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|Evolved Stars in the Galactic Globular Cluster M55 (NGC 6809)|
We have compiled the asymptotic giant, horizontal, and upper red giantbranch (AGB, HB, and RGB) stars in the globular cluster M55 (NGC 6809).Using the star counts and the R-parameter we compute the initial heliumabundance Y=0.274+/-0.016. TheR2=NAGB/NHB ratio (0.156+/-0.023) isunusually high for a globular cluster, being almost 2 σ away fromthe predicted values, and theR1=NAGB/NRGB ratio (0.272+/-0.047) isthe highest recorded for a massive globular cluster. We argue that M55'sparticular HB morphology and metallicity have produced long-lived HBstars that are not too blue to avoid producing AGB stars. This resulthints that we are able to map evolutionary effects on the HB. Finally,although we find no evidence of variations in HB morphology withdistance from the center of the cluster, the red HB stars aresignificantly less concentrated than the majority of HB stars, and thebluest HB stars are more centrally concentrated.
|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).
|Iron abundances of B-type post-asymptotic giant branch stars in globular clusters: Barnard29 in M13 and ROA5701 in ωCen|
High-resolution optical and ultraviolet (UV) spectra of two B-typepost-asymptotic giant branch (post-AGB) stars in globular clusters,Barnard29 in M13 and ROA5701 in ωCen, have been analysed usingmodel atmosphere techniques. The optical spectra have been obtained withFEROS on the ESO 2.2-m telescope and the 2d-Coudé spectrograph onthe 2.7-m McDonald telescope, while the UV observations are from theGoddard high-resolution spectrograph on the Hubble Space Telescope(HST). Abundances of light elements (C, N, O, Mg, Al and S) plus Fe havebeen determined from the optical spectra, while the UV data provideadditional Fe abundance estimates from FeIII absorption lines in the1875-1900 Å wavelength region. A general metal underabundancerelative to young B-type stars is found for both Barnard29 and ROA5701.These results are consistent with the metallicities of the respectiveclusters, as well as with previous studies of the objects. The derivedabundance patterns suggest that the stars have not undergone a gas-dustseparation, contrary to previous suggestions, although they may haveevolved from the AGB before the onset of the third dredge-up. However,the Fe abundances derived from the HST spectra are lower than thoseexpected from the metallicities of the respective clusters, by 0.5 dexfor Barnard29 and 0.8 dex for ROA5701. A similar systematicunderabundance is also found for other B-type stars in environments ofknown metallicity, such as the Magellanic Clouds. These results indicatethat the FeIII UV lines may yield abundance values which aresystematically too low by typically 0.6 dex and hence such estimatesshould be treated with caution.This paper includes data taken at the McDonald Observatory of theUniversity of Texas at Austin, and on the ESO 2.2-m La Silla, programme0077.D-025(A).E-mail: email@example.com
|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.
|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.
|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.
|The Observed Properties of Dark Matter on Small Spatial Scales|
We present a synthesis of recent photometric and kinematic data forseveral of the most dark matter dominated galaxies, the dwarf spheroidalGalactic satellites, and compare them to star clusters. There is abimodal distribution in half-light radii, with stable star clustersalways being smaller than ~30 pc, while stable galaxies are alwayslarger than ~120 pc. We extend the previously known observationalrelationships and interpret them in terms of a more fundamental pair ofintrinsic properties of dark matter itself: dark matter forms cored massdistributions, with a core scale length of greater than about 100 pc,and always has a maximum central mass density within a narrow range. Thedark matter in dSph galaxies appears to be clustered such that there isa mean volume mass density within the stellar distribution which has thevery low value of less than about 0.1 Msolar pc-3(about 5 GeV/c2 cm-3). All dSph's have velocitydispersions at the edge of their light distributions equivalent tocircular velocities of ~15 km s-1. The maximum central darkmatter density derived is model dependent but is likely to have acharacteristic value (averaged over a volume of radius 10 pc) of ~0.1Msolar pc-3 for the favored cored dark massdistributions (where it is similar to the mean value), or ~60 Msolar pc-3 (about 2 TeV/c2cm-3) if the dark matter density distribution is cusped.Galaxies are embedded in dark matter halos with these properties;smaller systems containing dark matter are not observed. These valuesprovide new information about the nature of the dominant form of darkmatter.
|The Multiplicity of the Subgiant Branch of ω Centauri: Evidence for Prolonged Star Formation|
We combine spectroscopic and photometric data for subgiant stars ofω Cen to extract results that neither data set could have providedon its own. GIRAFFE@VLT spectra of 80 stars at R=6400 give metallicitiesfor all of them and abundances of C, N, Ca, Ti, and Ba for a subset ofthem. The photometric data, which have unusually high accuracy, comefrom a ~10×10 arcmin2 mosaic of HST ACS images centeredon the cluster center and on multicolor images of a ~34×33arcmin2 field, taken with the WFI@ESO2.2m camera. Stars with[Fe/H]<-1.25 have a large magnitude spread on the flat part of theSGB. We interpret this as empirical evidence for an age spread, and fromtheoretical isochrones we derive a relative age for each star. Withinthe SGB region we identify four distinct stellar groups: (1) an old,metal-poor group ([Fe/H]~-1.7) (2) an old, metal-rich group([Fe/H]~-1.1) (3) a young (up to 4-5 Gyr younger than the old component)metal-poor group ([Fe/H]~-1.7) (4) a young, intermediate-metallicity([Fe/H]~-1.4) group, on average 1-2 Gyr younger than the old metal-poorpopulation, and with an age spread that we cannot properly quantify withthe present sample. In addition, many SGB stars are spread between theintermediate-metallicity and metal-rich branches. We tentatively proposeconnections between the SGB stars and both the multiple main-sequenceand the red giant branches. Finally, we discuss the implications of themultiple stellar populations on the formation and evolution of ωCen. The spread in age within each population establishes that theoriginal system must have had a composite nature.Based on FLAMES+GIRAFFE@VLT observations under the DDT program272.D-5065(A) and on observations with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by AURA, Inc., under NASA contract NAS 5-26555.
|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.
|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.
|Space Velocities of Southern Globular Clusters. V. A Low Galactic Latitude Sample|
We have measured the absolute proper motions of globular clusters NGC2808, 3201, 4372, 4833, 5927, and 5986. The proper motions are on theHipparcos system, and they are the first determinations ever made forthese low Galactic latitude clusters. The proper-motion uncertaintiesrange from 0.3 to 0.5 mas yr-1. The inferred orbits indicatethat (1) the single metal-rich cluster in our sample, NGC 5927,dynamically belongs to the thick disk; (2) the remaining metal-poorclusters have rather low-energy orbits of high eccentricity, and amongthese there appear to be two ``pairs'' of dynamically associatedclusters; (3) the most energetic cluster in our sample, NGC 3201, is ona highly retrograde orbit-which had already been surmised from radialvelocity alone-with an apocentric distance of 22 kpc; and (4) none ofthe metal-poor clusters appear to be associated with the recentlydetected SDSS streams or with the Monoceros structure. These are thefirst results of the Southern Proper Motion program where thesecond-epoch observations are taken with the recent CCD camera systeminstalled on the double astrograph at El Leoncito, Argentina.
|Hubble Space Telescope Photometry for the Halo Stars in the Leo Elliptical NGC 3377|
We have used the ACS camera on HST to obtain (V,I) photometry for 57,000red giant stars in the halo of the Leo elliptical NGC 3377, anintermediate-luminosity elliptical. We use this sample of stars toderive the metallicity distribution function (MDF) for its halo fieldstars and comment on its chemical evolution history compared with bothlarger and smaller E galaxies. Our ACS WFC field spans a radial rangeextending from 4 to 18 kpc projected distance from the center of NGC3377 and thus covers a significant portion of this galaxy's halo. Wefind that the MDF is broad, reaching a peak atlog(Z/Zsolar)~=-0.6, but containing virtually no stars moremetal-poor than log(Z/Zsolar)=-1.5. It may, in addition, haverelatively few stars more metal-rich thanlog(Z/Zsolar)~=-0.3, although interpretation of thehigh-metallicity end of the MDF is limited by photometric completenessthat affects the detection of the reddest, most metal-rich stars. NGC3377 appears to have an enrichment history intermediate between those ofnormal dwarf ellipticals and the much larger giants. As yet, we find noclear evidence that the halo of NGC 3377 contains a significantpopulation of ``young'' (<3 Gyr) stars.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 NAS 5-26555. These observations are associated withprogram 9811. Support for this work was provided in part by NASA throughgrant HST-GO-09811.01-A from the Space Telescope Science Institute,under NASA contract NAS 5-26555.
|Contrasting copper evolution in ω Centauri and the Milky Way|
Despite the many studies on stellar nucleosynthesis published so far,the scenario for the production of copper in stars remains elusive. Inparticular, it is still debated whether copper originates mostly inmassive stars or in Type Ia supernovae. To answer this question, wecompute self-consistent chemical evolution models taking into accountthe results of updated stellar nucleosynthesis. By contrasting copperevolution in ω Cen and the Milky Way, we end up with a picturewhere massive stars are the major factor responsible for the productionof copper in ω Cen as well as the Galactic disc.
|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.
|The Rotation of Subpopulations in ω Centauri|
We present the first result of the Ital-FLAMES survey of red giantbranch (RGB) stars in ω Cen. Radial velocities with a precision of~0.5 km s-1 are presented for 650 members of ω Cenobserved with FLAMES-GIRAFFE at the Very Large Telescope. We found thatstars belonging to the metal-poor (RGB-MP), metal-intermediate(RGB-MInt), and metal-rich (RGB-a) subpopulations of ω Cen are allcompatible with having the same rotational pattern. Our results appearto contradict past findings by Norris et al., who could not detect anyrotational signature for metal-rich stars. The slightly higher precisionof the present measurements and the much larger sample size, especiallyfor the stars richer in metals, appear as the most likely explanationsfor this discrepancy. The result presented here weakens the body ofevidence in favor of a merger event in the past history of ω Cen.Based on data obtained with the Giraffe-FLAMES facility of ESO VeryLarge Telescope during the Ital-FLAMES GTO program 71.D-0217(A). Alsobased on data from the VALD and GEISA databases.
|Updated Electron-Conduction Opacities: The Impact on Low-Mass Stellar Models|
We review the theory of electron-conduction opacity, a fundamentalingredient in the computation of low-mass stellar models; shortcomingsand limitations of the existing calculations used in stellar evolutionare discussed. We then present new determinations of theelectron-conduction opacity in stellar conditions for an arbitrarychemical composition that improve over previous works and, mostimportantly, cover the whole parameter space relevant to stellarevolution models (i.e., both the regime of partial and high electrondegeneracy). A detailed comparison with the currently used tabulationsis also performed. The impact of our new opacities on the evolution oflow-mass stars is assessed by computing stellar models along both the H-and He-burning evolutionary phases, as well as main sequence models ofvery low-mass stars and white dwarf cooling tracks.
|Tip of the Red Giant Branch Distances. II. Zero-Point Calibration|
The luminosity of the tip of the red giant branch (TRGB) provides anexcellent measure of galaxy distances and is easily determined in theresolved images of nearby galaxies observed with Hubble Space Telescope.There is now a large amount of archival data that are relevant to theTRGB methodology and offer comparisons with other distance estimators.Zero-point issues related to the TRGB distance scale are reviewed inthis paper. Consideration is given to the metallicity dependence of theTRGB, the transformations between HST flight systems and Johnson-Cousinsphotometry, the absolute magnitude scale based on horizontal branchmeasurements, and the effects of reddening. The zero point of the TRGBis established with a statistical accuracy of 1%, modulo the uncertaintyin the magnitude of the horizontal branch, with a typical rmsuncertainty of 3% in individual galaxy distances at high Galacticlatitude. The zero point is consistent with the Cepheidperiod-luminosity relation scale but invites reconsideration of theclaimed metallicity dependence with that method. The maser distance toNGC 4258 is consistent with TRGB but presently has lower accuracy.
|Near-Infrared Observations of Globular Clusters in NGC 4472, NGC 4594, NGC 3585, and NGC 5813 and Implications for Their Ages and Metallicities|
We present near-infrared photometry of the globular cluster systems ofthe early-type galaxies NGC 4472, NGC 4594, NGC 3585, and NGC 5813. Wecombine these near-infrared data, obtained with PANIC at the MagellanBaade 6.5 m telescope, with archival optical HST and FORS/VLT data, anduse the optical to near-infrared colors to constrain the ages andmetallicities of the globular clusters around the target galaxies. ForNGC 4472 we have the most extensive near-infrared and opticalphotometric data set. These colors show that the NGC 4472 globularcluster system has a broad metallicity distribution and that theclusters are predominantly old (i.e., ages of about 10 Gyr or more).This result agrees well with earlier spectroscopic work on NGC 4472, andis evidence that the combination of optical to near-infrared colors canidentify predominantly old systems and distinguish these from systemswith a substantial intermediate-age component. Based on the smallersample of combined optical and near-infrared data, NGC 4594 and NGC 3585appear to have predominantly old globular cluster systems, while that ofNGC 5813 may have a more significant age spread. We also match oursample of globular clusters with near-infrared and optical photometry toChandra X-ray source detections in these galaxies, and discuss how theprobability that a globular cluster hosts a low-mass X-ray binarydepends on metallicity and age.
|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.
|The Clusters Ages Experiment (CASE). I. V209 ω Cen: An Eclipsing Post-Common-Envelope Binary in the Globular Cluster ω Cen|
We use photometric and spectroscopic observations of the detachedeclipsing binary V209 ω Cen to derive the masses, radii, andluminosities of the component stars. The system exhibits total eclipsesand, based on the measured systemic velocity and the derived distance,is a member of the globular cluster ω Cen. We obtain 0.945+/-0.043Msolar, 0.983+/-0.015 Rsolar, and 6.68+/-0.88Lsolar for the cooler but larger and more luminous primarycomponent. The secondary component has 0.144+/-0.008 Msolar,0.425+/-0.008 Rsolar, and 2.26+/-0.28 Lsolar. Theeffective temperatures are estimated at 9370 K for the primary and at10,866 K for the secondary. On the color-magnitude diagram of thecluster, the primary component occupies a position between the tip ofthe blue straggler region and the extended horizontal branch, while thesecondary component is located close to the red border of the areaoccupied by hot subdwarfs. However, its radius is too large and itseffective temperature is too low for it to be an sdB star. We propose ascenario leading to the formation of a system with such unusualproperties, with the primary component ``reborn'' from a former whitedwarf that accreted a new envelope through mass transfer from itscompanion. The secondary star has lost most of its envelope whilestarting its ascent onto the subgiant branch. It failed to ignite heliumin its core and is currently powered by a hydrogen-burning shell.This paper uses data obtained with the 6.5 m Magellan Telescopes locatedat Las Campanas Observatory, Chile.
|On the effect of helium enhancement on bolometric corrections and Teff-colour relations|
We evaluate the effects that variations in He content have on bolometriccorrections and T_eff-colour relations. To do this, we computed ATLAS9model atmospheres and spectral energy distributions for effectivetemperatures ranging from 3500 K to 40 000 K for dwarfs and from 3500 Kto 8000 K for giants, considering both “He-non-enhanced” and“He-enhanced” compositions. The variations in He contentthat were considered are of Δ Y = +0.1 and +0.2 for themetallicity [M/H] = +0.5 and Δ Y = +0.1 for [M/H] = -0.5 and -1.5.Synthetic photometry was then carried out for the UBVRIJHK system. Weconclude that the changes in bolometric corrections, caused by theadopted He-enhancements are in general too small (less than 0.01 mag),for both dwarfs and giants, to be affecting present-day tables ofbolometric corrections at a significant level. The only possibleexceptions are found for the U-band at T_eff between 4000 K and 8000 K,where |Δ BCU| amounts to ~0.02 mag, and for T_eff equalto 3500 K, where |Δ BCS_λ| values clearly becomemuch higher (up to 0.06 mag for passbands from U to V). However, even inthe latter case the overall uncertainty caused by variations in the Hecontent may be not so significant, because the ATLAS9 results are stillapproximative at their lowest temperature limit.
|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.
|On the origin of mass-metallicity relations, blue tilts and scaling relations for metal-poor globular cluster systems|
We investigate formation processes and physical properties of globularcluster systems (GCSs) in galaxies based on high-resolution cosmologicalsimulations with globular clusters. We focus on metal-poor clusters(MPCs) and correlations with their host galaxies by assuming that MPCformation is truncated at a high redshift (ztrun >= 6). Wefind that the correlation between mean metallicities (Zgc) ofMPCs and their host galaxy luminosities (L) flattens from z =ztrun to 0. We also find that the observed relation(Zgc ~ L0.15) in MPCs can be reproduced well inthe models with Zgc ~ L0.5 at z = ztrunwhen ztrun ~ 10, if mass-to-light ratios are assumed to beconstant at z = ztrun. A flatter L-Zgc at z =ztrun is found to be required to explain the observedrelation for constant mass-to-light ratio models with lower z =ztrun. However, better agreement with the observed relationis found for models with different mass-to-light ratios between z =ztrun and 0. It is also found that the observedcolour-magnitude relation of luminous MPCs (i.e. `blue tilts') may onlyhave a small contribution from the stripped stellar nuclei of dwarfgalaxies, which have nuclei masses that correlate with their total massat z = ztrun. The simulated blue tilts are found to be seenmore clearly in more massive galaxies, which reflects the fact that moremassive galaxies at z = 0 are formed from a larger number of dwarfs withstellar nuclei formed at z > ztrun. The half-number radii(Re) of GCSs, velocity dispersions of GCSs (σ) andtheir host galaxy masses (Mh) are found to be correlated withone another such that Re ~ Mh0.57 andσ ~ Mh0.32. Based on these results, wediscuss the link between hierarchical merging histories of galaxies andthe physical properties of MPCs, the origin of the L-Zgcrelation and non-homology of GCSs.
|A Triple Main Sequence in the Globular Cluster NGC 2808|
Accurate photometry with HST's ACS shows that the main sequence (MS) ofthe globular cluster NGC 2808 splits into three separate branches. Thethree MS branches may be associated with the complexities of thecluster's horizontal branch and of its abundance distribution. Weattribute the MS branches to successive rounds of star formation, withdifferent helium abundances; we discuss possible sources of heliumenrichment. Some other massive globulars also appear to have complexpopulations; we compare them with NGC 2808.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.
|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.
|Remnant of a ``Wet'' Merger: NGC 34 and Its Young Massive Clusters, Young Stellar Disk, and Strong Gaseous Outflow|
This paper presents new images and spectroscopic observations of NGC 34(Mrk 938) obtained with the du Pont 2.5 m and Baade 6.5 m telescopes atLas Campanas, plus photometry of an archival V image obtained withHubble Space Telescope. This MV=-21.6 galaxy has often beenclassified as a Seyfert 2, yet recently published infrared spectrasuggest a dominant central starburst. We find that the galaxy features asingle nucleus, a main spheroid containing a blue central disk and muchouter fine structure, and tidal tails indicative of two former diskgalaxies. At present these galaxies appear to have completed merging.The remnant shows three clear optical signs that the merger was gas-rich(``wet'') and accompanied by a starburst: (1) It sports a rich system ofyoung star clusters, of which 87 have absolute magnitudes-10.0>=MV>=-15.4. Five clusters with available spectrahave ages in the range 0.1-1.0 Gyr and photometric masses of2×106Msolar<~M<~2×107 Msolarthey are gravitationally bound young globular clusters. (2) The bluecentral disk appears to be young. It is exponential, can be traced to>~10 kpc radius, and has a smooth structure and colors suggestingthat its optical light is dominated by a ~400 Myr old poststarburstpopulation. And (3), the center of NGC 34 drives a strong outflow ofcool, neutral gas, as revealed by broad blueshifted Na I D lines. Thecenter-of-line velocity of this gas is -620 km s-1, while themaximum detected outflow velocity reaches -1050 km s-1.Assessing all available evidence, we suggest that NGC 34 stems from tworecently merged gas-rich disk galaxies with an estimated mass ratio of1/3<~m/M<~2/3. The remnant seems to have first experienced agalaxy-wide starburst that then shrank to its current central andobscured state. The strong gaseous outflow came last.Based in part on observations with the 6.5 m Magellan Telescopes locatedat Las Campanas Observatory, Chile.
|The Globular Cluster NGC 1978 in the Large Magellanic Cloud|
We have used deep, high-resolution Hubble Space Telescope ACSobservations to image the cluster NGC 1978 in the Large MagellanicCloud. This high-quality photometric data set allowed us to confirm thehigh ellipticity (ε~0.30+/-0.02) of this stellar system. Thederived color-magnitude diagram allowed a detailed study of the mainevolutionary sequences; in particular, we have detected the so-calledbump along the red giant branch (at V555=19.10+/-0.10). Thisis the first detection of this feature in an intermediate-age cluster.Moreover, the morphology of the evolutionary sequence and the populationratios have been compared with the expectations of different theoreticalmodels (namely, BaSTI, Pisa Evolutionary Library [PEL], and Padua) inorder to quantify the effect of convective overshooting. The bestagreement (in terms of both morphology and star counts) has been foundwithin the PEL isochrone, with Z=0.008 (consistent with the most recentdetermination of the cluster metallicity, [M/H]=-0.37 dex) and a mildovershooting efficiency (Λos=0.1). By adopting thistheoretical set an age of τ=1.9+/-0.1 Gyr has been obtained.
|Neutron star observations: Prognosis for equation of state constraints|
Electronic Article Available from Elsevier Science.
|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.
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