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New abundances for neutron-capture (n-capture) elements in a large sample of metal-poor giants from the Bond survey are presented. The spectra were acquired with the KPNO 4-m echelle and coudé feed spectrographs, and have been analyzed using LTE fine-analysis techniques with both line analysis and spectral synthesis. Abundances of eight n-capture elements (Sr, Y, Zr, Ba, La, Nd, Eu, Dy) in 43 stars have been derived from blue (λλ4070-4710Å, R∼20,000, S/N ratio∼100-200) echelle spectra and red (λλ6100-6180Å, R∼22,000, S/N ratio∼100-200) coudé spectra, and the abundance of Ba only has been derived from the red spectra for an additional 27 stars.Overall, the abundances show clear evidence for a large star-to-star dispersion in the heavy element-to-iron ratios. This condition must have arisen from individual nucleosynthetic events in rapidly evolving halo progenitors that injected newly manufactured n-capture elements into an inhomogeneous early Galactic halo interstellar medium. The new data also confirm that at metallicities [Fe/H] ∼ <-2.4, the abundance pattern of the heavy (Z≥56) n-capture elements in most giants is well-matched to a scaled Solar System r-process nucleosynthesis pattern.The onset of the main r-process can be seen at [Fe/H]≈-2.9; this onset is consistent with the suggestion that low mass Type II supernovae are responsible for the r-process. Contributions from the s-process can first be seen in some stars with metallicities as low as [Fe/H]∼-2.75, and are present in most stars with metallicities [Fe/H]>-2.3. The appearance of s-process contributions as metallicity increases presumably reflects the longer stellar evolutionary timescale of the (low-mass) s-process nucleosynthesis sites.The lighter n-capture elements (Sr-Y-Zr) are enhanced relative to the heavier r-process element abundances. Their production cannot be attributed solely to any combination of the Solar System r-and main s-processes, but requires a mixture of material from the r-process and from an additional n-capture process which can operate at early Galactic time. This additional process could be the weak s-process in massive (∼25 M ⊙ ) stars, or perhaps a second r-process site, i.e. different than the site that produces the heavier (Z≥56) n-capture elements.

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The heavy elements formed by neutron capture processes have an interesting history from which we can extract useful clues to and constraints upon both the characteristics of the processes themselves and the star formation and nucleosynthesis history of Galactic matter. Of particular interest in this regard are the heavy element compositions of extremely metal-deficient stars. At metallicities [Fe/H] ≤ -2.5, the elements in the mass region past barium (A > ∼ 130-140) have been found (in non carbon-rich stars) to be pure r-process products. The identification of an environment provided by massive stars and associated Type II supernovae as an r-process site seems compelling. Increasing levels of heavy s-process (e.g., barium) enrichment with increasing metallicity, evident in the abundances of more metal-rich halo stars and disk stars, reflect the delayed contributions from the low-and intermediate-mass (M ∼ 1-3 M ⊙ ) stars that provide the site for the main s-process nucleosynthesis component during the AGB phase of their evolution. New abundance data in the mass region 60 < ∼ A < ∼ 130 is providing insight into the identity of possible alternative r-process sites. We review recent observational studies of heavy element abundances both in low metallicity halo stars and in disk stars, discuss the observed trends in light of nucleosynthesis theory, and explore some implications of these results for Galactic chemical evolution, nucleosynthesis, and nucleocosmochronology.We are concerned in this review with the interpretation of the heavy element (A > ∼ 60) abundance patterns observed in diverse stellar populations in the context of nucleosynthesis theory. Following upon the early discussions of nucleosynthesis mechanisms by Cameron (1957) and Burbidge et al. (1957), we understand that most of the heavy elements are log 10 (N A /N B ) ⊙ , and that log ǫ(A) ≡ log 10 (N A /N H ) + 12.0, for elements A and B. Also, metallicity will be assumed here to be equivalent to the stellar [Fe/H] value. which the 22 Ne(α,n) 25 Mg reaction can operate to produce s-process nuclei through the mass region A ≈ 90 (the "weak" component). First studied by Peters (1968) and by Lamb et al. (1977), this process can in principle provide a source of the lightest s-process nuclei during the early stages of Galactic evolution (as soon as significant production of iron seed nuclei has occurred). Recent studies (Raiteri, Gallino, & Busso 1992; Baraffe, El Eid, & Prantzos 1992; Heger et al. 2002) reveal that the efficiency of production of s-process nuclei decreases at low metallicities (below [Fe/H]≈ -2) due to the increased competition arising from the elevated levels of abundance of nuclei from Ne to Ca relative to iron. We will return to this issue when we discuss the evolution of s-process abundances in §4. -7 -• The thermally pulsing helium shells of asymptotic giant branch stars provide an environment in which the 13 C(α,n) 16 O reaction can operate to produce s-process nuclei in the heavy region through to lead and bismuth (the "main" compon...

Accepted for publication in The Astrophysical Journal 47 Tuc is an ideal target to study chemical evolution and GC formation in massive more metalrich GCs since is the closest, massive GC. We present chemical abundances for O, Na, Al, Si, Ca, Ti, Fe, Ni, La, and Eu in 164 red giant branch (RGB) stars in the massive globular cluster 47 Tuc using spectra obtained with both the Hydra multi-fiber spectrograph at the Blanco 4-m telescope and the FLAMES multi-object spectrograph at the Very Large Telescope. We find an average [Fe/H]=-0.79±0.09 dex, consistent with literature values, as well as over-abundances of alpha-elements ([α/Fe] ∼ 0.3 dex). The n-capture process elements indicate that 47 Tuc is r-process dominated ([Eu/La]=+0.24), and the light elements O, Na, and Al exhibit star-to-star variations. The Na-O anti-correlation, a signature typically seen in Galactic globular clusters, is present in 47 Tuc, and extends to include a small number of stars with [O/Fe] ∼ -0.5. Additionally, the [O/Na] ratios of our sample reveal that the cluster stars can be separated into three distinct populations. A KS-test demonstrates that the O-poor/Na-rich stars are more centrally concentrated than the O-rich/Na-poor stars. The observed number and radial distribution of 47 Tuc's stellar populations, as distinguished by their light element composition, agrees closely with the results obtained from photometric data. We do not find evidence supporting a strong Na-Al correlation in 47 Tuc, which is consistent with current models of AGB nucleosynthesis yields.

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A combined effort utilizing spectroscopy and photometry has revealed the existence of a new globular cluster class. These "anomalous" clusters, which we refer to as "iron-complex" clusters, are differentiated from normal clusters by exhibiting large (0.10 dex) intrinsic metallicity dispersions, complex sub-giant branches, and correlated [Fe/H] and s-process enhancements. In order to further investigate this phenomenon, we have measured radial velocities and chemical abundances for red giant branch stars in the massive, but scarcely studied, globular cluster NGC 6273. The velocities and abundances were determined using high resolution (R ∼ 27,000) spectra obtained with the Michigan/ Magellan Fiber System (M2FS) and MSpec spectrograph on the Magellan-Clay 6.5 m telescope at Las Campanas Observatory. We find that NGC 6273 has an average heliocentric radial velocity of +144.49 km s −1 (σ = 9.64 km s