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|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.
|Evolved Stars in the Galactic Globular Cluster M55 (NGC 6809)|
We have compiled the asymptotic giant, horizontal, and upper red giantbranch (AGB, HB, and RGB) stars in the globular cluster M55 (NGC 6809).Using the star counts and the R-parameter we compute the initial heliumabundance Y=0.274+/-0.016. TheR2=NAGB/NHB ratio (0.156+/-0.023) isunusually high for a globular cluster, being almost 2 σ away fromthe predicted values, and theR1=NAGB/NRGB ratio (0.272+/-0.047) isthe highest recorded for a massive globular cluster. We argue that M55'sparticular HB morphology and metallicity have produced long-lived HBstars that are not too blue to avoid producing AGB stars. This resulthints that we are able to map evolutionary effects on the HB. Finally,although we find no evidence of variations in HB morphology withdistance from the center of the cluster, the red HB stars aresignificantly less concentrated than the majority of HB stars, and thebluest HB stars are more centrally concentrated.
|A Panchromatic Study of the Globular Cluster NGC 1904. I. The Blue Straggler Population|
By combining high-resolution (HST/WFPC2) and wide-field ground-based(2.2 m ESO/WFI) and space (GALEX) observations, we have collected amultiwavelength photometric database (ranging from the far-UV to thenear infrared) of the galactic globular cluster NGC 1904 (M79). Thesample covers the entire cluster extension, from the very centralregions up to the tidal radius. In the present paper, such a data set isused to study the BSS population and its radial distribution. A totalnumber of 39 bright (m218<=19.5) BSSs have been detected,and they have been found to be highly segregated in the cluster core. Nosignificant upturn in the BSS frequency has been observed in theoutskirts of NGC 1904, in contrast to other clusters (M3, 47 Tuc, NGC6752, M5) studied with the same technique. Such evidence, coupled withthe large radius of avoidance estimated for NGC 1904(ravoid~30 core radii), indicates that the vast majority ofthe cluster heavy stars (binaries) has already sunk to the core.Accordingly, extensive dynamical simulations suggest that BSSs formed bymass transfer activity in primordial binaries evolving in isolation inthe cluster outskirts represent only a negligible (0%-10%) fraction ofthe overall population.Based on observations with the NASA/ESA HST, obtained at the SpaceTelescope Science Institute, which is operated by AURA, Inc., under NASAcontract NAS5-26555. Also based on GALEX observations (program GI-056)and WFI observations collected at the European Southern Observatory, LaSilla, Chile, within the observing programs 62.L-0354 and 64.L-0439.
|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.
|The Dynamical Implications of Multiple Stellar Formation Events in Galactic Globular Clusters|
Various Galactic globular clusters display abundance anomalies thataffect the morphology of their color-magnitude diagrams. In this paperwe consider the possibility of helium enhancement in the anomaloushorizontal branch of NGC 2808. We examine the dynamics of aself-enrichment scenario in which an initial generation of stars with atop-heavy initial mass function enriches the interstellar medium withhelium via the low-velocity ejecta of its asymptotic giant branch stars.This enriched medium then produces a second generation of stars whichare themselves helium-enriched. We use a direct N-body approach toperform five simulations and conclude that such two-generation clustersare both possible and would not differ significantly from theirsingle-generation counterparts on the basis of dynamics. We find,however, that the stellar populations of such clusters would differ fromsingle-generation clusters with a standard initial mass function and inparticular would be enhanced in white dwarf stars. We conclude, at leastfrom the standpoint of dynamics, that two-generation globular clustersare feasible.
|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.
|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.
|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).
|Chandra X-Ray Sources in the Collapsed-Core Globular Cluster M30 (NGC 7099)|
We report the detection of six discrete, low-luminosity(LX<1033 ergs s-1) X-ray sources,located within 12" of the center of the collapsed-core globular clusterM30 (NGC 7099), and a total of 13 sources within the half-mass radius,from a 50 ks Chandra ACIS-S exposure. Three sources lie within the verysmall upper limit of 1.9" on the core radius. The brightest of the threecore sources has a luminosity of LX(0.5-6keV)~6×1032 ergs s-1 and a blackbody-likesoft X-ray spectrum, which are both consistent with it being a quiescentlow-mass X-ray binary (qLMXB). We have identified optical counterpartsto four of the six central sources and a number of the outlying sources,using deep Hubble Space Telescope and ground-based imaging. While thetwo proposed counterparts that lie within the core may represent chancesuperpositions, the two identified central sources that lie outside ofthe core have X-ray and optical properties consistent with beingcataclysmic variables (CVs). Two additional sources outside of the corehave possible active binary counterparts. We discuss the X-ray sourcepopulation of M30 in light of its collapsed-core status.
|The Stellar Populations in the Outer Regions of M33. II. Deep ACS Imaging|
Studying the stellar populations in the outskirts of spiral galaxies canprovide important constraints on their structure, formation, andevolution. To that end, we present VI photometry obtained with theAdvanced Camera for Surveys for three fields located ~20'-30' inprojected distance southeast of M33's nucleus (corresponding toapproximately four to six visual scale lengths or ~9-13 kpc indeprojected radius). The color-magnitude diagrams reveal a mixed stellarpopulation whose youngest constituents have ages no greater than ~100Myr and whose oldest members have ages of at least several gigayears.The presence of stars as massive as 3-5 Msolar is consistentwith global star formation thresholds in disk galaxies but could arguefor a threshold in M33 that is on the low end of observational andtheoretical expectations. The metallicity gradient as inferred bycomparing the observed red giant branch (RGB) to the Galactic globularclusters is consistent with M33's inner disk gradient traced by severalother studies. The surface density of the RGB stars drops offexponentially with a radial scale length of4.7'+/-0.1'. The scale length increases with agein a manner similar to the vertical scale height of several nearbylate-type spirals. Based on the metallicity gradient, density gradient,and mixed nature of the stellar populations, we conclude that thesefields are dominated by a disk population, although we cannot rule outthe presence of a small halo component.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555. These observations are associated with program9479.
|Fast rotating massive stars and the origin of the abundance patterns in galactic globular clusters|
Aims.We propose the Wind of Fast Rotating Massive Stars scenario toexplain the origin of the abundance anomalies observed in globularclusters. Methods: We compute and present models of fast rotating starswith initial masses between 20 and 120 M_ȯ for an initialmetallicity Z = 0.0005 ([Fe/H]≃-1.5). We discuss thenucleosynthesis in the H-burning core of these objects and present thechemical composition of their ejecta. We consider the impact ofuncertainties in the relevant nuclear reaction rates. Results: Fastrotating stars reach critical velocity at the beginning of theirevolution and remain near the critical limit during the rest of the mainsequence and part of the He-burning phase. As a consequence they loselarge amounts of material through a mechanical wind which probably leadsto the formation of a slow outflowing disk. The material in this slowwind is enriched in H-burning products and presents abundance patternssimilar to the chemical anomalies observed in globular cluster stars. Inparticular, the C, N, O, Na and Li variations are well reproduced by ourmodel. However the rate of the 24Mg(p,γ) has to beincreased by a factor 1000 around 50 × 106 K in orderto reproduce the amplitude of the observed Mg-Al anticorrelation. Wediscuss how the long-lived low-mass stars currently observed in globularclusters could have formed out of the slow wind material ejected bymassive stars.
|Na-O anticorrelation and horizontal branches. VI. The chemical composition of the peculiar bulge globular cluster NGC 6388|
We present the LTE abundance analysis of high resolution spectra for redgiant stars in the peculiar bulge globular cluster NGC 6388. Spectra ofseven members were taken using the UVES spectrograph at the ESO VLT2 andthe multiobject FLAMES facility. We exclude any intrinsic metallicityspread in this cluster: on average, [Fe/H]=-0.44±0.01±0.03dex on the scale of the present series of papers, where the first errorbar refers to individual star-to-star errors and the second issystematic, relative to the cluster. Elements involved in H-burning athigh temperatures show large spreads, exceeding the estimated errors inthe analysis. In particular, the pairs Na and O, Al and Mg areanticorrelated and Na and Al are correlated among the giants in NGC6388, the typical pattern observed in all galactic globular clustersstudied so far. Stars in NGC 6388 shows an excess of α-processelements, similar to the one found in the twin bulge cluster NGC 6441.Mn is found underabundant in NGC 6388, in agreement with the averageabundance ratio shown by clusters of any metallicity. Abundances ofneutron-capture elements are homogeneously distributed within NGC 6388;the [Eu/Fe] ratio stands above the value found in field stars of similarmetallicity.Based on observations collected at ESO telescopes under programme073.D-0211. Full Table 3 is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/967
|Na-O anticorrelation and horizontal branches. V. The Na-O anticorrelation in NGC 6441 from Giraffe spectra|
Aims.We present an analysis of FLAMES-Giraffe spectra for several brightgiants in NGC 6441, to investigate the presence andextent of the Na-O anticorrelation in this anomalous globular cluster. Methods: The field of NGC 6441 is very crowded, withsevere contamination by foreground (mainly bulge) field stars.Appropriate membership criteria were devised to identify a group of 25likely cluster members among the about 130 stars observed. Combined withthe UVES data obtained with the same observations (Gratton et al. 2006),high dispersion abundance analyses are now available for a total of 30stars in NGC 6441, 29 of them having data for both Oand Na. The spectra were analyzed by a standard line analysis procedure;care was taken to minimize the impact of the differential interstellarreddening throughout the cluster, and to extract reliable informationfrom crowded, and moderately high S/N (30-70), moderately highresolution (R˜ 23 000) spectra. Results: NGC6441 has the typical abundance pattern seen in several otherglobular clusters. It is very metal-rich ([Fe/H] = -0.34 ± 0.02± 0.04 dex). There is no clear sign of star-to-star scatter inthe Fe-peak elements. The α-elements Mg, Si, Ca, and Ti areoverabundant by rather large factors, suggesting that the cluster formedfrom material enriched by massive core collapse SNe. The O-Naanticorrelation is well defined, with about 1/4 of the stars beingNa-rich and O-poor. One of the stars is a Ba-rich and moderately C-richstar. Such stars are rare in globular clusters. Conclusions: .Thedistribution of [Na/O] ratios among RGB stars in NGC6441 appears similar to the distribution of colors of starsalong the horizontal branch. The fraction of Na-poor, O-rich stars foundin NGC 6441 agrees well with that of stars on the redhorizontal branch of this cluster (in both cases about 80%), with asloping distribution toward lower values of [O/Na] (among RGB stars) andbluer colors (among HB stars).Based on data collected at the European Southern Observatory with theVLT-UT2, Paranal, Chile (ESO Program 073.D-0211). Full Table [see fulltextsee full textsee full textsee full text] is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(220.127.116.11) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/953
|Na-O anticorrelation and horizontal branches. IV. Detection of He-rich and He-poor stellar populations in the globular cluster NGC 6218|
We used the multifiber spectrograph FLAMES on the ESO Very LargeTelescope UT2 to derive atmospheric parameters, metallicities andabundances of O and Na for 79 red giant stars in the Galactic globularcluster NGC 6218 (M 12). We analyzed stars in the magnitude range fromabout 1 mag below the bump to the tip of the Red Giant Branch. Theaverage metallicity we derive is [Fe/H]=-1.31± 0.004±0.028 dex (random and systematic errors, respectively), with a verysmall star-to-star scatter (rms=0.033 dex), from moderatelyhigh-resolution Giraffe spectra. This is the first extensivespectroscopic abundance analysis in this cluster. Our results indicatethat NGC 6218 is very homogeneous as far as heavy elements areconcerned. On the other hand, light elements involved in the well knownproton-capture reactions of H-burning at high temperature, such as O andNa, show large variations, anticorrelated with each other, at allluminosities along the red giant branch. The conclusion is that the Na-Oanticorrelation must be established in early times at the clusterformation. We interpret the variation of Na found near the RGB-bump asthe effect of two distinct populations having different bumpluminosities, as predicted for different He content. To our knowledge,NGC 6218 is the first GC where such a signature has beenspectroscopically detected, when combined with consistent andhomogeneous data obtained for NGC 6752 to gain in statisticalsignificance.Based on observations collected at ESO telescopes under programme073.D-0211. Full Tables 2, 3, and 5 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/939
|Na-O anticorrelation and horizontal branches. II. The Na-O anticorrelation in the globular cluster NGC 6752|
We are studying the Na-O anticorrelation in several globular clusters ofdifferent Horizontal Branch (HB) morphology in order to derive apossible relation between (primordial) chemical inhomogeneities andmorphological parameters of the cluster population. We used themultifiber spectrograph FLAMES on the ESO Very Large Telescope UT2 andderived atmospheric parameters and elemental abundances of Fe, O and Nafor about 150 red giant stars in the Galactic globular cluster NGC 6752.The average metallicity we derive is [Fe/H] = -1.56, in agreement withother results from red giants, but lower than obtained for dwarfs orearly subgiants. In NGC 6752 there is not much space for an intrinsicspread in metallicity: on average, the rms scatter in [Fe/H] is 0.037± 0.003 dex, while the scatter expected on the basis of the majorerror sources is 0.039 ± 0.003 dex. The distribution of starsalong the Na-O anticorrelation is different to what was found in thefirst paper of this series for the globular cluster NGC 2808: in NGC6752 it is skewed toward more Na-poor stars, and it resembles more theone in M 13. Detailed modeling is required to clarify whether thisdifference may explain the very different distributions of stars alongthe HB.Based on observations collected at ESO telescopes under programme073.D-0211. Full Tables [see full textsee full textsee full text], [seefull textsee full textsee full text] and [see full textsee full textseefull text] are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/927
|Beryllium abundance in turn-off stars of NGC 6752|
Aims.To measure the beryllium abundance in two TO stars of the GlobularCluster NGC 6752, one oxygen rich and sodium poor, the other presumablyoxygen poor and sodium rich. Be abundances in these stars are used toput on firmer grounds the hypothesis of Be as cosmochronometer and toinvestigate the formation of Globular Clusters. Methods: We presentnear UV spectra with resolution R˜ 45 000 obtained with the UVESspectrograph on the 8.2 m VLT Kueyen telescope, analysed with spectrumsynthesis based on plane parallel LTE model atmospheres. Results: Be isdetected in the O rich star with log(Be/H) = -12.04 ±0.15, whileBe is not detected in the other star for which we obtain the upper limitlog(Be/H) < -12.2. A large difference in nitrogen abundance (1.6 dex)is found between the two stars. Conclusions: .The Be measurement iscompatible with what found in field stars with the same [Fe/H] and[O/H]. The "Be age" of the cluster is found to be 13.3 Gyr, in excellentagreement with the results from main sequence fitting and stellarevolution. The presence of Be confirms the results previously obtainedfor the cluster NGC 6397 and supports the hypothesis that Be can be usedas a clock for the early formation of the Galaxy. Since only an upperlimit is found for the star with low oxygen abundance, we cannot decidebetween competing scenarios of Globular Cluster formation, but we canexclude that "polluted" stars are substantially younger than"unpolluted" ones. We stress that the Be test might be the onlymeasurement capable of distinguishing between these scenarios.Based on observations collected at the ESO VLT, Paranal Observatory,Chile, program 075.D-0807(A).
|Why Haven't Loose Globular Clusters Collapsed Yet?|
We report on the discovery of a surprising observed correlation betweenthe slope of the low-mass stellar global mass function (GMF) of globularclusters (GCs) and their central concentration parameterc=log(rt/rc), i.e., the logarithmic ratio of tidaland core radii. This result is based on the analysis of a sample of 20Galactic GCs with solid GMF measurements from deep HST or VLT data. Allthe high-concentration clusters in the sample have a steep GMF, mostlikely reflecting their initial mass function. Conversely,low-concentration clusters tend to have a flatter GMF, implying thatthey have lost many stars via evaporation or tidal stripping. No GCs arefound with a flat GMF and high central concentration. This findingappears counterintuitive, since the same two-body relaxation mechanismthat causes stars to evaporate and the cluster to eventually dissolveshould also lead to higher central density and possibly core collapse.Therefore, more concentrated clusters should have lost proportionatelymore stars and have a shallower GMF than low-concentration clusters,contrary to what is observed. It is possible that severely depleted GCshave also undergone core collapse and have already recovered a normalradial density profile. It is, however, more likely that GCs with a flatGMF have a much denser and smaller core than that suggested by theirsurface brightness profile and may well be undergoing collapse atpresent. In either case, we may have so far seriously underestimated thenumber of post-core collapse clusters, and many may be lurking in theMilky Way.
|ACS Photometry of Newly Discovered Globular Clusters in the Outer Halo of M31|
We report the first results from deep ACS imaging of 10 classicalglobular clusters in the far outer regions (15kpc<~Rp<~100 kpc) of M31. Eight of the clusters,including two of the most remote M31 globular clusters presently known,are described for the first time. Our F606W, F814W color-magnitudediagrams extend ~3 mag below the horizontal branch and clearlydemonstrate that the majority of these objects are old (>~10 Gyr),metal-poor clusters. Five have [Fe/H] ~ -2.1, while an additional fourhave -1.9 <~ [Fe/H] <~ -1.5. The remaining object is moremetal-rich, with [Fe/H] ~ -0.70. Several clusters exhibit thesecond-parameter effect. Using aperture photometry, we estimateintegrated luminosities and structural parameters for all clusters.Many, including all four clusters with projected radii greater than 45kpc, are compact and very luminous, with -8.9 <~ MV <~-8.3. These four outermost clusters are thus quite unlike their MilkyWay counterparts, which are typically diffuse, subluminous (-6.0 <~MV <~ -4.7), and more metal-rich (-1.8 <~ [Fe/H] <~-1.3).Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555. These observations are associated with program10394.
|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.
|New catalogue of blue stragglers in open clusters|
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (126.96.36.199) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
|The MODEST questions: Challenges and future directions in stellar cluster research|
We present a review of some of the current major challenges in stellarcluster research, including young clusters, globular clusters, andgalactic nuclei. Topics considered include: primordial mass segregationand runaway mergers, expulsion of gas from clusters, the production ofstellar exotica seen in some clusters (e.g., blue stragglers and extremehorizontal-branch stars), binary populations within clusters, theblack-hole population within stellar clusters, the final parsec problem,stellar dynamics around a massive black hole, and stellar collisions.The Modest Questions posed here are the outcome of discussions whichtook place at the Modest-6A workshop held in Lund, Sweden, in December,2005. Modest-6A was organised as part of the activities of the ModestCollaboration (see www.manybody.org for further details).
|ACS Photometry of Extended, Luminous Globular Clusters in the Outskirts of M31|
A new population of extended, luminous globular clusters has recentlybeen discovered in the outskirts of M31. These objects have luminositiestypical of classical globular clusters, but much larger half-lightradii. We report the first results from deep ACS imaging of four suchclusters, one of which is a newly discovered example lying at aprojected distance of ~60 kpc from M31. Our F606W, F814W color-magnitudediagrams extend ~3 mag below the horizontal branch level, and clearlydemonstrate, for the first time, that all four clusters are composed of>~10 Gyr old, metal-poor stellar populations. No evidence formultiple populations is observed. From a comparison with Galacticglobular cluster fiducials we estimate metallicities in the range-2.2<~[Fe/H]<~-1.8. The observed horizontal branch morphologiesshow a clear second parameter effect between the clusters. Preliminaryradial luminosity profiles suggest integrated magnitudes in the range-7.7<~MV<~-6.6, near the median value of the globularcluster luminosity function. Our results confirm that these four objectsare bona fide old, metal-poor globular clusters, albeit with combinedstructures and luminosities unlike those observed for any other globularclusters in the Local Group or beyond.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. These observations are associatedwith program 10394.
|Timing of Millisecond Pulsars in NGC 6752. II. Proper Motions of the Pulsars in the Cluster Outskirts|
Exploiting a 5 year span of data, we present improved timing solutionsfor the five millisecond pulsars known in the globular cluster NGC 6752.These include proper-motion determinations for the two outermost pulsarsin the cluster, PSR J1910-5959A and PSR J1910-5959C. The values of theproper motions are in agreement with each other within currentuncertainties, but they do not match (at the 4 σ and 2 σlevels, respectively) the value of the proper motion of the entireglobular cluster derived in the optical band. The implications of theseresults for the cluster membership of the two pulsars are investigated.Prospects for the detection of the Shapiro delay in the binary systemJ1910-5959A are also discussed.
|The Chemical Evolution of Helium in Globular Clusters: Implications for the Self-Pollution Scenario|
We investigate the suggestion that there are stellar populations in someglobular clusters with enhanced helium (Y~0.28-0.40) compared to theprimordial value. We assume that a previous generation of massiveasymptotic giant branch (AGB) stars have polluted the cluster. Twoindependent sets of AGB yields are used to follow the evolution ofhelium and CNO using a Salpeter initial mass function (IMF) and twotop-heavy IMFs. In no case are we able to produce the postulated largeY~0.35 without violating the observational constraint that the CNOcontent is nearly constant.
|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.
|A Moderate Sample Size, Multielement Analysis of the Globular Cluster M12 (NGC 6218)|
We present chemical abundances of several proton-capture, α-,Fe-peak, and neutron-capture elements and radial velocities for 21 redgiant branch (RGB) and asymptotic giant branch members of the Galacticglobular cluster M12. Abundances are based on equivalent widthmeasurements and synthetic spectral analyses of moderate-resolutionspectra (R~15,000) obtained with the 3.5 m WIYN telescope and Hydramultifiber spectrograph. The stars observed range from the RGB tip(M0v=-2.47) down to about 0.50 mag above the levelof the horizontal branch (M0v=+0.11). Ourspectroscopic analysis suggests that M12 is a moderately metal-poorcluster with [Fe/H]=-1.54 (σ=0.09). While the Na abundancesexhibit a range of 0.90 dex, Mg and Al abundances are enhanced by 0.37and 0.54 dex and are nearly constant at all RGB luminosities, incontrast to the blue horizontal-branch cluster M13. The α- andFe-peak elements indicate that M12 has undergone a similar chemicalenrichment history to that of globular clusters and field stars ofcomparable metallicity, with <[α/Fe]>=+0.33 (σ=0.11).M12 also appears to be slightly r-process-rich, with<[Eu/Ba,La]>=+0.22 (σ=0.18).
|The radial distribution of blue straggler stars and the nature of their progenitors|
The origin of blue straggler stars (BSS) in globular clusters (GCs) isstill not fully understood: they can form from stellar collisions, orthrough mass transfer in isolated, primordial binaries (PBs). In thispaper we use the radial distribution of BSS observed in four GCs (M3,47Tuc, NGC6752 and ωCen) to investigate which formation processprevails. We find that both channels co-exist in all the considered GCs.The fraction of mass-transfer (collisional) BSS with respect to thetotal number of BSS is around ~0.4-0.5 (~0.5-0.6) in M3, 47Tuc andNGC6752. The case of ωCen is peculiar with an underproduction ofcollisional BSS. The relative lack of collisional BSS in ωCen canbe understood if mass segregation has not yet driven to the core asizeable number of PBs, which dominate stellar collisions through three-and four-body processes. The spatial distribution of BSS provides stronghints to their origin: the BSS in the cluster outskirts form almostexclusively from mass transfer in PBs, whereas the BSS found close tothe cluster core most likely have a collisional origin.
|Formation and evolution of compact binaries in globular clusters - I. Binaries with white dwarfs|
In this paper, the first of a series, we study the stellar dynamical andevolutionary processes leading to the formation of compact binariescontaining white dwarfs (WDs) in dense globular clusters (GCs). Weexamine the processes leading to the creation of X-ray binaries such ascataclysmic variables (CVs) and AM CVn systems. Using numericalsimulations, we identify the dominant formation channels and we predictthe expected numbers and characteristics of detectable systems,emphasizing how the cluster sources differ from the field population. Weexplore the dependence of formation rates on cluster properties and weexplain in particular why the distribution of CVs has only a weakdependence on cluster density. We also discuss the frequency of dwarfnova outbursts in GCs and their connection with moderately strong WDmagnetic fields. We examine the rates of Type Ia supernovae (SNe Ia) viaboth single and double degenerate channels in clusters and we argue thatthose rates may contribute to the total SN Ia rate in ellipticalgalaxies. Considering coalescing WD binaries, we discuss possibleconstraints on the common envelope evolution of their progenitors and wederive theoretical expectations for gravitational wave detection byLaser Interferometer Space Antenna (LISA).
|The Detailed Star Formation History in the Spheroid, Outer Disk, and Tidal Stream of the Andromeda Galaxy|
Using HST ACS, we have obtained deep optical images reaching stars wellbelow the oldest main-sequence turnoff in the spheroid, tidal stream,and outer disk of Andromeda. We have reconstructed the star formationhistory in these fields by comparing their color-magnitude diagrams to agrid of isochrones calibrated to Galactic globular clusters observed inthe same bands. Each field exhibits an extended star formation history,with many stars younger than 10 Gyr but few younger than 4 Gyr.Considered together, the star counts, kinematics, and populationcharacteristics of the spheroid argue against some explanations for itsintermediate-age, metal-rich population, such as a significantcontribution from stars residing in the disk or a chance intersectionwith the stream's orbit. Instead, it is likely that this population isintrinsic to the inner spheroid, whose highly disturbed structure isclearly distinct from the pressure-supported metal-poor halo thatdominates farther from the galaxy's center. The stream and spheroidpopulations are similar, but not identical, with the stream's mean agebeing ~1 Gyr younger; this similarity suggests that the inner spheroidis largely polluted by material stripped from either the stream'sprogenitor or similar objects. The disk population is considerablyyounger and more metal-rich than the stream and spheroid populations,but not as young as the thin-disk population of the solar neighborhood;instead, the outer disk of Andromeda is dominated by stars of age 4-8Gyr, resembling the Milky Way's thick disk. The disk data areinconsistent with a population dominated by ages older than 10 Gyr andin fact do not require any stars older than 10 Gyr.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated byAURA, Inc., under NASA contract NAS 5-26555. These observations areassociated with proposals 9453 and 10265. Some of the data presentedherein were obtained at the W. M. Keck Observatory, which is operated asa scientific partnership among the California Institute of Technology,the University of California, and NASA. The Observatory was madepossible by the generous financial support of the W. M. Keck Foundation.
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