Globular cluster orbits and second parameter effects

Bergh, Sidney van den

doi:10.1086/116485

Cited by 46 publications

(13 citation statements)

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“…This assumes the Grevesse & Sauval (1998) solar abundances, with the enhancements in the abundances of the individual α-elements (at low metallicities) given by Cayrel et al (2004), appropriately scaled to [Fe/H] Cayrel et al) As shown by VandenBerg et al (2010), these models appear to provide good fits to optical CMDs for both metal-poor and metal-rich GCs and they reproduce quite satisfactorily the properties of local subdwarfs that have −2.0 [Fe/H] −0.6 and welldetermined M V values, as derived from Hipparcos parallaxes (van Leeuwen 2007). Since, in this investigation, the adopted GC distances are based on the predicted luminosities of ZAHB models, we have generated fully consistent ZAHB loci in the canonical way (see VandenBerg et al 2000) using exactly the same stellar evolutionary code that is described in detail by VBD12.…”

Section: The Input Observational and Theoretical Data Basesmentioning

confidence: 99%

THE AGES OF 55 GLOBULAR CLUSTERS AS DETERMINED USING AN IMPROVED $\Delta V^{\rm HB}_{\rm TO}$ METHOD ALONG WITH COLOR-MAGNITUDE DIAGRAM CONSTRAINTS, AND THEIR IMPLICATIONS FOR BROADER ISSUES

VandenBerg

Brogaard

Leaman

et al. 2013

ApJ

438

407

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Ages have been derived for 55 globular clusters (GCs) for which Hubble Space Telescope Advanced Camera for Surveys photometry is publicly available. For most of them, the assumed distances are based on fits of theoretical zero-age horizontal-branch (ZAHB) loci to the lower bound of the observed distributions of HB stars, assuming reddenings from empirical dust maps and metallicities from the latest spectroscopic analyses. The age of the isochrone that provides the best fit to the stars in the vicinity of the turnoff (TO) is taken to be the best estimate of the cluster age. The morphology of isochrones between the TO and the beginning part of the subgiant branch (SGB) is shown to be nearly independent of age and chemical abundances. For well-defined color-magnitude diagrams (CMDs), the error bar arising just from the "fitting" of ZAHBs and isochrones is ≈ ± 0.25 Gyr, while that associated with distance and chemical abundance uncertainties is ∼±1.5-2 Gyr. The oldest GCs in our sample are predicted to have ages of ≈13.0 Gyr (subject to the aforementioned uncertainties). However, the main focus of this investigation is on relative GC ages. In conflict with recent findings based on the relative main-sequence fitting method, which have been studied in some detail and reconciled with our results, ages are found to vary from mean values of ≈12.5 Gyr at [Fe/H] −1.7 to ≈11 Gyr at [Fe/H] −1. At intermediate metallicities, the age-metallicity relation (AMR) appears to be bifurcated: one branch apparently contains clusters with disk-like kinematics, whereas the other branch, which is displaced to lower [Fe/H] values by ≈0.6 dex at a fixed age, is populated by clusters with halo-type orbits. The dispersion in age about each component of the AMR is ∼±0.5 Gyr. There is no apparent dependence of age on Galactocentric distance (R G ) nor is there a clear correlation of HB type with age. As previously discovered in the case of M3 and M13, subtle variations have been found in the slope of the SGB in the CMDs of other metal-poor ([Fe/H] −1.5) GCs. They have been tentatively attributed to clusterto-cluster differences in the abundance of helium. Curiously, GCs that have relatively steep "M13-like" SGBs tend to be massive systems, located at small R G , that show the strongest evidence of in situ formation of multiple stellar populations. The clusters in the other group are typically low-mass systems (with 2-3 exceptions, including M3) that, at the present time, should not be able to retain the matter lost by mass-losing stars due either to the development of GC winds or to ram-pressure stripping by the halo interstellar medium. The apparent separation of the two groups in terms of their present-day gas retention properties is difficult to understand if all GCs were initially ∼20 times their current masses. The lowest-mass systems, in particular, may have never been massive enough to retain enough gas to produce a significant population of second-generation stars. In this case, the observed light element abundance variations, ...

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Section: The Input Observational and Theoretical Data Basesmentioning

confidence: 99%

THE AGES OF 55 GLOBULAR CLUSTERS AS DETERMINED USING AN IMPROVED $\Delta V^{\rm HB}_{\rm TO}$ METHOD ALONG WITH COLOR-MAGNITUDE DIAGRAM CONSTRAINTS, AND THEIR IMPLICATIONS FOR BROADER ISSUES

VandenBerg

Brogaard

Leaman

et al. 2013

ApJ

438

407

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“…These properties have led many to suggest that the YH GCs have been accreted (e.g. van den Bergh 1993). On the basis of their spatial distribution and sizes, Mackey & Gilmore (2004) suggested that ∼16 per cent, or ∼11, of the OH GCs were also accreted.…”

Section: Zinn Horizontal Branch Morphologymentioning

confidence: 99%

Accreted versusin situMilky Way globular clusters

Forbes¹,

Bridges²

2010

Monthly Notices of the Royal Astronomical Society

212

171

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A large, perhaps dominant fraction, of the Milky Way's halo is thought to be built up by the accretion of smaller galaxies and their globular cluster (GC) systems. 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 data base of ages and metallicities for 93 Milky Way GCs from literature deep colour–magnitude data. The age–metallicity relation for 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 GCs 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 six to eight dwarf galaxies, were accreted to build the Milky Way GC system we see today.

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“…Thus, information is passed at slower pace in lbest topology than that in gbest topology. This has been supported by separate research conducted by van den Bergh and Engelbrecht [16].…”

Section: Description Of Pso and Bare-bones Pso Methodsmentioning

confidence: 59%