Metal-poor Clusters Research Articles

Low-resolution spectroscopy of main sequence stars belonging to 12 Galactic globular clusters

Globular clusters show abundance variations for light elements that are not yet well understood. The preferred explanation involves a self-enrichment scenario, with two subsequent generations of stars. Observations of main sequence stars allow us to investigate the signature of this chemically processed material without the complicating effects of internal mixing. Our goal is to investigate the C-N anti-correlation with low-resolution spectroscopy of 20-50 stars fainter than the first dredge-up in seven globular clusters (NGC288, NGC1851, NGC5927, NGC6352, NGC6388, and Pal12) with different properties. We complemented our observations with 47~Tuc archival data, with four additional clusters from the literature (M15, M22, M55, NGC362), and with additional literature data on NGC288. In this first paper, we measured the strength of CN and CH band indices, and we investigated the anti-correlation and bimodality of these indices. We compared r_CN, the ratio of stars belonging to the CN-strong and weak groups, with 15 different cluster parameters. We clearly see bimodal anti-correlation of the CH and CN band stregths in the metal-rich clusters (Pal12, 47Tuc, NGC6352, NGC5927). Only M15 among the metal-poor clusters shows a clearly bimodal anti-correlation. We found weak correlations (sligthly above 1 sigma) of r_CN with the cluster orbital parameters, present-day total mass, cluster concentration, and age. Our findings support the self-enrichment scenario, and suggest that the occurrence of more than two major generations of stars in a GGC should be rare. Small additional generations (<10-20% of the total) would be difficult to detect with our samples. The first generation, which corresponds to the CN-weak stars, usually contains more stars than the second one (<r_CN>=0.82+/-0.29), as opposed to results based on the Na-O anti-correlations.

SPACE VELOCITIES OF SOUTHERN GLOBULAR CLUSTERS. VI. NINE CLUSTERS IN THE INNER MILKY WAY

We have measured the absolute proper motions of nine low-latitude, inner Galaxy globular clusters, namely, NGC 6273 (M 19), NGC 6284, NGC 6287, NGC 6293, NGC 6333 (M 9), NGC 6342, NGC 6356, NGC 6388, and NGC 6441. These are the first determinations ever made for these clusters. The proper motions are on the International Celestial Reference System via Hipparcos. The proper-motion errors range between 0.4 and 0.9 mas yr−1 and are dominated by the number of measurable cluster members in these regions which are very crowded by the bulge/bar and the thick disk. This sample contains five metal-poor ([Fe/H] < −1.0) and four metal-rich ([Fe/H] ⩾ −1.0) clusters; seven clusters are located within ∼4 kpc from the Galactic center, while the remaining two, namely NGC 6356 and NGC 6284, are in the background of the bulge at ∼7.5 kpc from the Galactic center. By combining proper motions with radial velocities and distances from the literature we derive three-dimensional velocities. In a number of cases, distance uncertainties make the kinematical classification ambiguous. For the metal-poor group of clusters, we find that three clusters, namely NGC 6273, NGC 6287, and NGC 6293 are members of a kinematically hot system, the inner halo. As for the remaining two metal-poor clusters, NGC 6284 is located at ∼7.5 kpc from the Galactic center and kinematically belongs to the thick disk, while NGC 6333, located in the inner ∼2 kpc, has an uncertain membership (between halo and thick disk) due to the distance uncertainty. Within the metal-rich group of clusters, NGC 6356 and NGC 6342 have velocities compatible with membership in the thick disk; however, velocity uncertainties do not allow us to rule out their belonging to a hotter system. NGC 6342 is within ∼2 kpc from the Galactic center, and thus it may belong to the bulge. NGC 6356 is at ∼7.5 kpc from the Galactic center and thus its metallicity, kinematics, and location argue together in favor of thick-disk membership. The remaining two metal-rich clusters, NGC 6388 and NGC 6441, have velocities incompatible with membership in the thick disk or the bar of the Milky Way. They can be thought of as members of a kinematically hot system in the inner Galaxy. Curiously, both clusters have similar velocity components. Together with their similar Galactic location and peculiar but similar stellar-population characteristics, these two clusters may share a common origin. Their velocities are also very low indicating that the two clusters are now at apocenter, i.e., they will not leave the inner ∼4 kpc of the Galaxy.

Pre-main-sequence stars in the star-forming complex Sh 2-284

Located at the galactic anticenter, Sh 2-284 is a HII region which harbors several young open clusters; Dolidze 25, a rare metal poor (Z~0.004) young cluster, is one of these. Given its association with Sh 2-284, it is reasonable to assume the low metallicity for the whole HII region. Sh~2-284 is expected to host a significant population of Pre-Main Sequence (PMS) stars of both low and intermediate mass stars (Herbig Ae stars). We aim at characterizing these stars by means of a spectroscopic and photometric survey conducted with VIMOS@VLT and complemented with additional optical and infrared observations. In this survey we selected and characterized 23 PMS objects. We derived the effective temperature, the spectral energy distribution and luminosity of these objects; using theoretical PMS evolutionary tracks, with the appropriate metallicity, we estimated the mass and the age of the studied objects. We also estimated a distance of 4 Kpc for Sh 2-284 by using spectroscopic parallax of 3 OB stars. From the age determination we concluded that triggered star formation is in act in this region. Our results show that a significant fraction of the young stellar objects (YSOs) may have preserved their disk/envelopes, in contrast with what is found in other recent studies of low-metallicity star forming regions in the Galaxy. Finally, among the 23 bona fide PMS stars, we identified 8 stars which are good candidates to pulsators of the delta Scuti type.

MODELING THE METALLICITY DISTRIBUTION OF GLOBULAR CLUSTERS

Observed metallicities of globular clusters reflect physical conditions in the interstellar medium of their high-redshift host galaxies. Globular cluster systems in most large galaxies display bimodal color and metallicity distributions, which are often interpreted as indicating two distinct modes of cluster formation. The metal-rich and metal-poor clusters have systematically different locations and kinematics in their host galaxies. However, the red and blue clusters have similar internal properties, such as the masses, sizes, and ages. It is therefore interesting to explore whether both metal-rich and metal-poor clusters could form by a common mechanism and still be consistent with the bimodal distribution. We present such a model, which prescribes the formation of globular clusters semi-analytically using galaxy assembly history from cosmological simulations coupled with observed scaling relations for the amount and metallicity of cold gas available for star formation. We assume that massive star clusters form only during mergers of massive gas-rich galaxies and tune the model parameters to reproduce the observed distribution in the Galaxy. A wide, but not entire, range of model realizations produces metallicity distributions consistent with the data. We find that early mergers of smaller hosts create exclusively blue clusters, whereas subsequent mergers of more massive galaxies create both red and blue clusters. Thus bimodality arises naturally as the result of a small number of late massive merger events. This conclusion is not significantly affected by the large uncertainties in our knowledge of the stellar mass and cold gas mass in high-redshift galaxies. The fraction of galactic stellar mass locked in globular clusters declines from over 10% at z>3 to 0.1% at present.

Properties of stellar generations in globular clusters and relations with global parameters

ABRIDGED) We revise the formation of Galactic GCs by adding the detailed chemical composition of their different stellar generations (from 1200 giants in 19 GCs) to their global parameters. We propose to identify as GCs those showing the Na-O anticorrelation, and we classify the GCs according to kinematics and location in the Galaxy in disk/bulge, inner, and outer halo. We find that the LF of GCs is fairly independent of their population, suggesting that it is imprinted by the formation mechanism, and only marginally affected by the ensuing evolution. We show that a large fraction of the primordial population should have been lost by the proto-GCs. The extremely low Al abundances found for the primordial population of massive GCs indicate a very fast enrichment process before the formation of the primordial population. We suggest a scenario for the formation of GCs including at least 3 main phases: i) the formation of a precursor population (likely due to the interaction of cosmological structures similar to those leading to dwarf spheroidals, but residing at smaller Rgc, with the early Galaxy or with other structures), ii) which triggers a large episode of star formation (the primordial population), and iii) the formation of the current GC, mainly within a cooling flow formed by the slow winds of a fraction of the primordial population. The precursor population is very effective in raising the metal content in massive and/or metal poor (mainly halo) clusters, while its role is minor in small and/or metal rich (mainly disk) ones. Finally, we use PCA and multivariate relations to study the phase of metal-enrichment from 1st to 2nd generation. Most of the chemical signatures of GCs may be ascribed to a few parameters, the most important being [Fe/H], mass, and age of the cluster, with the location within the Galaxy also playing some role.

SPECTROSCOPY OF M81 GLOBULAR CLUSTERS

We obtained spectra of 74 globular clusters in M81. These globular clusters had been identified as candidates in an HST ACS I-band survey. 68 of these 74 clusters lie within 7' of the M81 nucleus. 62 of these clusters are newly spectroscopically confirmed, more than doubling the number of confirmed M81 GCs from 46 to 108. We determined metallicities for our 74 observed clusters using an empirical calibration based on Milky Way globular clusters. We combined our results with 34 M81 globular cluster velocities and 33 metallicities from the literature and analyzed the kinematics and metallicity of the M81 globular cluster system. The mean of the total sample of 107 metallicities is -1.06 +/- 0.07, higher than either M31 or the Milky Way. We suspect this high mean metallicity is due to an overrepresentation of metal-rich clusters in our sample created by the spatial limits of the HST I-band survey. The metallicity distribution shows marginal evidence for bimodality, with metal-rich and metal-poor peaks approximately matching those of M31 and the Milky Way. The GC system as a whole, and the metal-poor GCs alone, show evidence of a radial metallicity gradient. The M81 globular cluster system as a whole shows strong evidence of rotation, with V_r(deprojected) = 108 +/- 22 km/s overall. This result is likely biased toward high rotational velocity due to overrepresentation of metal-rich, inner clusters. The rotation patterns among globular cluster subpopulations are roughly similar to those of the Milky Way: clusters at small projected radii and metal-rich clusters rotate strongly, while clusters at large projected radii and metal-poor clusters show weaker evidence of rotation.

THE LOW-MASS INITIAL MASS FUNCTION IN THE 30 DORADUS STARBURST CLUSTER

We present deep Hubble Space Telescope (HST) NICMOS 2 F160W band observations of the central 56*57" (14pc*14.25pc) region around R136 in the starburst cluster 30 Dor (NGC 2070) located in the Large Magellanic Cloud. Our aim is to derive the stellar Initial Mass Function (IMF) down to ~1 Msun in order to test whether the IMF in a massive metal-poor cluster is similar to that observed in nearby young clusters and the field in our Galaxy. We estimate the mean age of the cluster to be 3 Myr by combining our F160W photometry with previously obtained HST WFPC2 optical F555W and F814W band photometry and comparing the stellar locus in the color-magnitude diagram with main sequence and pre-main sequence isochrones. The color-magnitude diagrams show the presence of differential extinction and possibly an age spread of a few megayears. We convert the magnitudes into masses adopting both a single mean age of 3 Myr isochrone and a constant star formation history from 2 to 4 Myr. We derive the IMF after correcting for incompleteness due to crowding. The faintest stars detected have a mass of 0.5 Msun and the data are more than 50% complete outside a radius of 5 pc down to a mass limit of 1.1 Msun for 3 Myr old objects. We find an IMF of dN/dlog(M) M^(-1.20+-0.2) over the mass range 1.1--20 Msun only slightly shallower than a Salpeter IMF. In particular, we find no strong evidence for a flattening of the IMF down to 1.1 Msun at a distance of 5 pc from the center, in contrast to a flattening at 2 Msun at a radius of 2 pc, reported in a previous optical HST study. We examine several possible reasons for the different results. If the IMF determined here applies to the whole cluster, the cluster would be massive enough to remain bound and evolve into a relatively low-mass globular cluster.

STRUCTURAL PARAMETERS OF THE MESSIER 87 GLOBULAR CLUSTERS

We derive structural parameters for ~2000 globular clusters in the giant Virgo elliptical M87 using extremely deep Hubble Space Telescope images in F606W (V) and F814W (I) taken with the ACS/WFC. The cluster scale sizes (half-light radii r_h) and ellipticities are determined from PSF-convolved King-model profile fitting. We find that the r_h distribution closely resembles the inner Milky Way clusters, peaking at r_h~2.5 pc and with virtually no clusters more compact than r_h ~ 1 pc. The metal-poor clusters have on average an r_h 24% larger than the metal-rich ones. The cluster scale size shows a gradual and noticeable increase with galactocentric distance. Clusters are very slightly larger in the bluer waveband V a possible hint that we may be beginning to see the effects of mass segregation within the clusters. We also derived a color magnitude diagram for the M87 globular cluster system which show a striking bimodal distribution.

VLT-FLAMES analysis of 8 giants in the bulge metal-poor globular cluster NGC 6522: oldest cluster in the Galaxy?

NGC 6522 has been the first metal-poor globular cluster identified in the bulge by W. Baade. Despite its importance, very few high resolution abundance analyses of stars in this cluster are available in the literature. The bulge metal-poor clusters may be important tracers of the early chemical enrichment of the Galaxy. The main purpose of this study is the determination of metallicity and elemental ratios in individual stars of NGC 6522. High resolution spectra of 8 giants of the bulge globular cluster NGC 6522 were obtained at the 8m VLT UT2-Kueyen telescope with the FLAMES+GIRAFFE spectrograph. Multiband V,I,J,Ks} photometry was used to derive effective temperatures as reference values. Spectroscopic parameters are derived from FeI and FeII lines, and adopted for the derivation of abundance ratios. The present analysis provides a metallicity [Fe/H] = -1.0+-0.2. The alpha-elements Oxygen, Magnesium and Silicon show [O/Fe]=+0.4, [Mg/Fe]=[Si/Fe]= +0.25, whereas Calcium and Titanium show shallower ratios of [Ca/Fe]=[Ti/Fe]=+0.15. The neutron-capture r-process element Europium appears to be overabundant by [Eu/Fe]=+0.4. The neutron-capture s-elements La and Ba are enhanced by [La/Fe]=+0.35 and [Ba/Fe]=+0.5. The large internal errors, indicating the large star-to-star variationin the Ba and Eu abundances, are also discussed. The moderate metallicity combined to a blue Horizontal Branch (BHB), are characteristics similar to those of HP~1 and NGC 6558, pointing to a population of very old globular clusters in the Galactic bulge. Also, the abundance ratios in NGC 6522 resemble those in HP 1 and NGC 6558. The ultimate conclusion is that the bulge is old, and went through an early prompt chemical enrichment.

On the origin of globular cluster bimodality

AbstractGlobular cluster systems in most large galaxies display bimodal color and metallicity distributions, which are frequently interpreted as indicating two distinct modes of cluster formation. The metal-rich (red) and metal-poor (blue) clusters have systematically different locations and kinematics in their host galaxies. However, the red and blue clusters have similar internal properties, such as their masses, sizes, and ages. It is therefore interesting to explore whether both metal-rich and metal-poor clusters could form by a common mechanism and still be consistent with the bimodal distribution. We show that if all globular clusters form only during mergers of massive, gas-rich protogalactic disks, their metallicity distribution could be statistically consistent with that of the Galactic globulars. We take the galaxy assembly history from cosmological dark-matter simulations and couple it with the observed scaling relations for the amount of cold gas available for star formation. In the best-fitting model, early mergers of smaller hosts create exclusively blue clusters, while subsequent mergers of progenitor galaxies with a range of masses create both red and blue clusters. Thus, bimodality arises naturally as the result of a small number of late, massive merger events. We calculate cluster mass loss, including the effects of two-body scattering and stellar evolution, and find that more blue than red clusters are disrupted by the present time because of their lower initial masses and older ages. The present-day mass function in the best-fitting model is consistent with the Galactic distribution. However, the spatial distribution of model clusters is much more extended than observed and is independent of the parameters of our model.

Nuclear clusters in dwarf irregular galaxies and their connection to massive galactic clusters

AbstractWe compare nuclear globular clusters (nGCs) in dwarf galaxies and Galactic GCs with extended (hot) horizontal branches (EHB–GCs) to test the suggested external origin of the latter and the conditions under which GC self-enrichment can operate. We show that the present-day escape velocity (vesc) of stellar ejecta to reach the cluster tidal radius compares with those of EHB–GCs. For EHB–GCs, we find a correlation between the present-day vesc and their metallicity as well as (V − I) colour. The similar vesc and (V − I) distribution of nGCs and EHB–GCs implies that nGCs could also have complex stellar populations. The vesc–[Fe/H] relation could reflect the known relation of increasing stellar-wind velocity with metallicity, which in turn explains why more metal-poor clusters typically show more peculiarities in their stellar population than more metal-rich clusters of the same mass.

MASS OUTFLOW FROM RED GIANT STARS IN M13, M15, AND M92

Chromospheric model calculations of the Halpha line for selected red giant branch (RGB) and asymptotic giant branch (AGB) stars in the globular clusters M13, M15, and M92 are constructed to derive mass loss rates. The model spectra are compared to the observations obtained with the Hectochelle on the MMT telescope. These stars show strong Halpha emissions and blue-shifted Halpha cores signaling that mass outflow is present in all stars. Outflow velocities of 3-19 km/s, larger than indicated by Halpha profiles, are needed in the upper chromosphere to achieve good agreement between the model spectra and the observations. The resulting mass loss rates range from 0.6*10^{-9} to 5*10^{-9} Msun/yr, which are about an order of magnitude lower than predicted from Reimers' law or inferred from the infrared excess of similar stars. The mass loss rate increases slightly with luminosity and with decreasing effective temperature. Stars in the more metal-rich M13 have higher mass loss rates by a factor of ~2 than in the metal-poor clusters M15 and M92. A fit to the mass loss rates is given by: M [Msun/yr] = 0.092 * L^{0.16} * Teff^{-2.02} * A^{0.37} where A=10^[Fe/H]. Multiple observations of stars revealed one object in M15, K757, in which the mass outflow increased by a factor of 6 between two observations separated by 18 months. Other stars showed changes in mass loss rate by a factor of 1.5 or less.

GLOBULAR CLUSTER SYSTEMS IN GIANT ELLIPTICALS: THE MASS/METALLICITY RELATION

Hubble Space Telescope ACS/WFC data in (B,I) are used to investigate the globular cluster populations around 6 gE galaxies ~40 Mpc distant. The total comprises a sample of ~8000 high-probability globular clusters. PSF-convolved King-model profiles are used to measure their individual total magnitudes, colors, and effective radii. The classic bimodal form of the GC color-magnitude distribution shows up unambiguously in all the galaxies, allowing an accurate definition of the mean colors along each of the two sequences as a function of magnitude (the mass/metallicity relation or MMR). The blue, metal-poor cluster sequence shows a clearly defined but nonlinear MMR, changing smoothly from a near-vertical sequence at low luminosity to an increasingly redward slope at higher luminosity, while the red, metal-rich sequence is nearly vertical at all luminosities. All the observed features of the present data agree with the interpretation that the MMR is created primarily by GC self-enrichment, along the lines of the quantitative model of Bailin and Harris (2009): The threshold mass at which this effect should become noticeable is near 1 million Solar masses, which is closely consistent with the transition region that is seen in the data. Correlation of the median half-light radii of the GCs with other parameters shows that the metal-poor clusters are consistently 17% larger than those of the metal-rich clusters, at all galactocentric distances and luminosities. At the same time, cluster size scales with halo location as r_h ~ R_gc^0.11, indicating that both metallicity and the external tidal environment play roles in determining the scale size of a given cluster. Lastly, both the red and blue GC components show metallicity gradients with galactocentric distance, following Z ~ R_gc^-0.1.

STOCHASTIC SELF-ENRICHMENT, PRE-ENRICHMENT, AND THE FORMATION OF GLOBULAR CLUSTERS

We develop a model for stochastic pre-enrichment and self-enrichment in globular clusters (GCs) during their formation process. GCs beginning their formation have an initial metallicity determined by the pre-enrichment of their surrounding protocloud, but can also undergo internal self-enrichment during formation. Stochastic variations in metallicity arise because of the finite numbers of supernova. We construct an analytic formulation of the combined effects of pre-enrichment and self-enrichment and use Monte Carlo models to verify that the model accurately encapsulates the mean metallicity and metallicity spread among real GCs. The predicted metallicity spread due to self-enrichment alone, a robust prediction of the model, is much smaller than the observed spread among real GCs. This result rules out self-enrichment as a significant contributor to the metal content in most GCs, leaving pre-enrichment as the viable alternative. Self-enrichment can, however, be important for clusters with masses well above 106 M☉, which are massive enough to hold in a significant fraction of their SN ejecta even without any external pressure confinement. This transition point corresponds well to the mass at which a mass–metallicity relationship (MMR, "blue tilt") appears in the metal-poor cluster sequence in many large galaxies. We therefore, suggest that self-enrichment is the primary driver for the MMR. Other predictions from our model are that the cluster-to-cluster metallicity spread decreases amongst the highest mass clusters; and that the red GC sequence should also display a more modest mass–metallicity trend if it can be traced to similarly high mass.

MASS OUTFLOW AND CHROMOSPHERIC ACTIVITY OF RED GIANT STARS IN GLOBULAR CLUSTERS. II. M13 AND M92

High resolution spectra of 123 red giant stars in the globular cluster M13 and 64 red giant stars in M92 were obtained with Hectochelle at the MMT telescope. Emission and line asymmetries in Halpha, and Ca K are identified, characterizing motions in the extended atmospheres and seeking differences attributable to metallicity in these clusters and M15. On the red giant branch, emission in Halpha generally appears in stars with T_eff 2.75. Fainter stars showing emission are asymptotic giant branch (AGB) stars or perhaps binary stars. The line-bisector for Halpha reveals the onset of chromospheric expansion in stars more luminous than log L/L_sun ~ 2.5 in all clusters, and this outflow velocity increases with stellar luminosity. However, the coolest giants in the metal-rich M13 show greatly reduced outflow in Halpha most probably due to decreased T_eff and changing atmospheric structure. The Ca K_3 outflow velocities are larger than shown by Halpha at the same luminosity and signal accelerating outflows in the chromospheres. Stars clearly on the AGB show faster chromospheric outflows in Halpha than RGB objects. While the Halpha velocities on the RGB are similar for all metallicities, the AGB stars in the metal-poor M15 and M92 have higher outflow velocities than in the metal-rich M13. Comparison of these chromospheric line profiles in the paired metal-poor clusters, M15 and M92 shows remarkable similarities in the presence of emission and dynamical signatures, and does not reveal a source of the `second-parameter' effect.

Massive AGB models of low metallicity: the implications for the self-enrichment scenario in metal-poor globular clusters

Context: We present the physical and chemical properties of intermediate-mass stars models of low metallicity, evolved along the thermal pulse phase. Aims: The target of this work is to extend to low metallicities, Z=1,2 and 6 x 10^{-4}, the models previously computed for chemistries typical of Globular Clusters of an intermediate metallicity (Z=0.001), and for the most metal-rich clusters found in our Galaxy (Z=0.004); the main goal is to test the self-enrichment scenario also for metal poor Globular Clusters Methods: We calculated three grids of intermediate-mass models with metallicities Z=10^{-4}, 2x10^{-4}, and 6x10^{-4}; the evolutionary sequences are followed from the pre-main sequence throughout the AGB phase, almost until the ejection of the whole envelope. We discuss the chemistry of the ejecta, and in particular the mass fractions of those elements that have been investigated during the many, deep, spectrocopic surveys of Globular Clusters Results: Although the data for oxygen and sodium are scarce for low metallicity Globular Clusters, the few data for the unevolved stars in NGC6397 are compatible with the models. Further, we find good agreement with the C--N anticorrelation of unevolved stars in the cluster M15. In this cluster, however, no stars having low oxygen ([O/Fe] = -1) have been detected. The most massive, very metal poor clusters, should contain such stars, according to the present models. At the lowest metallicity Z=10^{-4}, the ejecta of the most massive AGBs have C/O>1, due to the dramatic decrease of the oxygen abundance. We discuss the possible implications of this prediction.

Globular cluster systems in nearby dwarf galaxies - II. Nuclear star clusters and their relation to massive Galactic globular clusters

(Abridged) Using luminosities and structural parameters of globular clusters (GCs) in the nuclear regions (nGCs) of low-mass dwarf galaxies from HST/ACS imaging we derive the present-day escape velocities (v_esc) of stellar ejecta to reach the cluster tidal radius and compare them with those of Galactic GCs with extended (hot) horizontal branches (EHBs-GCs). For EHB-GCs, we find a correlation between the present-day v_esc and their metallicity as well as (V-I)_0 colour. The similar v_esc, (V-I)_0 distribution of nGCs and EHB-GCs implies that nGCs could also have complex stellar populations. The v_esc-[Fe/H] relation could reflect the known relation of increasing stellar wind velocity with metallicity, which in turn could explain why more metal-poor clusters typically show more peculiarities in their stellar population than more metal-rich clusters of the same mass do. Thus the cluster v_esc can be used as parameter to describe the degree of self-enrichment. The nGCs populate the same Mv vs. rh region as EHB-GCs, although they do not reach the sizes of the largest EHB-GCs like wCen and NGC 2419. We argue that during accretion the rh of an nGC could increase due to significant mass loss in the cluster vicinity and the resulting drop in the external potential in the core once the dwarf galaxy dissolves. Our results support the scenario in which Galactic EHB-GCs have originated in the centres of pre-Galactic building blocks or dwarf galaxies that were later accreted by the Milky Way.

LOW-MASS X-RAY BINARIES AND GLOBULAR CLUSTERS IN EARLY-TYPE GALAXIES. II. GLOBULAR CLUSTER CANDIDATES AND THEIR MASS-METALLICITY RELATION

We present an astrometry and photometry catalogue of globular cluster (GC) candidates detected with the Hubble Space Telescope Wide Field Planetary Camera 2 in a sample of 19 early-type galaxies, appropriate for comparison with the low-mass X-ray binary populations observed with Chandra. In a companion paper, we present the Chandra data and investigate the relation between these populations. We demonstrate that, although there is little evidence of a color-magnitude correlation for the GCs, after estimating mass and metallicity from the photometry under the assumption of a single-age simple stellar population, there is a significant positive correlation between mass and metallicity. We constrained [Z/H] = (-2.1 ± 0.2) + (0.25 ± 0.04)log10M, with a 1σ intrinsic scatter of 0.62 dex in metallicity. If GCs are bimodal in metallicity, this relation is consistent with recent suggestions of a mass-metallicity relation only for metal-poor clusters. Adopting a new technique to fit the GC luminosity function (GCLF) accounting for incompleteness and the Eddington bias, we compute the V-band local GC specific frequency (SN ) and specific luminosity (SL ) of each galaxy. We show that SL is the more robust measure of the richness of a GC population where a significant fraction is undetected due to source detection incompleteness. We find that the absolute magnitude of the GCLF turnover exhibits intrinsic scatter from galaxy to galaxy of ~0.3 mag (1σ), limiting its accuracy as a standard distance measure.

Relative and absolute ages of Galactic globular clusters

AbstractWe present a review of the latest work concerned with the relative and absolute ages of the Galactic globular clusters (GCs). Relative age-dating techniques generally divide into two types - those that measure a magnitude difference between two features in the color-magnitude diagram (i.e. Vertical Methods) and those that rely on color differences in the color-magnitude diagram (i.e. Horizontal Methods). Both types of diagnostics have been successfully applied and generally reach the same conclusions. Galactic GCs exhibit a mean age range of ~3 Gyr, smaller (or nonexistent) for metal-poor clusters and larger (as much as 6 Gyr) for metal-rich ones. Generally speaking, the inner-halo GCs are older and more uniform in age as compared with those outside of the solar circle. Furthermore, the tendency of GCs with predominantly red horizontal branches (HBs) located in the outer halo to be preferentially younger than those with bluer HBs closer to the Galactic center suggests that age is the second parameter which, in addition to metal abundance, controls the HB morphology. In particular, we present additional compelling evidence supporting this assertion using a detailed examination of new photometry for the classic second-parameter cluster pair NGC 288 and NGC 362. Moving on to the absolute ages, we note that the absolute ages of the most metal-poor Galactic GCs sets a lower limit on the age of the Universe. The preferred age indicator for absolute ages is the luminosity of the main-sequence turnoff because most theoretical models agree on the onset of hydrogen exhaustion in the cores of low-mass stars. Based on the technique of main-sequence fitting to field subdwarfs with Hipparcos parallaxes, we find an age of 11.6+1.4−1.1 Gyr for four metal-poor GCs with deep color-magnitude diagrams on a consistent photometric scale; this age is consistent with the results of a number of previous investigations.

CHEMICAL COMPARISON OF SAGITTARIUS AND FORNAX GLOBULAR CLUSTERS

Both Sagittarius (Sgr) and Fornax have a complex structure and a system of globular clusters (GCs) at variance with the classic concept of dwarf spheroidal galaxies: simple, uniform spherical systems. Sgr and Fornax are of similar total mass but present great differences in some aspects. We compare the GCs of these two galaxies using the results from spectroscopic studies available in the literature. The Sgr GCs present a wide range in metallicity and they can be divided in two groups: young, metal-rich clusters with a red horizontal branch (HB) and old, metal-poor clusters with a blue HB. On the other hand, the Fornax clusters show similar metallicity and have essentially the same age (with the possible exception of cluster 4 that seems to be 3 Gyr younger) but have a wide range of HB types. The most metal-poor cluster in the Sgr system, Ter 8, has a metallicity comparable with the Fornax globular results. Oxygen and α-elements abundances of the Fornax clusters and the metal-poor Sgr globulars are similar and, with the metal-rich Sgr clusters, they appear to follow the same patterns found in Galactic GC stars. The behavior of the light and heavy s-process elements with the iron content shows analogous results for the Sgr metal-poor clusters and the Fornax clusters. We can say that the Sgr GCs are more like those in the Milky Way (MW) with a range in metallicities while Fornax clusters are more similar to the very outer MW clusters with second parameter problems.