Horizontal Branch Morphology Research Articles

ASSESSING THE MILKY WAY SATELLITES ASSOCIATED WITH THE SAGITTARIUS DWARF SPHEROIDAL GALAXY

Numerical models of the tidal disruption of the Sagittarius (Sgr) dwarf galaxy have recently been developed that for the first time simultaneously satisfy most observational constraints on the angular position, distance, and radial velocity trends of both leading and trailing tidal streams emanating from the dwarf. We use these dynamical models in combination with extant 3-D position and velocity data for Galactic globular clusters and dSph galaxies to identify those Milky Way satellites that are likely to have originally formed in Sgr and been stripped from it during its extended interaction with the Milky Way. We conclude that the globular clusters Arp 2, M 54, NGC 5634, Terzan 8, and Whiting 1 are likely associated with the Sgr dwarf, and that Berkeley 29, NGC 5053, Pal 12, and Terzan 7 may be as well. The initial Sgr system therefore may have contained 5-9 globular clusters, corresponding to a specific frequency S_N = 5 - 9 for an initial Sgr luminosity M_V = -15.0. Our result is consistent with the 8\pm2 Sgr globular clusters expected from statistical modeling of the Galactic globular cluster distribution and the corresponding false-association rate due to chance alignments with the Sgr streams. These clusters are consistent with previous reconstructions of the Sgr age-metallicity relation, and show no evidence for a second-parameter effect shaping their horizontal branch morphologies. We find no statistically significant evidence to suggest that any of the recently discovered population of ultra-faint dwarf galaxies are conclusively associated with the Sgr tidal streams. (Abridged).

TERZAN 5: AN ALTERNATIVE INTERPRETATION FOR THE SPLIT HORIZONTAL BRANCH

We consider the horizontal branch (HB) of the Globular Cluster Terzan 5, recently shown to be split into two parts, the fainter one (delta M_K ~ 0.3mag) having a lower metallicity than the more luminous. Both features show that it contains at least two stellar populations. The separation in magnitude has been ascribed to an age difference of ~6 Gyr and interpreted as the result of an atypical evolutionary history for this cluster. We show that the observed HB morphology is also consistent with a model in which the bright HB is composed of second generation stars that are metal enriched and with a helium mass fraction larger (by delta Y ~ 0.07) than that of first generation stars populating the fainter part of the HB. Terzan 5 would therefore be anomalous, compared to most "normal" clusters hosting multiple populations, only because its second generation is strongly contaminated by supernova ejecta; the previously proposed prolonged period of star formation, however, is not required. The iron enrichment of the bright HB can be ascribed either to contamination from Type Ia supernova ejecta of the low-iron, helium rich, ejecta of the massive asympotic giant branch stars of the cluster, or to its mixing with gas, accreting on the cluster from the environment, that has been subject to fast metal enrichment due to its proximity with the galactic bulge. The model here proposed requires only a small age difference, of ~100Myr.

Accreted versusin situMilky Way globular clusters

Here we examine the Milky Way's GC system to estimate the fraction of accreted versus in situ formed GCs. We first assemble a high quality database of ages and metallicities for 93 Milky Way GCs from literature deep colour-magnitude data. The age-metallicity relation for the Milky Way's GCs reveals two distinct tracks -- one with near constant old age of ~12.8 Gyr and the other branches to younger ages. We find that the latter young track is dominated by globular clusters associated with the Sagittarius and Canis Major dwarf galaxies. Despite being overly simplistic, its age-metallicity relation can be well represented by a simple closed box model with continuous star formation. The inferred chemical enrichment history is similar to that of the Large Magellanic Cloud, but is more enriched, at a given age, compared to the Small Magellanic Cloud. After excluding Sagittarius and Canis Major GCs, several young track GCs remain. Their horizontal branch morphologies are often red and hence classified as Young Halo objects, however they do not tend to reveal extended horizontal branches (a possible signature of an accreted remnant nucleus). Retrograde orbit GCs (a key signature of accretion) are commonly found in the young track. We also examine GCs that lie close to the Fornax-Leo-Sculptor great circle defined by several satellite galaxies. We find that several GCs are consistent with the young track and we speculate that they may have been accreted along with their host dwarf galaxy, whose nucleus may survive as a GC. Finally, we suggest that 27-47 GCs (about 1/4 of the entire system), from 6-8 dwarf galaxies, were accreted to build the Milky Way GC system we seen today.

ON THE ANOMALOUS BALMER LINE STRENGTHS IN GLOBULAR CLUSTERS

Spectral feature index diagrams with integrated globular clusters and simple stellar population models often show that some clusters have weak H beta, so weak that even the oldest models cannot match the observed feature depths. In this work, we rule out the possibility that abundance mixture effects are responsible for the weak indices unless such changes operate to cool the entire isochrone. We discuss this result in the context of other explanations, including horizontal branch morphology, blue straggler populations, and nebular or stellar emission fill-in, finding a preference for flaring in M giants as an explanation for the H beta anomaly.

THE ACS SURVEY OF GALACTIC GLOBULAR CLUSTERS. IX. HORIZONTAL BRANCH MORPHOLOGY AND THE SECOND PARAMETER PHENOMENON

The horizontal branch (HB) morphology of globular clusters (GCs) is most strongly influenced by metallicity. The second parameter phenomenon acknowledges that metallicity alone is not enough to describe the HB morphology of all GCs. In particular, the outer Galactic halo contains GCs with redder HBs at a given metallicity than are found inside the Solar circle. Thus, at least a second parameter is required to characterize HB morphology. Here we analyze the median color difference between the HB and the red giant branch (RGB), d(V-I), measured from HST ACS photometry of 60 GCs within ~20 kpc of the Galactic Center. Analysis of this homogeneous data set reveals that, after the influence of metallicity has been removed, the correlation between d(V-I) and age is stronger than that of any other parameter considered. Expanding the sample to include HST photometry of the 6 most distant Galactic GCs lends additional support to the correlation between d(V-I) and age. This result is robust with respect to the adopted metallicity scale and the method of age determination, but must bear the caveat that high quality, detailed abundance information is not available for a significant fraction of the sample. When a subset of GCs with similar metallicities and ages are considered, a correlation between d(V-I) and central luminosity density is exposed. With respect to the existence of GCs with anomalously red HBs at a given metallicity, we conclude that age is the second parameter and central density is most likely the third. Important problems related to HB morphology in GCs, notably multi-modal distributions and faint blue tails, remain to be explained. (Abridged)

COMPARISON OF ALPHA-ELEMENT-ENHANCED SIMPLE STELLAR POPULATION MODELS WITH MILKY WAY GLOBULAR CLUSTERS

We present simple stellar population (SSP) models with scaled-solar and alpha-element enhanced abundances. The SSP models are based on the Dartmouth Stellar Evolution Database, our library of synthetic stellar spectra, and a detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. In order to test the relative importance of a variety of SSP model ingredients, we compare our SSP models with integrated spectra of 41 Milky Way Globular Clusters (MWGCs) from Schiavon et al. (2005). Using the Mg b and Ca4227 indices, we confirm that Mg and Ca are enhanced by about +0.4 and +0.2 dex, respectively, in agreement with results from high resolution spectra of individual stars in MWGCs. Balmer lines, particularly Hgamma and Hdelta, of MWGCs are reproduced by our alpha-enhanced SSP models not only because of the combination of isochrone and spectral effects but also because of our reasonable HB treatment. Moreover, it is shown that the Mg abundance significantly influences Balmer and iron line indices. Finally, the investigation of power-law initial mass function (IMF) variations suggests that an IMF much shallower than Salpeter is unrealistic because the Balmer lines are too strong on the metal-poor side to be compatible with observations.

A new method to measure stellar mass-loss rates

AbstractThe total stellar mass loss that a star suffers through post-main-sequence evolution is of vital importance to understand its subsequent evolution. The mass-loss rate along the first-ascent red-giant branch alone determines the upper red-giant-branch luminosity function and horizontal-branch morphology. The distribution of stars in these phases directly affects our interpretation of the integrated colors of distant galaxies, and is therefore of fundamental importance for galaxy formation and evolution studies in the higher-redshift Universe. Yet, these mass-loss rates, especially as a function of age and metallicity, are very poorly constrained in current models. I present new constraints on this field based on imaging and spectroscopic observations of the end products from this evolution, white dwarfs. By studying the mass distribution of these dead stars in nearby star clusters with a range of (known) ages and metallicities, we can directly constrain the mass-loss rates of stars across a range of environments. These observations directly impact several fields in astrophysics, including our knowledge of the enrichment of the interstellar medium, our ability to construct population synthesis models to interpret galaxy colors and the general interpretation of the sources and processes responsible for the observed ultraviolet upturn in elliptical galaxies.

An optical/near-infrared survey of GCs in early-type galaxies

AbstractOptical/near-infrared observations of 14 globular cluster (GC) systems in early-type galaxies are presented. We investigate the recent claims (Yoon et al. 2006) of colour bimodality in GC systems being an artefact of the nonlinear colour–metallicity transformation driven by horizontal-branch morphology. Taking advantage of the fact that the combination of optical and near-infrared colours can, in principle, break the age/metallicity degeneracy we also analyse age distributions in these systems.

HORIZONTAL BRANCH MORPHOLOGY OF GLOBULAR CLUSTERS: A MULTIVARIATE STATISTICAL ANALYSIS

The proper interpretation of horizontal branch (HB) morphology is crucial to the understanding of the formation history of stellar populations. In the present study a multivariate analysis is used (principal component analysis) for the selection of appropriate HB morphology parameter, which, in our case, is the logarithm of effective temperature extent of the HB (log Teff HB). Then this parameter is expressed in terms of the most significant observed independent parameters of Galactic globular clusters (GGCs) separately for coherent groups, obtained in a previous work, through a stepwise multiple regression technique. It is found that, metallicity ([Fe/H]), central surface brightness (μv), and core radius (rc) are the significant parameters to explain most of the variations in HB morphology (multiple R 2 ∼ 0.86) for GGC elonging to the bulge/disk while metallicity ([Fe/H]) and absolute magnitude (Mv) are responsible for GGC belonging to the inner halo (multiple R 2 ∼ 0.52). The robustness is tested by taking 1000 bootstrap samples. A cluster analysis is performed for the red giant branch (RGB) stars of the GGC belonging to Galactic inner halo (Cluster 2). A multi-episodic star formation is preferred for RGB stars of GGC belonging to this group. It supports the asymptotic giant branch (AGB) model in three episodes instead of two as suggested by Carretta et al. for halo GGC while AGB model is suggested to be revisited for bulge/disk GGC.

Red Clump stars in the Boötes III stellar system

Abstract We report on the detection of a population of Red Clump (RC) stars probably associated with the recently discovered stellar system Boötes III. The RC is identified as a 3σ peak in the luminosity function (LF) of colour-selected stars extracted from the Sloan Digital Sky Survey data base. The peak is consistently detected in the g, r, i and z LFs at the expected luminosity of a typical RC at the distance of Boötes III. Moreover, the stars around the LF peak show a maximum of surface density nearly coincident with the reported centre of the system. Assuming that the detected feature is the genuine RC of Boötes III, we find that the system has the horizontal branch morphology typical of old and metal-poor dwarf spheroidals, it has an integrated magnitude MV≃−5.8 ± 0.5 and an ellipticity ε∼ 0.5, quite typical of the recently identified new class of very faint dwarf galaxies.

REVEALING PROBABLE UNIVERSAL FEATURES IN THE LOWER RED GIANT BRANCH LUMINOSITY FUNCTIONS OF GALACTIC GLOBULAR CLUSTERS

This paper aims at demonstrating, for the first time, very probable universal peculiarities of the evolution of stars in the lower red giant branch (RGB) of Galactic globular clusters (GCs), reflected in two corresponding dips in the luminosity functions (LFs). By relying on the database of Hubble Space Telescope photometry of GCs, we analyze the lower RGB LFs of a sample of 18 GCs in a wide metallicity range, Δ[Fe/H] ≈ 1.9 dex. We first show that in the F555W–(F439W–F555W) color-magnitude diagrams (CMDs), the lower RGB of GCs, except for the most metal-poor of them, frequently shows an apparent knee. It reveals itself as a fairly abrupt change of the RGB slope. At the same luminosity level, the RGB LFs show a feature in the form of a more or less pronounced dip. We find that the magnitude difference between the RGB base and the given feature is, on average, around Δ F555Wdip base≈ 1.4 mag. It shows a marginal variation with metallicity, if any, comparable to the error. At the same time, the magnitude difference between the dip and the RGB bump, Δ F555Wbump dip, decreases with increasing metallicity and falls within the range 0.8 Δ F555Wbump dip 1.7 mag. Generalized LFs (GLFs) have been obtained for three subsamples of GCs within limited metallicity ranges and with different horizontal branch (HB) morphology. They reproduce the knee-related dip that is statistically significant in two of the GLFs. This feature turns out to be more pronounced in the GLFs of GCs with either the blue or red HB morphology than with the intermediate one. The same GLFs also reveal an additional probable universal dip. It shows up below the RGB bump at Δ F555W slightly increasing from ~0.3 to ~0.5 mag with increasing metallicity. Also, the statistical significance of this prebump dip increases, on average, toward higher metallicity. Except for the well known RGB bump, no other universal features corresponding to those found here were so far empirically revealed or theoretically predicted in the lower RGB of GCs.

The helium content of globular clusters: light element abundance correlations and HB morphology

Helium has been proposed as the key element to interpret the observed multiple main sequences (MS), subgiant branches (SGB) and red giant branches (RGB), as well as the complex horizontal branch (HB) morphology in Globular Clusters (GC). However, up to now, He was never directly measured in suitable GC stars (8500<Teff<11500 K) with the purpose of verify this hypothesis. We studied 7 hot blue horizontal branch (BHB) stars (Teff<11500 K) in the GC NGC 6752 with the purpose to measure their Helium content. In addition Fe,Cr,Si,Ti,O,Na, and Ba abundances were measured. We could measure He abundance only for stars warmer than Teff=8500 K. All our targets with measurable He are zero age HB (ZAHB) objects and turned out to have a homogeneous He content with a mean value of Y=0.245+-0.012, compatible with the most recent measurements of the primordial He content of the Universe (Y~0.25). The whole sample of stars have a metallicity of [Fe/H]=-1.56+-0.03 and [alpha/Fe]=+0.21+-0.03. Our HB targets show the same Na-O anticorrelation identified among the TO-SGB-RGB stars. This is the first direct measurement of the He abundance for a significative sample of GC stars in a temperature regime where the He content is not altered by sedimentation processing or extreme mixing as suggested for the hottest, late helium flasher HB stars.

Ground-based CCD astrometry with wide field imagers

Context. ω Centauri is the most well studied Galactic Globular Cluster because of its numerous puzzling features: significant dispersion in metallicity, multiple populations, triple main-sequence, horizontal branch morphology, He-rich population(s), and extended star-formation history. Intensive spectroscopic follow-up observing campaigns targeting stars at different positions in the color–magnitude diagram promises to clarify some of these peculiarities.

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.

Horizontal-Branch Morphology and Mass Loss in Globular Clusters

The connection between mass loss on the red giant branch (RGB) and horizontal-branch (HB) morphology in globular clusters (GCs) has long been acknowledged but the mechanisms governing mass loss remain poorly understood from a theoretical perspective. The present study uses synthetic HB models to demonstrate for the first time that α-enhancement and a simple relation between mass loss and metallicity can explain the entire range of HB morphology (characterized by the HB type index) observed in old, coeval GCs. The mass loss-metallicity relation accounts naturally for the fact that the most metal-poor GCs ([Fe/H] < –2) have redder HBs than is typical of GCs with –2 < [Fe/H] < –1.5 without invoking younger ages. These results may prove useful in studying the contribution of HB stars to integrated light via stellar population synthesis.

Globular Cluster Abundances from High‐Resolution Integrated‐Light Spectra. I. 47 Tuc

We describe the detailed chemical abundance analysis of a high-resolution ( -->R ~ 35,000), integrated-light (IL), spectrum of the core of the Galactic globular cluster 47 Tuc, obtained using the du Pont echelle at Las Campanas. We develop an abundance analysis strategy that can be applied to spatial unresolved extragalactic clusters. We have computed abundances for Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Ba, La, Nd, and Eu. For an analysis with the known color-magnitude diagram (CMD) for 47 Tuc we obtain a mean [Fe/H] value of -->-0.75 ? 0.026 ? 0.045 dex (random and systematic error), in good agreement with the mean of five recent high-resolution abundance studies, at ?0.70 dex. Typical random errors on our mean [X/Fe] ratios are 0.07-0.10 dex, similar to studies of individual stars in 47 Tuc. Na and Al appear enhanced, perhaps due to proton burning in the most luminous cluster stars. Our IL abundance analysis with an unknown CMD employed theoretical Teramo isochrones; however, we apply zero-point abundance corrections to account for the factor of 3 underprediction of stars at the AGB bump luminosity. While line diagnostics alone provide only mild constraints on the cluster age (ruling out ages younger than ~2 Gyr), when theoretical IL -->B ? V colors are combined with metallicity derived from the Fe I lines, the age is constrained to 10-15 Gyr and we obtain -->[ Fe/H ] = ? 0.70 ? 0.021 ? 0.052 dex. We find that Fe I line diagnostics may also be used to constrain the horizontal-branch morphology of an unresolved cluster. Lastly, our spectrum synthesis of 5.4 million TiO lines indicates that the 7300-7600 ? TiO window should be useful for estimating the effect of M giants on the IL abundances, and important for clusters more metal-rich than 47 Tuc.

GLOBULAR CLUSTERS IN THE OUTER GALACTIC HALO: AM-1 AND PALOMAR 14

AM-1, at ~120 kpc, and Pal 14, at ~70 kpc, are two of the most distant Galactic globular clusters known. We present Hubble Space Telescope WFPC2 photometry of AM-1 and Pal 14 that reveals unprecedented depth and detail in the color-magnitude diagrams of these two clusters. Absolute and relative age measurements confirm that both are younger than the inner halo globular cluster M 3 by 1.5-2 Gyr, assuming all three clusters have similar compositions. Thus AM-1 and Pal 14 join Pal 3, Pal 4, and Eridanus (studied by Stetson et al.) as distant Galactic globular clusters with red horizontal branches and young ages relative to the inner halo. Within the context of the entire body of research on the ages of second parameter globular clusters, the observed correlation between age and horizontal branch morphology suggests that age is the best candidate for the second parameter. However, this conclusion is tempered by the lack of precise chemical abundance determinations for a significant fraction of second parameter globular clusters.

THE FLATTENING OF GLOBULAR CLUSTERS

In the three nearest luminous galaxies, the Milky Way System, the Andromeda Galaxy and NGC 5128 the brightest globular clusters are rounder than the faintest ones. On the other hand (contrary to some previous results) the flattening of individual LMC clusters is found to be independent of their luminosities. This suggests the possibility that the relationship between the flattening and luminosity of clusters might depend on host galaxy luminosity. No significant differences are found between the intrinsic flattening distributions of Galactic old halo, Galactic young halo and Galactic bulge/disk clusters. Such a dependence might perhaps have been expected if tidal forces (which are largest at small Galactocentric distances) removed angular momentum from globular clusters. The preliminary conclusion by Norris that clusters with blue horizontal branches are more flattened than red HB clusters is not confirmed by the larger data base that is now available. In other words there is no evidence for the puzzling claimed correlation between the flattening and the horizontal branch morphology of Galactic globular clusters.

On the Origin of Bimodal Horizontal Branches in Massive Globular Clusters: The Case of NGC 6388 and NGC 6441

Despite the efforts of the past decade, the origin of the bimodal horizontal-branch (HB) found in some globular clusters (GCs) remains a conundrum. Inspired by the discovery of multiple stellar populations in the {\it most massive} Galactic GC, $\omega$ Centauri, we investigate the possibility that two distinct populations may coexist and are responsible for the bimodal HBs in the {\it third} and {\it fifth} brightest GCs, NGC 6388 and NGC 6441. Using the population synthesis technique, we examine two different chemical ``self-enrichment'' hypotheses in which a primordial GC was sufficiently massive to contain two or more distinct populations as suggested by the populations found in $\omega$ Cen: (1) the age-metallicity relation scenario in which two populations with different metallicity and age coexist, following an internal age-metallicity relation, and (2) the super-helium-rich scenario in which GCs contain a certain fraction of helium-enhanced stars, for instance, the second generation stars formed from the helium-enriched ejecta of the first. The comparative study indicates that the detailed color-magnitude diagram morphologies and the properties of the RR Lyrae variables in NGC 6388 and NGC 6441 support the latter scenario; i.e., the model which assumes a minor fraction ($\sim$ 15 %) of helium-excess (Y $\simeq$ 0.3) stars. The results suggest that helium content is the main driver behind the HB bimodality found most often in massive GCs. If confirmed, the GC-to-GC variation of helium abundance should be considered a {\it local} effect, further supporting the argument that age is the {\it global} second parameter of HB morphology.

Self-Enrichment in Globular Clusters: Is There a Role for the Super-Asymptotic Giant Branch Stars?

In four globular clusters (GCs) a non negligible fraction of stars can be interpreted only as a very helium rich population. The evidence comes from the presence of a "blue" main sequence in $\omega$ Cen and NGC 2808, and from the the very peculiar horizontal branch morphology in NGC 6441 and NGC 6388. Although a general consensus is emerging on the fact that self--enrichment is a common feature among GCs, the helium content required for these stars is Y$\simgt$0.35, and it is difficult to understand how it can be produced without any --or, for $\omega$ Cen, without a considerable--associated metal enhancement. We examine the possible role of super--AGB stars, and show that they may provide the required high helium. However, the ejecta of the most massive super--AGBs show a global CNO enrichment by a factor of $\simeq$4, due to the dredge--out process occurring at the second dredge up stage. If these clusters show no evidence for this CNO enrichment, we can rule out that at least the most massive super--AGBs evolve into O--Ne white dwarfs and take part in the formation of the second generation stars. This latter hypothesis may help to explain the high number of neutron stars present in GCs. The most massive super--AGBs would in fact evolve into electron--capture supernovae. Their envelopes would be easily ejected out of the cluster, but the remnant neutron stars remain into the clusters, thanks to their small supernova natal kicks.