STAR CLUSTERS IN M31. V. EVIDENCE FOR SELF-ENRICHMENT IN OLD M31 CLUSTERS FROM INTEGRATED SPECTROSCOPY (original) (raw)

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The Chemical Properties of Milky Way and M31 Globular Clusters. I. A Comparative Study

The Astronomical Journal, 2004

A comparative analysis is performed between high-quality integrated spectral indices of 30 globular clusters in M31, 20 Milky Way globular clusters, and a sample of field and cluster elliptical galaxies. We find that the Lick CN indices in the M31 and Galactic clusters are enhanced relative to the bulges of the Milky Way, M31, and elliptical spheroids, in agreement with Burstein et al. Although not particularly evident in the Lick CN indices, the near-UV cyanogen feature (λ3883) is strongly enhanced with respect to the Galactic globulars at metallicities, -1.5<[Fe/H]<-0.3. Carbon shows signs of varying amongst these two groups. For [Fe/H]>-0.8, we observe no systematic differences in the Hδ, Hγ, or Hβ indices between the M31 and Galactic globulars, in contrast to previous studies. The elliptical galaxy sample lies offset from the loci of the globular clusters in the both the Cyanogen-[MgFe], and Balmer-line-[MgFe] planes. Six of the M31 clusters appear young, and are projected onto the M31 disk. Population synthesis models suggest that these are metal-rich clusters with ages 100-800 Myr, metallicities -0.20 ≤ [Fe/H] ≤ 0.35, and masses 0.7 ∼ 7.0 × 10 4 M ⊙ . Two other young clusters are Hubble V in NGC 205, observed as a template, and an older (∼3 Gyr) cluster some 7 kpc away from the plane of the disk. The six clusters projected onto the disk show signs of rotation similar to the HI gas in M31, and three clusters exhibit thin disk kinematics (Morrison et al.). Dynamical mass estimates and detailed structural parameters are required for these objects to determine whether they are massive open clusters or globular clusters. If they are the latter, our findings suggest globular clusters may trace the build up of galaxy disks. In either case, we conclude that these clusters are part of a young, metal-rich disk cluster system in M31, possibly as young as 1 Gyr old.

Chemical abundances in the old LMC globular cluster

2012

Context. The study of globular clusters is one of the most powerful ways to learn about a galaxy's chemical evolution and star formation history. They preserve a record of chemical abundances at the time of their formation and are relatively easy to age date. The most detailed knowledge of the chemistry of a star is given by high resolution spectroscopy, which provides accurate abundances for a wide variety of elements, yielding a wealth of information on the various processes involved in the cluster's chemical evolution. Aims. We studied red giant branch (RGB) stars in an old, metal-poor globular cluster of the Large Magellanic Cloud (LMC), Hodge 11 (H11), in order to measure as many elements as possible. The goal is to compare its chemical trends to those in the Milky Way halo and dwarf spheroidal galaxies in order to help understand the formation history of the LMC and our own Galaxy. Methods. We have obtained high resolution VLT/FLAMES spectra of eight RGB stars in H11. The spectral range allowed us to measure a variety of elements, including Fe,

Old open clusters as key tracers of Galactic chemical evolution

Astronomy and Astrophysics, 2006

Aims. We obtained high-resolution UVES/FLAMES observations of a sample of nine old open clusters spanning a wide range of ages and Galactocentric radii. The goal of the project is to investigate the radial metallicity gradient in the disk, as well as the abundance of key elements (α and Fe-peak elements). In this paper we present the results for the metallicity of three clusters: NGC 2660 (age ∼ 1 Gyr, Galactocentric distance of 8.68 kpc), NGC 3960 (∼1 Gyr, 7.80 kpc), and Be 32 (∼6-7 Gyr, 11.30 kpc). For Be 32 and NGC 2660, our study provides the first metallicity determination based on high-resolution spectra. Methods. We performed equivalent width analysis with the spectral code MOOG, which allows us to define a metallicity scale and build a homogeneous sample. Results. We find that NGC 3960 and NGC 2660 have a metallicity that is very close to solar ([Fe/H]=+0.02 and +0.04, respectively), while the older Be 32 turns out to have [Fe/H]=−0.29.

Chemical evolution of star clusters

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2010

I discuss the chemical evolution of star clusters, with emphasis on old Galactic globular clusters (GCs), in relation to their formation histories. GCs are clearly formed in a complex fashion, under markedly different conditions from any younger clusters presently known. Those special conditions must be linked to the early formation epoch of the Galaxy and must not have occurred since. While a link to the formation of GCs in dwarf galaxies has been suggested, present-day dwarf galaxies are not representative of the gravitational potential wells within which the GCs formed. Instead, a formation deep within the proto-Galaxy or within dark-matter mini-haloes might be favoured. Not all GCs may have formed and evolved similarly. In particular, we may need to distinguish Galactic Halo from Galactic Bulge clusters.

Chemical Evolution of Clusters of Galaxies

The high metallicity of the intra-cluster medium (ICM) is generally interpreted on the base of the galactic wind scenario for elliptical galaxies. In this framework, we develop a toy-model to follow the chemical evolution of the ICM, formulated in analogy to chemical models for individual galaxies. Just as the ingredients for usual models are (a) the stellar yields, amount of metals newly synthesized and re-ejected by stars; (b) the Star Formation Rate and (c) the stellar Initial Mass Function (IMF), our model for clusters involves: (a') "galactic yields" derived from galactic wind models of ellipticals; (b') a parametric Galactic Formation Rate; (c') a Press-Schechter-like Galactic Initial Mass Function. The model is used to test the response of the predicted metal content and abundance evolution of the ICM to varying input galactic models. The resulting luminosity function of cluster galaxies is also calculated, in order to constrain model parameters.

THE AGES AND ABUNDANCES OF A LARGE SAMPLE OF M87 GLOBULAR CLUSTERS1

A subset of 150 globular clusters (GCs) in M87 has been selected for abundance and age determinations from the sample of Paper I . This has been done solely on the basis of the signal-to-noise ratios of the spectra. Indices that measure the strength of the strongest spectral features were determined for the M87 GCs and from new data for twelve galactic GCs. Combining the new and existing data for the galactic GCs and comparing the (U − R) colors and the line indices gave qualitative indications for the ages and abundances of the M87 GC system.

The Ages and Abundances of the M87 Globular Clusters

1997

We find that the M87 GCs span a wide range in metallicity, from very metal poor to somewhat above solar metallicity. The mean [Fe/H] of -0.95 dex is higher than that of the galactic GC system, and there is a metal rich tail that reaches to higher [Fe/H] than one finds among the galactic GCs. The mean metallicity of the M87 GC system is about a factor of four lower than that of the M87 stellar halo at a fixed projected radius RRR. The metallicity inferred from the X-ray studies is similar to that of the M87 stellar halo, not to that of GCs. We infer the relative abundances of Na, Mg, and Fe in the M87 GCs from the strength of their spectral features. The behavior of these elements between the metal rich and metal poor M87 GCs is similar to that shown by the galactic GCs and by halo stars in the Galaxy. The pattern of chemical evolution in these disparate old stellar systems is indistinguishable. We obtain a median age for the M87 GC system of 13 Gyr, similar to that of the galactic GCs, with a small dispersion about this value.

The Ages and Abundances of a Large Sample of M87 Globular Clusters

Astrophysical Journal, 1998

A subset of 150 globular clusters (GCs) in M87 has been selected for abundance and age determinations from the sample of Paper I . This has been done solely on the basis of the signal-to-noise ratios of the spectra. Indices that measure the strength of the strongest spectral features were determined for the M87 GCs and from new data for twelve galactic GCs. Combining the new and existing data for the galactic GCs and comparing the (U − R) colors and the line indices gave qualitative indications for the ages and abundances of the M87 GC system.

Abundance variations in the globular cluster M71 (NGC 6838)

Astronomy and Astrophysics, 2008

Context. Abundance variations in moderately metal-rich globular clusters can give clues about the formation and chemical enrichment of globular clusters. Aims. CN, CH, Na, Mg and Al indices in spectra of 89 stars of the template metal-rich globular cluster M71 are measured and implications on internal mixing are discussed. Methods. Stars from the turn-off up to the Red Giant Branch (0.87 < log g < 4.65) observed with the GMOS multi-object spectrograph at the Gemini-North telescope are analyzed. Radial velocities, colours, effective temperatures, gravities and spectral indices are determined for the sample. Results. Previous findings related to the CN bimodality and CN-CH anticorrelation in stars of M71 are confirmed. We also find a CN-Na correlation, and Al-Na, as well as an Mg 2-Al anticorrelation. Conclusions. A combination of convective mixing and a primordial pollution by AGB or massive stars in the early stages of globular cluster formation is required to explain the observations.