Contents
Images
Upload your image
DSS Images Other Images
Related articles
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: h.thompson@qub.ac.uk
| Period, Amplitude and Light Curve of V38 in M13
We have measured the period and amplitude of the new red variable starV38 (L414), of M13 (NGC 6205, C 1639+365), using CCDs and amateurtelescopes (35.5-, 30.5- and 20.3-cm) from Spain in the years 2001-2005.The analysis of our CCD frames shows that V38 is a red variable starwith a period of approximately 81 days, and low amplitude nearly 0.08magnitudes: the period agrees with Osborn' s (1977) measures, but theperiod and amplitude with Kopacki' s et al. (2003) data. Also for 14radial velocities published we find a pulsating period P = 83 days,consistent with the others three periods founded.
| 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.
| Deciphering the globular-cluster code.
Not Available
| 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.
| CN Abundance Inhomogeneities in the Globular Cluster Messier 13 (NGC 6205): Results Based on Merged Data Sets from the Literature
The globular cluster Messier 13 (NGC 6205) has been shown by a number ofdifferent studies to be chemically inhomogeneous with regard to elementsranging from C through Al. A database of λ3883 CN-band indices,carbon, oxygen, and sodium abundances for red giants in M13 has beencompiled and homogenized from a variety of literature sources. The dataare used to document the distribution of CN band strength among both redgiant branch (RGB) and asymptotic giant branch (AGB) stars brighter thanMV=+0.8, as well as the relationships between the CNinhomogeneities and the dispersions in other elements. The CNdistribution among the M13 RGB stars is bimodal, although a fewintermediate-CN red giants are present in the cluster. The AGB starsshow uniformly weak CN bands. The spread in CN band strength, onceempirically corrected for effective temperature and gravity differencesamong the red giants, is very well correlated with the sodium abundanceand is anticorrelated with oxygen. Above MV=+0.8, the carbonabundance of red giants decreases with increasing luminosity, as firstfound by N. Suntzeff. There is a rather modest anticorrelation betweenCN band strength and [C/Fe] among RGB stars of comparable magnitude. Theabundance patterns within M13 can be interpreted as a primordial (orpre-RGB phase) abundance spread, coupled with the later effects of deepmixing within the red giants. In the CN-weak giants and those CN-strongstars with [O/Fe]~-0.2+/-0.2 dex on the upper RGB, deep mixing seems tohave been limited largely to the dredge-up of C-->N-processedmaterial. By contrast, there are some M13 stars, most notably a group ofvery oxygen-deficient giants near the RGB tip, but perhaps also afraction of CN-strong giants with lesser oxygen depletions, in whichsome measure of O-->N-processed material also appears to have beenbrought to the surface.
| Abundances of Baade's Window Giants from Keck HIRES Spectra. II. The Alpha and Light Odd Elements
We report detailed abundances of O, Na, Mg, Al, Si, Ca, and Ti-elementsproduced by massive stars-for 27 red giants toward the Galactic bulge inBaade's window. These species are overabundant in the bulge relative tothe disk, consistent with enhancement by Type II SN ejecta. [Mg/Fe]=+0.3dex over the full range of [Fe/H], while O, Si, Ca, and Ti are elevatedbut follow more disklike trends. We propose that the decline in [O/Fe]is due to metallicity-dependent oxygen yields from massive stars,perhaps connected with the Wolf-Rayet phenomenon. The elements Si, Ca,and Ti, believed to be produced during explosive nucleosynthesis,possess identical trends with [Fe/H]. We attribute the decline of theseelements to metallicity-dependent yields in Type II SNe. The trend of[Al/Fe] is found to vary strikingly with environment; the range from theSgr dwarf to the bulge is 0.7 dex. The disjoint composition of thethick/thin disk and bulge stars is inconsistent with models in which thebulge formed from the thickening of the disk, while the elevated alphaelements are consistent with a rapid bulge formation timescale. Thestarkly smaller scatter of [/Fe] with [Fe/H] in the bulgecompared with the halo is consistent with the expectation that the bulgeshould have efficiently mixed. The metal-poor bulge [/Fe]ratios are higher than ~80% of the halo; the bulge could not have formedfrom gas with the present-day halo composition.Based on data obtained at the W. M. Keck Observatory, which is operatedas a scientific partnership among the California Institute ofTechnology, the University of California, and NASA and was made possibleby the generous financial support of the W. M. Keck Foundation.
| 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.
| A probable stellar solution to the cosmological lithium discrepancy
The measurement of the cosmic microwave background has stronglyconstrained the cosmological parameters of the Universe. When themeasured density of baryons (ordinary matter) is combined with standardBig Bang nucleosynthesis calculations, the amounts of hydrogen, heliumand lithium produced shortly after the Big Bang can be predicted withunprecedented precision. The predicted primordial lithium abundance is afactor of two to three higher than the value measured in the atmospheresof old stars. With estimated errors of 10 to 25%, this cosmologicallithium discrepancy seriously challenges our understanding of stellarphysics, Big Bang nucleosynthesis or both. Certain modifications tonucleosynthesis have been proposed, but found experimentally not to beviable. Diffusion theory, however, predicts atmospheric abundances ofstars to vary with time, which offers a possible explanation of thediscrepancy. Here we report spectroscopic observations of stars in themetal-poor globular cluster NGC6397 that reveal trends of atmosphericabundance with evolutionary stage for various elements. Theseelement-specific trends are reproduced by stellar-evolution models withdiffusion and turbulent mixing. We thus conclude that diffusion ispredominantly responsible for the low apparent stellar lithium abundancein the atmospheres of old stars by transporting the lithium deep intothe star.
| 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
| A non-LTE abundance analysis of the post-AGB star ROA5701
An analysis of high-resolution Anglo-Australian Telescope(AAT)/University College London Échelle Spectrograph(UCLÉS) optical spectra for the ultraviolet (UV)-bright starROA5701 in the globular cluster ω Cen (NGC5139) is performed,using non-local thermodynamic equilibrium (non-LTE) model atmospheres toestimate stellar atmospheric parameters and chemical composition.Abundances are derived for C, N, O, Mg, Si and S, and compared withthose found previously by Moehler et al. We find a general metalunderabundance relative to young B-type stars, consistent with theaverage metallicity of the cluster. Our results indicate that ROA5701has not undergone a gas-dust separation scenario as previouslysuggested. However, its abundance pattern does imply that ROA5701 hasevolved off the asymptotic giant branch (AGB) prior to the onset of thethird dredge-up.
| 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.
| Optical/near-infrared colours of early-type galaxies and constraints on their star formation histories
We introduce and discuss the properties of a theoretical (B-K)(J-K)integrated colour diagram for single-age, single-metallicity stellarpopulations. We show how this combination of integrated colours is ableto largely disentangle the well-known age-metallicity degeneracy whenthe age of the population is greater than ~300Myr, and thus providesvaluable estimates of both age and metallicity of unresolved stellarsystems. We discuss in detail the effect on this colour-colour diagramof α-enhanced metal abundance ratios (typical of the oldestpopulations in the Galaxy), the presence of blue horizontal branch starsunaccounted for in the theoretical calibration and of statistical colourfluctuations in low-mass stellar systems. In the case of populationswith multiple stellar generations, the luminosity-weighted mean ageobtained from this diagram is shown to be heavily biased towards theyoungest stellar components. We then apply this method to several datasets for which optical and near-infrared photometry are available in theliterature. We find that Large Magellanic Cloud and M31 clusters havecolours which are consistent with the predictions of the models, butthese do not provide a sensitive test due to the fluctuations which arepredicted by our modelling of the Poisson statistics in such low-masssystems. For the two Local Group dwarf galaxies NGC 185 and 6822, themean ages derived from the integrated colours are consistent with thestar formation histories inferred independently from photometricobservations of their resolved stellar populations.The methods developed here are applied to samples of nearby early-typegalaxies with high-quality aperture photometry in the literature. Asample of bright field and Virgo cluster elliptical galaxies is found toexhibit a range of luminosity-weighted mean ages from 3 to 14Gyr, with amean of ~8Gyr, independent of environment, and mean metallicities at orjust above the solar value. Colour gradients are found in all of thegalaxies studied, in the sense that central regions are redder. Apartfrom two radio galaxies, where the extreme central colours are clearlydriven by the active galactic nucleus, and one galaxy which also shows aradial age gradient, these colour changes appear consistent withmetallicity changes at a constant mean age. Finally, aperture data forfive Virgo early-type dwarf galaxies show that these galaxies appear tobe shifted to lower mean metallicities and lower mean ages (range1-6Gyr) than their higher luminosity counterparts.
| 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.
| Multi-band photometry in Omega Centauri .
We present new medium-band uvby Strömgren and broad-band VIphotometry for the central regions of the globular cluster omega Cen.From this we have obtained differential reddening estimates relative totwo other globulars (M 13 and NGC 288) using a metallicity-independent,reddening-free temperature index, [c]equiv (u-v)-(v-b) - 0.2(b-y), forhot horizontal-branch (HB) stars (T_e> 8,500 K). We estimate colorexcesses of these hot HB stars using optical and near-infrared colors,and find clumpy extinction variations of almost a factor of two withinthe area of the cluster core. In particular, the greatest density ofmore highly reddened objects appears to be shifted along the rightascension axis by 1-1.5' when compared with less reddened ones. Thesefindings complicate photometric efforts to investigate the starformation history of omega Cen.
| Multiple helium abundances in globular clusters stars: consequences for the horizontal branch and RR Lyrae
Most inhomogeneities in the chemical composition of GC stars are due toprimordial enrichment. The model today most credited is that the windslost by high mass Asymptotic Giant Branch (AGB) stars, evolving during thefirst ≲ 200Myr of the Clusters life, directly form a second generation of stars with abundance anomalies. The best indirect hint towards this suggestion is the recognition that some peculiarities in the Horizontal Branch (HB) starsdistribution (blue tails, gaps, anomalous luminosity slope of the flatpart of the HB) can be attributed to the larger helium abundance in thematter, processed through Hot Bottom Burning, from which these stars areborn. The model has been reinforced by finding a peculiar main sequencedistribution in the cluster NGC 2808, which also has a bimodal HBdistribution and an extended blue tail: the excess of blue objects onthe main sequence has been interpreted as stars with very high helium.We remark that the RR Lyr distribution may be affected by the heliumspread, and this can be at the basis of the very long periods of theRRab variables of the metal rich clusters NGC 6388 and NGC 6441, longerthan for the very metal poor Oosterhoff II clusters. These periods implythat the RR Lyr are brighter than expected for their metallicities,consistent with a larger helium abundance.
| 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.
| Low mass stars
Low-mass stars dominate the population of clusters and old galaxies andserve as important tools for understanding the history of galaxies. Iwill review some recent developments and open questions concerning ourunderstanding of these comparatively simple stellar objects.
| 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.
| Beyond the big Galaxy: the structure of the stellar system 1900 - 1952
Not Available
| Different Characteristics of the Bright Branches of the Globular Clusters M15 and M92
We carried out relatively wide-field BVI CCD photometric observations ofthe globular clusters M15 (NGC 7078) and M92 (NGC 6341) using the 1.8 mtelescope of the Bohyun Optical Astronomy Observatory. We presentcolor-magnitude diagrams (V vs. B-V, V vs. V-I, and V vs. B-I) of M15and M92. We found asymptotic giant branch (AGB) bumps atVbumpAGB=15.20+/-0.05 mag andVbumpAGB=14.50+/-0.05 mag for M15 and M92,respectively. We identified the red giant branch (RGB) bumps of the twoclusters. We have estimated the population ratios R and R2for M15 and M92 in two cases: when only normal horizontal-branch (HB)stars are used and when all the HB stars are used. We have compared theobserved RGB luminosity functions of M15 and M92 with the theoreticalRGB luminosity functions of Bergbusch & VandenBerg and found nosignificant ``extra stars'' in the comparisons. This implies that the HBmorphology difference between M15 and M92 is not certain due to theresults of deep mixing in the RGB sequence.
| Vertical abundance stratification in the blue horizontal branch star HD 135485
Context: It is commonly believed that the observed overabundances ofmany chemical species relative to the expected cluster metallicity inblue horizontal branch (BHB) stars appear as a result of atomicdiffusion in the photosphere. The slow rotation of BHB stars (with T_eff> 11 500 K), typically v sin{i} < 10 km s-1, isconsistent with this idea. Aims: In this work we search forobservational evidence of vertical chemical stratification in theatmosphere of HD 135485. If this evidence exists, it will demonstratethe importance of atomic diffusion processes in the atmospheres of BHBstars. Methods: We undertake an extensive abundance stratificationanalysis of the atmosphere of HD 135485, based on recently acquired highresolution and S/N CFHT ESPaDOnS spectra and a McDonald-CE spectrum. Results: Our numerical simulations show that nitrogen and sulfur revealsignatures of vertical abundance stratification in the stellaratmosphere. It appears that the abundances of these elements increasetoward the upper atmosphere. This fact cannot be explained by theinfluence of microturbulent velocity, because oxygen, carbon, neon,argon, titanium and chromium do not show similar behavior and theirabundances remain constant throughout the atmosphere. It seems that theiron abundance may increase marginally toward the lower atmosphere. Thisis the first demonstration of vertical abundance stratification ofmetals in a BHB star.Based on observations made with ESPaDOnS at the Canada-France-HawaiiTelescope (CFHT) operated by the National Research Council (NRC) ofCanada, the Institut des Sciences de l'Univers of the Centre National dela Recherche Scientifique (CNRS) and the University of Hawaii and onobservations made with Echelle Spectrograph on the McDonald Observatory2.1-m Otto Struve Telescope. Full Table 2 is only available inelectronic form at http://www.aanda.org
| A Spectroscopic Search for New Sdb Stars from the Galex Survey
We have recently initiated a systematic search for the UV-bright,subdwarf B (sdB) stars in the Milky Way. The sdB stars are coreHe-burning stars with very thin H envelopes and are known to beimmediate progenitors of white dwarfs, but their formation mechanism isstill enigmatic. For instance, it is not clear whether such objects areborn as single stars or can form only in binary systems. The numberratio of sdB stars in each Galactic stellar population (i.e., thin disk,thick disk and halo) may give a clue to which of the suggested formationmechanisms dominates: the binary scenario or the RGB-peel-off scheme.This approach was hampered by the lack of identified sdB stars belongingto the thin disk and halo. Thus, it is of primary importance to find newsdB stars that are faint (halo) and lie at lower Galactic latitudes(thin disk). In this contribution, we will describe the motivation andplan for our spectroscopic survey and preliminary results based on pilotobservations for 34 sdB star candidates from the GALEX All-sky ImagingSurvey.
| Neutron star observations: Prognosis for equation of state constraints
Electronic Article Available from Elsevier Science.
| Do EC 14026 Stars Exist in Clusters?
We have began a program to search for pulsating sdB stars in open andglobular clusters. Detection of pulsators in clusters would aid theunderstanding of their evolution, since the masses and metal content ofthe main-sequence progenitors would be known. Here we describe thebeginning of our search during which we plan to observe about 100 sdBstars. A failure to detect any pulsators would suggest differencesbetween sdB stars in clusters and in the field of the Milky Way (e.g.,metallicity, binarity) which would need to be taken into account by anypulsation/evolution scenario.
| The Nature of the Hot Stars in the Bulge of Globular Cluster NGC 6388
The metal-rich bulge globular clusters NGC 6388 and NGC 6441 showdistinct blue horizontal-branch tails in their color-magnitude diagrams.They are thus strong cases of the well known Second Parameter Problem.In addition, the horizontal branches in these globular clusters show anupward tilt toward bluer colors, which cannot be explained by canonicalevolutionary models. We will discuss several scenarios which have beenproposed to explain these two features and present observations obtainedto test these scenarios.
| An Orphan in the ``Field of Streams''
We use Sloan Digital Sky Survey Data Release 5 photometry andspectroscopy to study a tidal stream that extends over ~50° in thenorth Galactic cap. From the analysis of the path of the stream and thecolors and magnitudes of its stars, the stream is~20+7-5 kpc away at its nearest detection (thecelestial equator). We detect a distance gradient: the stream is fartheraway from us at higher declination. The contents of the stream are madeup from a predominantly old and metal-poor population that is similar tothe globular clusters M13 and M92. The integrated absolute magnitude ofthe stream stars is estimated to be Mr~-7.5. There istentative evidence for a velocity signature, with the stream moving at~-40 km s-1 at low declinations and ~+100 km s-1at high declinations. The stream lies on the same great circle asComplex A, a roughly linear association of H I high-velocity cloudsstretching over ~30° on the sky, and as Ursa Major II, a recentlydiscovered dwarf spheroidal galaxy. Lying close to the same great circleare a number of anomalous, young, and metal-poor globular clusters,including Palomar 1 and Ruprecht 106.
| Dynamical Formation of Close Binaries in Globular Clusters: Cataclysmic Variables
We answer the long-standing question of which production mechanism isresponsible for the cataclysmic variables (CVs) in globular clusters.Arguments have been given that range from mostly primordial presence toa significant contribution of later dynamical formation in close stellarencounters. We conclude, based on a thorough analysis of a homogeneousChandra data set, that the majority of CVs in dense globular clustershave a dynamical origin.
| The Global Kinematics of the Globular Cluster M92
We report the determination of high-accuracy radial velocities for 306members of the globular cluster M92 using the Hydra multiobjectspectrograph on the WIYN telescope. We have concentrated on starsoutside of the central region of the cluster, located up to 14.4' fromthe cluster center. Candidate members were selected for spectroscopybased on a photometric metallicity index determined from three-bandWashington photometry, also obtained with the WIYN telescope. The medianerror in the velocities is 0.35 km s-1. We find theheliocentric radial velocity of the cluster to be -121.2+/-0.3 kms-1. We have used an improved Bayesian analysis to determinethe velocity dispersion profile of M92. The most probable profile is acored power law with a scale radius of 2', a velocity dispersion at 1'of 6.3 km s-1, and an outer power law with a slope of -0.6.We have also reanalyzed the M15 radial velocities of Drukier et al. andfind that a pure power law with a 1' velocity dispersion of 8 kms-1 and a slope of -0.5 and the combination of a power lawwith a slope of -0.4 and a scale of 7.5 km s-1 inside 9' anda dispersion of 4 km s-1 outside are equally likely. In bothclusters there is evidence that the samples include escaping stars. Wepresent results from a GRAPE-based N-body simulation of an isolatedcluster that demonstrates this effect. We suggest additional tests todetermine the relative importance of tidal heating and stellar ejectionfor establishing the velocity field in globular cluster halos.
| Detection of a 60°-long Dwarf Galaxy Debris Stream
We report on a 60°-long stream of stars, extending from Ursa Majorto Sextans, in the Sloan Digital Sky Survey. The stream is approximately2° wide and is clearly distinct from the northern tidal arm of theSagittarius dwarf galaxy. The apparent width of the stream indicates aprogenitor with a size and mass similar to that of a dwarf galaxy. Thestream is about 21 kpc distant and appears to be oriented almostperpendicular to our line of sight. The visible portion of the streamdoes not pass near any known dwarf galaxies, although we cannot rule outthat the stream may form the inner part of a known dwarf galaxy's orbit.The most likely explanation is that the stream constitutes the remainsof a dwarf galaxy that has been completely disrupted at some point inthe past. We also briefly report on the discovery of a diminutiveGalactic satellite that lies near the projected path of the new streambut is unlikely to be related to it.
| 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.
| On the Primordial Scenario for Abundance Variations within Globular Clusters: The Isochrone Test
Self-enrichment processes occurring in the early stages of a globularcluster lifetime are generally invoked to explain the observed CNONaMgAlabundance anticorrelations within individual Galactic globular clusters.We have tested, with fully consistent stellar evolution calculations,whether theoretical isochrones for stars born with the observedabundance anticorrelations satisfy the observational evidence thatobjects with different degrees of these anomalies lie on essentiallyidentical sequences in the color-magnitude diagram (CMD). To thispurpose, we have computed for the first time low-mass stellar models andisochrones with an initial metal mixture that includes the extremevalues of the observed abundance anticorrelations and varying initial Hemass fractions. Comparisons with ``normal'' α-enhanced isochronesand suitable Monte Carlo simulations that include photometric errorsshow that a significant broadening of the CMD sequences occurs only ifthe helium enhancement is extremely large (in this study, when Y=0.35)in the stars showing anomalous abundances. Stellar luminosity functionsup to the red giant branch tip are also very weakly affected, apartfrom-depending on the He content of the polluting material-the red giantbranch bump region. We also study the distribution of stars along thezero-age horizontal branch and derive general constraints on therelative location of objects with and without abundance anomalies alongthe observed horizontal branches of globular clusters.
| 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 (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/939
| Detection of a 63° Cold Stellar Stream in the Sloan Digital Sky Survey
We report on the detection in Sloan Digital Sky Survey data of a63°-long tidal stream of stars, extending from Ursa Major to Cancer.The stream has no obvious association with the orbit of any knowncluster or galaxy. The contrast of the detected stream is greatest whenusing a star count filter that is matched to the color-magnitudedistribution of stars in M13, which suggests that the stars making upthe stream are old and metal-poor. The visible portion of the stream isvery narrow and about 8.5 kpc above the Galactic disk, suggesting thatthe progenitor is or was a globular cluster. While the surface densityof the stream varies considerably along its length, its path on the skyis very smooth and uniform, showing no evidence of perturbations bylarge mass concentrations in the nearby halo. While definitiveconstraints cannot be established without radial velocity information,the stream's projected path and estimates of its distance suggest thatwe are observing the stream near the perigalacticon of its orbit.
| 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/927
| Various Modes of Helium Mixing in Globular Cluster Giants and Their Possible Effects on the Horizontal Branch Morphology
It has been known for a long time that some red giants in globularclusters exhibit large star-to-star variations in the abundances oflight elements that are not exhibited by field giants. This fact can betaken as evidence that the extra mixing mechanism(s) that operate inglobular cluster giants may be consequences of star-star interactions inthe dense stellar environment. In order to constrain the extra mixingmechanism(s), we study the influence of helium enrichment along the redgiant branch on the evolution of stars through the horizontal branch(HB). Three possible modes of helium enrichment are considered,associated with close encounters of stars in the globular clusters. Weshow that as a consequence of the variations in the core mass, as wellas in the total mass due to mass loss, the colors of horizontal branchmodels are distributed over almost the entire range of the horizontalbranch. The results are discussed in relation to a scenario for theorigin of the abundance anomalies and for the effects on the morphologyof the horizontal branch. We argue that the star-star interactions cannot only explain the source of the angular momentum of rapid rotationbut also provide a mechanism for the bimodal distribution of rotationrates in some globular clusters. We also propose the time elapsed fromthe latest core-collapse phase during the gravothermal oscillations asthe second parameter to explain the variations in HB morphology amongthe globular clusters.
| NGC 6441: another indication of very high helium content in globular cluster stars
Context: The metal-rich bulge globular cluster NGC6441 shows both a well-developed blue horizontal branch (Richet al.), and a strong slope upward from the red clump to the blue of theRR Lyrae region. The former corresponds to the well-known secondparameter problem. Both features are not explained by conventionalevolutionary models. Aims: Helium self-enrichment is proposed as apossible solution to both questions, a mechanism already invoked for theinterpretation of the peculiarities in NGC 2808 andM 13. Methods: We make use of horizontal branchsimulations that cover a wide range of main sequence helium abundance toinvestigate whether the main features of the NGC 6441 horizontal branchpopulation, including the RR Lyrae variables period, can be reproduced. Results: To describe the horizontal branch of NGC 6441, the followingmust be met. The helium content Y in the red clump must reach at least0.35; values up to Y˜0.37 are necessary to populate the RR Lyrregion, also reproducing the observed mean long periods; depending onthe dispersion in mass loss assumed in the simulations, values up toY˜0.38-0.40 are necessary to populate the blue HB. The totalself-enriched population amounts to ~60% of the whole stellar content. Conclusions: . Self-enrichment and multiple star-formation episodes inthe early evolution of globular clusters appear more and more able toaccount for many of the chemical and population peculiarities observedin these systems. The very large helium abundances (Y⪆0.35) requiredfor ~14% of the horizontal branch population pose some problems for theenrichment mechanisms.
| 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.
| A Large Stellar Evolution Database for Population Synthesis Studies. II. Stellar Models and Isochrones for an α-enhanced Metal Distribution
We present a large, new set of stellar evolution models and isochronesfor an α-enhanced metal distribution typical of Galactic halo andbulge stars; it represents a homogeneous extension of our stellar modellibrary for a scaled-solar metal distribution already presented byPietrinferni et al. The effect of the α-element enhancement hasbeen properly taken into account in the nuclear network, opacity,equation of state, and for the first time in the bolometric correctionsand color transformations. This allows us to avoid the inconsistentuse-common to all α-enhanced model libraries currentlyavailable-of scaled-solar bolometric corrections and colortransformations for α-enhanced models and isochrones. We show howbolometric corrections to magnitudes obtained for the U, B portion ofstellar spectra for Teff<=6500 K are significantlyaffected by the metal mixture, especially at the higher metallicities.Our models cover both an extended mass range (between 0.5 and 10Msolar, with a fine mass spacing) and a broad metallicityrange, including 11 values of the metal mass fraction Z, correspondingto the range -2.6<=[Fe/H]<=0.05. The initial He mass fraction isY=0.245 for the most metal-poor models, and it increases with Z,according to ΔY/ΔZ=1.4. Models with and without theinclusion of overshoot from the convective cores during the centralH-burning phase are provided, as well as models with different mass lossefficiencies. We also provide complete sets of evolutionary models forlow-mass, He-burning stellar structures cover the whole metallicityrange. This database, used in combination with our scaled-solar modellibrary, is a valuable tool for investigating both Galactic andextragalactic simple and composite stellar populations, using stellarpopulation synthesis techniques.
| An Empirical Calibration of the Mixing-Length Parameter α
We present an empirical calibration of the mixing-length free parameterα based on a homogeneous infrared database of 28 Galactic globularclusters spanning a wide metallicity range (-2.15<[Fe/H]<-0.2).Empirical estimates of the red giant effective temperatures have beenobtained from infrared colors. Suitable relations linking thesetemperatures to the cluster metallicity have been obtained and comparedto theoretical predictions. An appropriate set of models for the Sun andPopulation II giants has been computed by using both the standard solarmetallicity (Z/X)solar=0.0275 and the most recently proposedvalue (Z/X)solar=0.0177. We find that when the standard solarmetallicity is adopted, a unique value of α=2.17 can be used toreproduce both the solar radius and the Population II red gianttemperature. Conversely, when the new solar metallicity is adopted, twodifferent values of α are required: α=1.86 to fit the solarradius and α~2.0 to fit the red giant temperatures. However, itmust be noted that regardless the adopted solar reference, theα-parameter does not show any significant dependence onmetallicity.Based on observations collected at the European Southern Observatory(ESO), La Silla, Chile. Also based on observations made with the ItalianTelescopio Nazionale Galileo (TNG) operated on the island of La Palma bythe Fundacion Galileo Galilei of the INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
| 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| From Canonical to Enhanced Extra Mixing in Low-Mass Red Giants: Tidally Locked Binaries
Stellar models that incorporate simple diffusion or shear-induced mixingare used to describe canonical extra mixing in low-mass red giants oflow and solar metallicity. These models are able to simultaneouslyexplain the observed Li and CN abundance changes along the upper redgiant branch (RGB) in field low-metallicity stars and match photometry,rotation, and 12C/13C ratios for stars in the oldopen cluster M67. The shear mixing model requires that main-sequence(MS) progenitors of upper RGB stars possessed rapidly rotating radiativecores and that specific angular momentum was conserved in each of theirmass shells during their evolution. We surmise that solar-type starswill not experience canonical extra mixing on the RGB because their moreefficient MS spin-down resulted in solid-body rotation, as revealed byhelioseismological data for the Sun. Thus, RGB stars in the old,high-metallicity cluster NGC 6791 should show no evidence for mixing intheir 12C/13C ratios. We develop the idea thatcanonical extra mixing in a giant component of a binary system may beswitched to its enhanced mode with much faster and somewhat deepermixing as a result of the giant's tidal spin-up. This scenario canexplain photometric and composition peculiarities of RS CVn binaries.The tidally enforced enhanced extra mixing might contribute to thestar-to-star abundance variations of O, Na, and Al in globular clusters.This idea may be tested with observations of12C/13C ratios and CN abundances in RS CVnbinaries.
| Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters
We present a homogeneous set of stellar atmospheric parameters(Teff, logg, [Fe/H]) for MILES, a new spectral stellarlibrary covering the range λλ 3525-7500Å at2.3Å (FWHM) spectral resolution. The library consists of 985 starsspanning a large range in atmospheric parameters, from super-metal-rich,cool stars to hot, metal-poor stars. The spectral resolution, spectraltype coverage and number of stars represent a substantial improvementover previous libraries used in population synthesis models. Theatmospheric parameters that we present here are the result of aprevious, extensive compilation from the literature. In order toconstruct a homogeneous data set of atmospheric parameters we have takenthe sample of stars of Soubiran, Katz & Cayrel, which has very welldetermined fundamental parameters, as the standard reference system forour field stars, and have calibrated and bootstrapped the data fromother papers against it. The atmospheric parameters for our clusterstars have also been revised and updated according to recent metallicityscales, colour-temperature relations and improved set of isochrones.
| Chemical Compositions of Red Giant Stars in Old Large Magellanic Cloud Globular Clusters
We have observed 10 red giant stars in four old Large Magellanic Cloudglobular clusters with the high-resolution spectrograph MIKE on theMagellan Landon Clay 6.5 m telescope. The stars in our sample have up to20 elemental abundance determinations for the α-, iron peak, andneutron-capture element groups. We have also derived abundances for thelight odd-Z elements Na and Al. We find NGC 2005 and NGC 2019 to be moremetal-rich than previous estimates from the Ca II triplet, and we derive[Fe/H] values closer to those obtained from the slope of the red giantbranch. However, we confirm previous determinations for Hodge 11 and NGC1898 to within 0.2 dex. The LMC cluster [Mg/Fe] and [Si/Fe] ratios arecomparable to the values observed in old Galactic globular clusterstars, as are the abundances [Y/Fe], [Ba/Fe], and [Eu/Fe]. The LMCclusters do not share the low-Y behavior observed in some dwarfspheroidal galaxies. [Ca/Fe], [Ti/Fe], and [V/Fe] in the LMC, however,are significantly lower than what is seen in the Galactic globularcluster system. Neither does the behavior of [Cu/Fe] as a function of[Fe/H] in our LMC clusters match the trend seen in the Galaxy, stayinginstead at a constant value of roughly -0.8. Because not all[α/Fe] ratios are suppressed, these abundance ratios cannot beattributed solely to the injection of Type Ia supernova material andinstead reflect the differences in star formation history of the LMCversus the Milky Way. An extensive numerical experimental study wasperformed, varying both input parameters and stellar atmosphere models,to verify that the unusual abundance ratios derived in this study arenot the result of the adopted atomic parameters, stellar atmospheres, orstellar parameters. We conclude that many of the abundances in the LMCglobular clusters we observed are distinct from those observed in theMilky Way, and these differences are intrinsic to the stars in thosesystems.
| Cats and Dogs, Hair and a Hero: A Quintet of New Milky Way Companions
We present five new satellites of the Milky Way discovered in SloanDigital Sky Survey (SDSS) imaging data, four of which were followed upwith either the Subaru or the Isaac Newton Telescopes. They include fourprobable new dwarf galaxies-one each in the constellations of ComaBerenices, Canes Venatici, Leo, and Hercules-together with one unusuallyextended globular cluster, Segue 1. We provide distances, absolutemagnitudes, half-light radii, and color-magnitude diagrams for all fivesatellites. The morphological features of the color-magnitude diagramsare generally well described by the ridge line of the old, metal-poorglobular cluster M92. In the past two years, a total of 10 new Milky Waysatellites with effective surface brightness μv>~28 magarcsec-2 have been discovered in SDSS data. They are lessluminous, more irregular, and apparently more metal-poor than thepreviously known nine Milky Way dwarf spheroidals. The relationshipbetween these objects and other populations is discussed. We note thatthere is a paucity of objects with half-light radii between ~40 and ~100pc. We conjecture that this may represent the division between starclusters and dwarf galaxies.Based in part on data collected at Subaru Telescope, which is operatedby the National Astronomical Observatory of Japan.
| Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular Clusters
We have mosaicked Two Micron All Sky Survey (2MASS) images to derivesurface brightness profiles in J, H, and Ks for 104 Galacticglobular clusters. We fit these with King profiles and show that thecore radii are identical to within the errors for each of these IRcolors and are identical to the core radii at V in essentially allcases. We derive integrated-light colors V-J, V-H, V-Ks, J-H,and J-Ks for these globular clusters. Each color shows areasonably tight relation between the dereddened colors and metallicity.Fits to these are given for each color. The IR - IR colors have verysmall errors, due largely to the all-sky photometric calibration of the2MASS survey, while the V-IR colors have substantially largeruncertainties. We find fairly good agreement with measurements ofintegrated-light colors for a smaller sample of Galactic globularclusters by M. Aaronson, M. Malkan, and D. Kleinmann from 1977. Ourresults provide a calibration for the integrated light of distantsingle-burst old stellar populations from very low to solarmetallicities. A comparison of our dereddened measured colors withpredictions from several models of the integrated light of single-burstold populations shows good agreement in the low-metallicity domain forV-Ks colors but also shows an offset at a fixed [Fe/H] of~0.1 mag in J-Ks, which we ascribe to photometric systemtransformation issues. Some of the models fail to reproduce the behaviorof the integrated-light colors of the Galactic globular clusters nearsolar metallicity.
| Rubidium and Lead Abundances in Giant Stars of the Globular Clusters M13 and NGC 6752
We present measurements of the neutron-capture elements Rb and Pb infive giant stars of the globular cluster NGC 6752 and Pb measurements infour giants of the globular cluster M13. The abundances were derived bycomparing synthetic spectra with high-resolution, high signal-to-noiseratio spectra obtained using HDS on the Subaru telescope and MIKE on theMagellan telescope. The program stars span the range of the O-Alabundance variation. In NGC 6752, the mean abundances are[Rb/Fe]=-0.17+/-0.06 (σ=0.14), [Rb/Zr]=-0.12+/-0.06(σ=0.13), and [Pb/Fe]=-0.17+/-0.04 (σ=0.08). In M13 the meanabundance is [Pb/Fe]=-0.28+/-0.03 (σ=0.06). Within the measurementuncertainties, we find no evidence for star-to-star variation for eitherRb or Pb within these clusters. None of the abundance ratios [Rb/Fe],[Rb/Zr], or [Pb/Fe] are correlated with the Al abundance. NGC 6752 mayhave slightly lower abundances of [Rb/Fe] and [Rb/Zr] compared to thesmall sample of field stars at the same metallicity. For M13 and NGC6752 the Pb abundances are in accord with predictions from a Galacticchemical evolution model. If metal-poor intermediate-mass asymptoticgiant branch stars did produce the globular cluster abundance anomalies,then such stars do not synthesize significant quantities of Rb or Pb.Alternatively, if such stars do synthesize large amounts of Rb or Pb,then they are not responsible for the abundance anomalies seen inglobular clusters.Based in part on data collected at the Subaru Telescope, which isoperated by the National Astronomical Observatory of Japan, and onobservations made with the Magellan Clay Telescope at Las CampanasObservatory.
| Colour-colour diagrams and extragalactic globular cluster ages. Systematic uncertainties using the (V - K) - (V - I) diagram
Context: Age and metallicity estimates for extragalactic globularclusters, from integrated colour-colour diagrams, are examined. Aims:We investigate biases in cluster ages and [Fe/H] estimated from the(V-K)-(V-I) diagram, arising from inconsistent Horizontal Branchmorphology, metal mixture, treatment of core convection between observedclusters and the theoretical colour grid employed for age andmetallicity determinations. We also study the role played by statisticalfluctuations of the observed colours, caused by the low total mass oftypical globulars. Methods: Synthetic samples of globular clustersystems are created, by means of Monte-Carlo techniques. Each sampleaccounts for a different possible source of bias, among the onesaddressed in this investigation. Cumulative age and [Fe/H] distributionsare then retrieved by comparisons with a reference theoreticalcolour-colour grid, and analyzed. Results: Horizontal Branch morphologyis potentially the largest source of uncertainty. A single-age systemharbouring a large fraction of clusters with an HB morphologysystematically bluer than the one accounted for in the theoreticalcolour grid, can simulate a bimodal population with an age difference aslarge as ~8 Gyr. When only the redder clusters are considered, thisuncertainty is almost negligible, unless there is an extreme mass lossalong the Red Giant Branch phase. The metal mixture affects mainly theredder clusters; the effect of colour fluctuations becomes negligiblefor the redder clusters, or when the integrated MV isbrighter than ~-8.5 mag. The treatment of core convection is relevantfor ages below ~4 Gyr. The retrieved cumulative [Fe/H] distributions areoverall only mildly affected. Colour fluctuations and convective coreextension have the largest effect. When 1σ photometric errorsreach 0.10 mag, all biases found in our analysis are erased, and bimodalage populations with age differences of up to ~8 Gyr go undetected. Theuse of both (U-I)-(V-K) and (V-I)-(V-K) diagrams may help disclosing thepresence of blue HB stars unaccounted for in the theoretical colourcalibration.
| Mg Isotope Ratios in Giant Stars of the Globular Clusters M13 and M71
We present Mg isotope ratios in four red giants of the globular clusterM13 and one red giant of the globular cluster M71 based onhigh-resolution, high signal-to-noise ratio spectra obtained with HDS onthe Subaru Telescope. We confirm earlier results by Shetrone that forM13 the ratio varies from(25Mg+26Mg)/24Mg~=1 in stars with thehighest Al abundance to(25Mg+26Mg)/24Mg~=0.2 in stars with thelowest Al abundance. However, we separate the contributions of all threeisotopes and find a considerable spread in the ratio24Mg:25Mg:26Mg, with values rangingfrom 48:13:39 to 78:11:11. As in NGC 6752, we find a positivecorrelation between 26Mg and Al, an anticorrelation between24Mg and Al, and no correlation between 25Mg andAl. In M71, our one star has a Mg isotope ratio of 70:13:17. For bothclusters, even the lowest ratios of 25Mg/24Mg and26Mg/24Mg exceed those observed in field stars atthe same metallicity, a result also found in NGC 6752. The contributionof 25Mg to the total Mg abundance is constant within a givencluster and between clusters with25Mg/(24Mg+25Mg+26Mg)~=0.13.For M13 and NGC 6752, the ranges of the Mg isotope ratios are similarand both clusters show the same correlations between Al and Mg isotopes,suggesting that the same process is responsible for the abundancevariations in these clusters. While existing models fail to reproduceall the observed abundances, we continue to favor the scenario in whichtwo generations of asymptotic giant branch (AGB) stars produce theobserved abundances. A first generation of metal-poor AGB stars pollutesthe entire cluster and is responsible for the large ratios of25Mg/24Mg and 26Mg/24Mgobserved in cluster stars with compositions identical to field stars atthe same metallicity. Differing degrees of pollution by a secondgeneration of AGB stars of the same metallicity as the cluster providesthe star-to-star scatter in Mg isotope ratios.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| 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 Pure Noncollisional Blue Straggler Population in the Giant Stellar System ω Centauri
We have used high spatial resolution data from the Hubble SpaceTelescope (HST) and wide-field ground-based observations to search forblue straggler stars (BSSs) over the entire radial extent of the largestellar system ω Centauri. We have detected the largest populationof BSSs ever observed in any stellar system. Even though the sample isrestricted to the brightest portion of the BSS sequence, more than 300candidates have been identified. BSSs are thought to be produced by theevolution of binary systems (formed either by stellar collisions or massexchange in binary stars). Since systems like Galactic globular clusters(GGCs) and ω Cen evolve dynamically on timescales significantlyshorter than their ages, binaries should have settled toward the center,showing a more concentrated radial distribution than the ordinary, lessmassive single stars. Indeed, in all GGCs that have been surveyed forBSSs, the BSS distribution is peaked at the center. Conversely, inω Cen we find that the BSSs share the same radial distribution asthe adopted reference populations. This is the cleanest evidence everfound that such a stellar system is not fully relaxed even in thecentral region. We further argue that the absence of centralconcentration in the BSS distribution rules out a collisional origin.Thus, the ω Cen BSSs are the purest and largest population ofnoncollisional BSSs ever observed. Our results allow the first empiricalquantitative estimate of the production rate of BSSs via this channel.BSSs in ω Cen may represent the best local template for modelingthe BSS populations in distant galaxies where they cannot beindividually observed.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 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.
| The Discovery of Tidal Tails around the Globular Cluster NGC 5466
We report the discovery of tidal tails around the high-latitude Galacticglobular cluster NGC 5466 in Sloan Digital Sky Survey data. Neuralnetworks are used to reconstruct the probability distribution of clusterstars in ugriz space. The tails are clearly visible once extragalacticcontaminants and field stars have been eliminated. They extend ~4°on the sky, corresponding to ~1 kpc in projected length. The orientationof the tails is in good agreement with the cluster's Galactic orbit, asjudged from the proper-motion data.
| 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).
| Color and Variability Characteristics of Point Sources in the Faint Sky Variability Survey
We present an analysis of the color and variability characteristics forpoint sources in the Faint Sky Variability Survey (FSVS). The FSVScataloged ~23 deg2 in BVI filters from ~16 to 24 mag toinvestigate variability in faint sources at moderate-to-high Galacticlatitudes. Point-source completeness is found to be >83% for aselected representative sample (V=17.5-22.0 mag, B-V=0.0-1.5) containingboth photometric B, V detections and 80% of the time-sampled V dataavailable compared to a basic internal source completeness of 99%.Multiepoch (10-30) observations in V spanning minutes to years modeledby light-curve simulations reveal amplitude sensitivities to~0.015-0.075 mag over a representative V=18-22 mag range. Periodicitydeterminations appear viable to timescales of order 1 day or less usingthe most sampled fields (~30 epochs). The fraction of point sources isfound to be generally variable at 5%-8% over V=17.5-22.0 mag. For Vbrighter than 19 mag, the variable population is dominated bylow-amplitude (<0.05 mag) and blue (B-V<0.35) sources, possiblyrepresenting a population of γ Doradus stars. Overall, thedominant population of variable sources are bluer than B-V=0.65 and havemain-sequence colors, likely reflecting larger populations of RR Lyrae,SX Phe, γ Doradus, and W UMa variables.
| Empirical color transformations between SDSS photometry and other photometric systems
Aims.We present empirical color transformations between the SloanDigital Sky Survey (SDSS) ugriz photometry and the Johnson-Cousins UBVRIsystem and Becker's RGU system, respectively. Owing to the magnitude ofdata that is becoming available in the SDSS photometric system it isparticularly important to be able to convert between this new system andtraditional photometric systems. Unlike earlier publishedtransformations we based our calculations on stars actually measured bythe SDSS with the SDSS 2.5-m telescope. The photometric database of theSDSS provides in a sense a single-epoch set of "tertiary standards"covering more than one quarter of the sky. Our transformations shouldfacilitate their use to easily and reliably derive the correspondingapproximate Johnson-Cousins or RGU magnitudes. Methods: .The SDSSsurvey covers a number of areas that were previously established asstandard fields in the Johnson-Cousins system, in particular, fieldsestablished by Landolt and by Stetson. We used these overlapping fieldsto create well-photometered star samples on which our calculatedtransformations are based. For the RGU photometry we used fieldsobserved in the framework of the new Basel high-latitude field starsurvey. Results: .We calculated empirical color transformationsbetween SDSS photometry and Johnson-Cousins UBVRI and Becker's RGUsystem. For all transformations we found linear relations to besufficient. Furthermore we showed that the transformations between theJohnson-Cousins and the SDSS system have a slight dependence onmetallicity.
| Surface Brightness Profiles of Galactic Globular Clusters from Hubble Space Telescope Images
The Hubble Space Telescope (HST) allows us to study the central surfacebrightness profiles of globular clusters at unprecedented detail. Wehave mined the HST archives to obtain 38 WFPC2 images of Galacticglobular clusters with adequate exposure times and filters, which we useto measure their central structure. We outline a reliable method toobtain surface brightness profiles from integrated light that we test onan extensive set of simulated images. Most clusters have central surfacebrightness about 0.5 mag brighter than previous measurements made fromground-based data, with the largest differences around 2 mag. Includingthe uncertainties in the slope estimates, the surface brightness slopedistribution is consistent with half of the sample having flat cores andthe remaining half showing a gradual decline from 0 to -0.8[dlogΣ/dlogr)]. We deproject the surface brightness profiles in anonparametric way to obtain luminosity density profiles. Thedistribution of luminosity density logarithmic slopes shows similarfeatures, with half of the sample between -0.4 and -1.8. These resultsare in contrast to our theoretical bias that the central regions ofglobular clusters are either isothermal (i.e., flat central profiles) orvery steep (i.e., luminosity density slope approximately -1.6) forcore-collapse clusters. With only 50% of our sample having centralprofiles consistent with isothermal cores, King models appear torepresent most globular clusters in their cores poorly.
| 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.
| Manganese Abundances in Cluster and Field Stars
We have derived Mn abundances for more than 200 stars in 19 globularclusters. In addition, Mn abundance determinations have been made for acomparable number of halo field and disk stars possessing an overlappingrange of metallicities and stellar parameters. Our primary data set wascomprised of high-resolution spectra previously acquired at theMcDonald, Lick, and Keck Observatories. To enlarge our data pool, weacquired globular and open cluster spectra from several otherinvestigators. Data were analyzed using synthetic spectra of the 6000Å Mn I triplet. Hyperfine structure parameters were included inthe synthetic spectra computations. Our analysis shows that for themetallicity range -0.7>[Fe/H]>-2.7, stars of 19 globular clustershave a mean relative abundance of <[Mn/Fe]>=-0.37+/-0.01(σ=0.10), a value in agreement with that of the field stars,<[Mn/Fe]>=-0.36+/-0.01 (σ=0.08). Despite the 2 orders ofmagnitude span in metallicity, the <[Mn/Fe]> ratio remainsconstant in both stellar populations. Our Mn abundance data indicatethat there is no appreciable variation in the relative nucleosyntheticcontribution from massive stars that undergo core-collapse supernovaeand thus no significant change of the associated initial mass functionin the specified metallicity range.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
INHABITED SKY
