Galactic Globular Cluster Metallicity Research Articles

Stellar population models at high spectral resolution

We present new, high-to-intermediate spectral resolution stellar population models, based on four popular libraries of empirical stellar spectra, namely Pickles, ELODIE, STELIB and MILES. These new models are the same as our previous models, but with higher resolution and based on empirical stellar spectra, while keeping other ingredients the same including the stellar energetics, the atmospheric parameters and the treatment of the Thermally-Pulsating Asymptotic Giant Branch and the Horizontal Branch morphology. We further compute very high resolution (R=20,000) models based on the theoretical stellar library MARCS which extends to the near-infrared. We therefore provide merged high resolution stellar population models, extending from ~1000 AA to 25,000 AA. We compare how these libraries perform in stellar population models and highlight spectral regions where discrepancies are found. We confirm our previous findings that the flux around the V-band is lower (in a normalised sense) in models based on empirical libraries than in those based on the BaSeL-Kurucz library, which results in a bluer B-V colour. Most noticeably the theoretical library MARCS gives results fully consistent with the empirical libraries. This same effect is also found in other models using MILES, namely Vazdekis et al. and Conroy & Gunn, even though the latter authors reach the opposite conclusion. The bluer predicted B-V colour (by 0.05 magnitudes in our models) is in better agreement with both the colours of Luminous Red Galaxies and globular cluster data. We test the models on their ability to reproduce, through full spectral fitting, the ages and metallicities of galactic globular clusters as derived from CMD fitting and find overall good agreement. {Abridged}

Some Systematics of Galactic Globular Clusters

ABSTRACT The global properties of all known Galactic globular clusters are examined. The relationship between the luminosities and the metallicities of Galactic globular clusters is found to be complex. Among luminous clusters there is a correlation in the sense that the oldest clusters are slightly more metal-deficient than younger clusters. However, no such clear-cut relationship is found among the faintest globular clusters. The central concentration index C of globular clusters is seen to be independent of metallicity. The dependence of the half-light radii of globular clusters on their galactocentric distances can be approximated by the relation . Clusters with collapsed cores are mostly situated close to the Galactic nucleus. For Rgc < 10 kpc the luminosities and the radii of clusters appear to be uncorrelated. The Galaxy differs from the LMC and the SMC in that it appears to lack highly flattened luminous clusters. Galactic globular clusters with ages ≥13.0 Gyr are all of Oosterhoff type II, whereas almost all of those with ages < 13.0 Gyr have been assigned to Oosterhoff type I. Globular clusters with ages < 11.5 Gyr are all located in the outer Galactic halo and have below-average luminosities and above-average radii. On the other hand, the very old globular cluster NGC 6522 is situated close to the Galactic nucleus.

A DIFFERENTIAL CHEMICAL ELEMENT ANALYSIS OF THE METAL-POOR GLOBULAR CLUSTER NGC 6397

We present chemical abundances in three red giants and two turn-off stars in the metal poor Galactic globular cluster (GC) NGC 6397 based on spectroscopy obtained with the MIKE high resolution spectrograph on the Magellan 6.5-m Clay telescope. Our results are based on a line-by-line differential abundance analysis relative to the well-studied red giant Arcturus and the Galactic halo field star Hip 66815. At a mean of -2.10 +/- 0.02 (stat.) +/- 0.07 (sys.) the differential iron abundance is in good agreement with other studies in the literature based on gf-values. As in previous, differential works we find a distinct departure from ionization equilibrium in that the abundances of Fe I and Fe II differ by ~0.1 dex, with opposite sign for the RGB and TO stars. The alpha-element ratios are enhanced to 0.4 (RGB) and 0.3 dex (TO), respectively, and we also confirm strong variations in the O, Na, and Al/Fe abundance ratios. Accordingly, the light-element abundance patterns in one of the red giants can be attributed to pollution by an early generation of massive SNe II. TO and RGB abundances are not significantly different, with the possible exception of Mg and Ti, which is, however, amplified by the patterns in one TO star, additionally belonging to this early generation of GC stars. We discuss interrelations of these light elements as a function of the GC metallicity.

The light-weighted age and metallicity of Galactic globular clusters

Spectra synthesis is entering a new era with the availability of large and high-quality spectroscopic data and full spectrum fitting methods. Establishing the reliability whether the STARLIGHT code (a full spectrum fitting code, Cid Fernandes et al. (Mon. Not. R. Astron. Soc. 358:363, 2005)) is suitable to study the properties of various types of galaxies is vital to galaxy study. In testing this possibility, Galactic globular clusters (GCs) provide an excellent medium. Taking the 40 Galactic GCs as an example in our work, we analyze their integrated spectra from Schiavon et al. (Astrophys. J. Suppl. 160:163, 2005) and obtain their mixtures of stellar populations that best represents their composite nature. Through analyzing the distributions of stellar ages and metallicities respectively, we find that the stellar metallicity distribution is centered at a low value and the stellar age distribution is very scattered. To compare with the literature, we also calculate the light-weighted ages and metallicities of GCs, and we find that the light-weighted metallicities agree with the literature, while the light-weighted ages are lower than those of the literature. This demonstrates that STARLIGHT can be used to obtain the metallicities of GCs.

Na-O anticorrelation and HB

We present homogeneous abundances for Fe and some of the elements involved in the proton-capture reactions (O, Na, Mg, Al, and Si) for 202 red giants in 17 Galactic globular clusters (GCs) from the analysis of high resolution UVES spectra obtained with FLAMES@ESO-VLT2. Our programme clusters span almost the whole range in metallicity of GCs and were selected to sample the widest range of global parameters (HB morphology, masses, concentration, etc). Here we focus on the discussion of the Na-O and Mg-Al anticorrelations and related issues. Our study finds clear Na and O star-to-star abundance variations in all GCs. Variations in Al are present in all but a few GCs. Finally, a spread in abundances of Mg and Si are also present in a few clusters. Mg is slightly less overabundant and Si slightly more overabundant in the most Al-rich stars. The correlation between Si and Al abundances is a signature of production of 28Si leaking from the Mg-Al cycle in a few clusters. The cross sections required for the proper reactions to take over in the cycle point to temperatures in excess of about 65 MK for the favoured site of production. We used a dilution model to infer the total range of Al abundances starting from the Al abundances in the UVES spectra, and the Na abundance distributions found from analysis of the much larger set of stars for which GIRAFFE spectra were available. We found that the maximum amount of additional Al produced by first generation polluters contributing to the composition of the second generation stars in each cluster is closely correlated with the same combination of metallicity and cluster luminosity that reproduced the minimum O abundances found from GIRAFFE spectra. We then suggest that the high temperatures required for the Mg-Al cycle are only reached in the most massive and most metal-poor polluters.

The Relation between Near-infrared Luminosity of Red Giant Branch Bumps and Metallicity of Galactic Globular Clusters

Using photometric data from the Two Micron All Sky Survey Second Incremental Release Point Source Catalog, we constructed Ks versus J-Ks color-magnitude diagrams (CMDs) of Galactic globular clusters (GGCs) for which the JKs photometric data have been made available up to now. On the CMDs of 13 GGCs, we identified red giant branch bump features and derived luminosities of bumps in Ks (K) and in M (M) for 11 of them. We reconfirm the relation between M and metallicity [Fe/H]CG97 or [M/H], such that the luminosity of bump becomes brighter as metallicity [Fe/H]CG97 or [M/H] decreases. This result is similar to that obtained by Ferraro et al. (published in 1999) for the relation between M and [M/H] based on observations of 47 GGCs that were conducted in the optical region (V vs. B-V CMDs). Our results show the same trend as those found by Ferraro et al. (published in 2000) for the relations between M and metallicity [Fe/H]CG97 and [M/H] derived from observations of eight GGCs in the same near-infrared region (K vs. J-K CMDs). Combining with the 2000 data of Ferraro et al., we derive a robust relation for the metal-dependent luminosities of the bumps.