all the observed fields.
| Radial Velocity Studies of Close Binary Stars. XII.
Radial velocity measurements and sine-curve fits to the orbital radialvelocity variations are presented for 10 close binary systems: OO Aql,CC Com, V345 Gem, XY Leo, AM Leo, V1010 Oph, V2612 Oph, XX Sex, W UMa,and XY UMa. Most of these binaries have been observed spectroscopicallybefore, but our data are of higher quality and consistency than in theprevious studies. While most of the studied eclipsing pairs are contactbinaries, V1010 Oph is probably a detached or semidetached double-linedbinary, and XY UMa is a detached, chromospherically active system whosebroadening functions clearly show well-defined and localized dark spotson the primary component. A particularly interesting case is XY Leo,which is a member of visually unresolved quadruple system composed of acontact binary and a detached, noneclipsing, active binary with an 0.805day orbital period. V345 Gem and AM Leo are known members of visualbinaries. We found faint visual companions at about 2"-3" from XX Sexand XY UMa.Based on data obtained at the David Dunlap Observatory, University ofToronto.
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